RECENT VOLCANISM on TERCEIRA, AZORES a Thesis Submitted for the Degree of Doctor of Philosophy by Stephen Self December 1973

RECENT VOLCANISM ON TERCEIRA, AZORES

A thesis submitted for the degree of Doctor of Philosophy by

Stephen Self

December 1973 Geology Department, Imperial College, London, S.W.7. This thesis is dedicated to Col. Jose Agostinho. ABSTRACT: Terceira is one of the totally volcanic islands in the Azores, which straddle the Mid-Atlantic Ridge. The island shows a great diversity of lavas and pyroclastics; it has been the site of a large number of effusive and explosive eruptions. There are four volcanoes forming the island. Three are composed of both basaltic and salic rocks and one has only salic types exposed. All the rocks are in the alkali olivine basalt suite. Since the early stages of the island there has been a range of compositions from alkali olivine basalt to comendite existing at one time.

A 20,000 year old ignimbrite covers two-thirds of the island. Above this prominent horizon the products of over 100 eruptions have been recogw. nised. Most conspicuous are the comenditic ignimbrites (produced by pyroclastic flow eruptions), pumice-fall deposits and thick lava extrusions. Strombolian basaltic deposits and lavas are concentrated along the Fissure Zone which bisects the island diagonally from NW to SE. Crustal spreading on a small scale can be demonstrated along the Fissure Zone.

The pumice•fall deposits are of the sub-plinian type which have moderate dispersal areas and often show internal stratification. The ignimbrites display a layering of grain-size which has been recognized in ignimbrites from other areas.

There is a bimbdal population of rocks represented on the island. The most common compositions,both in number of eruptions and in total volume, are in the basalt-hawaiite and comendite groups. There are negligible lavas of benmoreite and trachyte composition lying chemically between the other two groups. Of the 5 cubic kilometres of new material erupted on the island in the past 20,000 years, over 3 are of salic composition. For an oceanic island the range of compositions of volcanic rooks and the range of eruptive styles is considerable. CONTENTS Page 1 Introduction' 1 1.1 Terceira 5 1.2 Structure 8 1.3 Previous studies 9 1.4 Objects rand methods 11 2 Stratigraphy and volcanology of Terceira 12 2.1.. Introduction 12 2.2 Pyroclastic-fall-deposits 13 2.2i, Pyroclastic-fall deposits from Pico Alto 18 2.2ii Pyroclastic-fall deposits from Sta. Barbara 26 2.2iii Basaltic scoria-fall deposits 36 2.3 Ignimbrite 46 2.4 Lava flows 55 2.4i Salle lava flows 57 2.4ii Basaltic lava flows 64 2.5 Discussion 70 2.5i Volume relationships 70 2.511 Dispersal of fall deposits 76 2.5111 Distribution of eruptions 79 2.5iv Styles of eruption 81_ 2.5v Conclusions 83 3 Ignimbrites on Terceira 85 3.1 Introduction 85 3.2 Stratigraphy 88 3.3 The Lajes and Angra Ignimbrites 95 3.3i Vertical grain-size variation 97 3.3ia) Layer 2b 100 3.3ib) Layer 2a 110 3.3ic) Layer 1 116 3.3id) Layer 3 122 3.3ii Areal grain-size distribution 123 3.4 The older ignimbrites of Terceira 128 3.-4i The Fanal and Vila Nova Ignimbrites 128 3.-4ii The Caldeira and Castelinho Ignimbrites 132 3.4iii The Porto do Pipas Ignimbrite 139 3.4iv Other ignimbrites 139 3.5 Fossil fumarole pipes 143 3.6 The effect of rising topography 147 3.7 Pumice and fiamme in the Lajes Ignimbrite 152 3.8 An eruptive mechanism model 155 3.9 Debris flows and other deposits 160 3.10 Ignimbrite on Sao Miguel 164 3.11 Conclusions 167 4 Petrology and geochemistry of Terceira 170 4.1 Introduction 170 4.2 Eruptive centres 172 4.3 Petrography 173 4.4 Feldspars 176 4.5 Chemistry 177 4.5i Analyses 178 4.5ii Geochemistry 180 4.5iia) Basaltic and intermediate lavas and pyroclastics 183 4.5iib) Peralkaline lavas and pyroclastics 191 Page 4.6 Volumetric and time considerations 198 4.6i Proportions of basaltic to saiic types 198 4.611 The composition gap 205 4.6111 Composition changes with time :208 4.6iv Discussion 208 4.7 Petrogenetic models 210 4.8 Comparison with other islands ' 213 4.9 Pyroclastic rock analyses, 216 5 The early history of Terceira 219 6 Summary 223 Acknowledgements 226 Bibliography 228

TEXT FIGURES

1.1 The Azores 2 1.2 Terceira, structure 4 1.3 Terceira, localities 7 1.4 Azores bathymetry 7 2.1 Tephrochronological chart 14 2.2 Pyroclastic-fall deposit "B" 17 2.3 Pyroclastic-fall deposit "C" and "I" 19 2.4 Pyroclastic-fall deposit "E" 21 2.5 Granulometric data for "B" 24 2.6 Pyroclastic-fall deposits "A","D" and "F" 27 2.7 Pyroclastic-fall deposit "G" . - 29 2.8 Pyroclastic-fall deposit "H" and palaeowinds 34 2.9 Basaltic scoria-fall deposits 39 2.10 Basaltic scoria-fall deposits 40) 2.11 Quantitative data 41 2.12 The Lajes and Angra Ignimbrites 47 2.13 Granulometric data for Angra Ignimbrite 51 2.14 Salle lavas 58 2.15 Salle lavas, quantitative data 59 2.16 Basaltic lavas 65 2.17 Scattered basaltic eruptions 67 2.18 Volumektric data 73 2.19 Grain-size/distance relationships 75 2.20 Eruptive centres of the past 23,000 years 80 3.1 Lajes and Angra Ignimbriteouterop map 86 3.2 North coast section 91 3.3 Angra harbour section 94 3.4 Schematic section through an ignimbrite 96 3.5 Lajes Ignimbrite granolometric data 98 Page 3.6 Angra Ignimbrite granulometric data 99 3.7 Inman parameters 101 3.8 Layer 2b frequency curves 102 3.9 Crystal concentration 104 3.10 Component cumulative curves 106 3.11 Laminar shear zone 112 3.12 Ground surge beds 117 3.13 Ground surge deposits, granulometric data 119 3.14 Grain-size distribution of pumice clasts 124 3.15 Grain-size distribution of lithic clasts 125 3,16 Fanal and Vila Nova Ignimbrites 129 3.17 Inman parameters 133 3.18 Castelinho and Caldeira Ignimbrites 138 3.19 Basaltic ignimbrite 140 3.20 Individual component triangle 142 3,21 So Mateus ignimbrite outcrop 153 3.22 Fluidisation phases 157 3.23 Debris flow 161 3.24 Vulcanian deposit 163 3.25 Combined Terceira Inman parameters 166 3.26 Block diagram 168 4.1 Major element oxides plotted against Mg0 179 4.2 CaO-NA O-K 0 diagram 181 4.3 Saturat2ion 2 state plotted against Differentiation Index 182 4.4, Ne-01-Q diagram 184 4.5 Alkalis plotted against SiO 185 4.6 AFM for Terceira rocks 2 187 4.7 AFM forCinquo Picos and recent volcanics 188 4.8 AFM for Santa Barbara 190 4.9 Femics plotted against normative quartz 192 4.10 A1 ,O, plotted against Si0,, 194 4.11 Sib Alkalis diagtiam 196 4.12 Quaarilateral of Al203 against differentiation index 197 4.13 Volumetric data 203 4.14 Frequency and volume plotted against differentiation index 204 4.15 Time plotted against peralkalinity index 207 4.16 AFM for Graciosa and sac Miguel 214 4.17 Ignimbrite section 217 5.1 Sections through the old volcanics of Terceira 220

LIST OF PLATES

1.1. Santa Barbara Volcano 3 1.2 Pico Alto Volcano 3 2.1 Pyroclastic-fall deposit "E" 22 2.2 Pyroclastic-fall deposit "G" 31 2.3 Pyroclastic-fall deposit "H" 33 2.4 Pico do Gaspar 37 2.5 Monte Brasil 38 2.6 Pico Gordo 43 Page

2.7 Monte Brasil tuff 44 2.8 Monte Brasil tuff and older compound lava flow 45 2.9 Ignimbrites near Furnas da Agua 48 2.10 Misterios Negros 54 2.11 Pico Alto Caldera 56 2.12 Pico Raehado 60 2.13 Lava levee 62 2.14 Pico Alto lavas 62 2.15 Flow structures in comendite lava 63 2.16 Lava structures on the Fissure Zone 66 2.17 2,000 year old basalt and Cinquo Picos Caldera 68 2.18 1761 Fissure 69 3.1 Cliff section east of Angra 90 3.2 The north coast plain 93 3.3 Pumice concentration 93 3.4 Basal layer, Angra harbour 109 3.5 Welded basal layer near Angra 109 3.6 Welded basal layer at So Mateus 111 3.7 Basal layer in Italian ignimbrite 115 3.8 Ground surge bed in Italian ignimbrite 115 3.9 Vila Nova Ignimbrite 127 3.10 Ground surge bed, Fanal 127 3.11 Small flow unit in Vila Nova Ignimbrite 130 3.12 Cliff section east'of Angra 131 3.13 Ground surge bed, north coast 134 3.14 Ground surge bed 135 3.15 Debris flow, Caldeirs Ignimbrite 136 3.16 Pipe near Sovannal jtaly 144 3.17 Pipe in Castelinho Ignimbrite 144 3.18 Lajes Ignimbrite 148 3.19 Juvenile material from Lajes Ignimbrite 150 3.20 Pumice in the Lajes Ignimbrite, Sao Mateus 151 4.1 Basaltic lavas on the coast north of Santa- Barbara Volcano 199 5.1 Dyke at Baia da Mina 222

• 1. INTRODUCTION

Modern theories of global tectonics tell us that at the lending edges of mobile lithosphere plates we must expect to encounter zones of orogerly, earthquakes, and paroxyamal volcanic activity. The other mar-. gins of these rigid plates, where new crust is produced, also have their share of spectacular phenomena. However, by the very fact of their being at the centre of ocean basins, these constructive plate margins are very difficult to study in detail. Where an island occurs near the crest of a mid-oceanic ridge it therefore' provides particularly valuable evidence of the processes at work in such an environment.

The widely scattered volcanic islands along the mid-Atlantic Ridge include some with a record of violent explosive activity. Jan Mayen, Iceland, the Azores, Ascension, Tristan da Cunha and Bouvet all lie on or near the crest of the mid-Atlantic Ridge and all have at least one active volcano producing explosive eruptions. The other centres of Quaternary volcanic activity in the Atlantic basin are on the eastern margin i.e. the Canaries and Cape Verde Islands.

The Azores group (Fig. 1.1) straddles the Mid-Atlantic Rift, in the latitudes 37-39°N and longitude 26-31°W, 1300km from the Portuguese mainland. It is a widely spaced archipelago of nine islands, 6001en from east to west, situated on the Azores Platform, an approximately triangular region of 2000m depth, extending mainly to the east of the Rift. The islands tend to be older the further they are situated from the Rift. The Azores are divided into 3 groups; a western group of Corvo and Flores, a central group of Faial, Pico, Sio Jorge, Graciosa and Terceira and an eastern group of Sic Miguel and Santa Maria. 1 Conic). 0 25'W *Flores Graciosa 4 39N- ,444446.- si Terceira . Falai Sao Jorge Pico Sao Miguel tag?

' Santa Maria 3714-

200km

Fig. 1.1 The Azores. Plate 1.1 Santa Barbara Volcano seen from the SE. Note truncated top, which rrntks the caldera rim; left side, in cloud,is highest point on island, 1021m.

•

Plate 1.2 Pico Alto Volcano looking NE from the fissure zone. The light-coloured slopes are comenditic lava domes covered by pyroclastic fall deposits. Darker lavas in distance are younger domes in the caldera; on the left is Pico dn. Pardelas dome.

i • 27.20. 2710*

•

11 I „, 32°W 111 26- I 1 o -40°N . rl 40 38.46- : . 4 k ,acoRA 4. .. Iv , -380 , 0„9„ .., cs. • 5krn e"...*i 1

Fig. 1.2 Map of Terceira. The fissure zone is stippled. A is Santa Barbara Volcano with Old and New Calderas represented by wide-spaced and close-spaced triangles respectively. B is PiCo Alto Volcano. C and D are the old calderas of Guilherme Moniz and Cinquo Picos volcanoes. Faults are shown by lines and towns as open squares. Contours at 250 and 500m, Inset are the Azores and the Mid-Atlantic Rift (MAR) and Terceira Rift (TR) after Krause and Watkins (1971). The islands, discovered by the Portuguese in 1450 AD, were colonised In order from east to west; Terceira, literally 'the third' (discovered), was colonised in 1485. It is the third largest of the group; it measures 28km wide by 18km from north to south and covers 2 401km . For its size, Terceira is perhaps the most complex of all the islands in the, Azores and shows a wide variety of rock types.

1.1 Terceira Terceira consists of a group of four strato-volcanoes lying on a prominent fissure zone Fig. 1.2) which traverses the island from NW to SE. The volcano with the most youthful form is Santa Barbara lying at the NW end of the island, nearest to the mid-Atlantic Rift. It is an active volcano, with a truncated profile, diversified by trachytie domes in and Found a young caldera. There is an older caldera with the highest point on the island, 1021m above sea level, on the rim, (Plate 1.1).

In the centre of the island another young volcano, Pico Alto (798m) lies immediately to the north of the Fissure Zone. It is a chaotic assemblage of trachytiodomes•and coul4es, lacking any symmetry, overspilling a young caldera, (Plate 1.2).

The two older volcanoes are incomplete, caldera remnants of extinct centres. Guilheme Moniz Caldera lies directly on the Fissure Zone south of Pico Alto Volcano. The caldera walls are trachytielavas, up to 150m thick. The floor of the caldera is covered by young basalt from the adjacent Fissure Zone. c The oldest caldera, Cinquo Picos, is the largest in the Azores with a maximum diameter of 7km. Exposures in the caldera rim are mpAnly 6 basaltic lavas. Scattered on the flanks of all the volcanoes, except Pico Alto, are numerous adventive basalt scoria cones and lava flows. Scoria and spatter cones are also prominent along the Fissure Zone.

The Azores islands are situated in the warm temperate oceanic climatic belt and have warm, dry summers and mild, wet winters. There is a peripheral sub—tropical zone at sea level. In the mountainous interior of Terceira there is high rainfall (over 5000mm per annum). Under these conditions soils develop rapidly, especially on pyroclastic deposits, a fact which has proved useful in estimating time periods between pyroclastic deposits. Despite the high rainfall there are only 2 permanent streams on the island, due to the high porosity of the rocks. Consequently the island is little dissected.

Beneath Guilherme Moniz Caldera there is an extensive underground stream system, utilising basalt lava tubes; the main water supply for the town of Angra do. Heroismo comes from this underground system. Angra is situated in the shadow of the large Monte Brasil tuff—ring, which aff- ords an excellent natural harbour. It is a town of some 22,000 people. The remainder of the 90,000 population of Terceira live,• almost without exception, in a 2km coastal strip around the circumference of the island and in the Lajes Plain. There is a NATO air force base occupying most of the Lajes Graben (Fig. 1.3), making most of this area out of bounds to the geologist. The island is well served by roads and mostis easily accessible except the caldera floors of Pico Alto and Santa Barbara. Here many of the young salic lavas are covered with dense, thick vegetation, making access virtually impossible and all but the very youngest have negligible outcrop. Apart from the higher parts of the two youngest volcanoes and the youngest lavas, the land is under grass, maize or vines. There is a large population of cattle and dairy farming 7

., ~ A aiaoib- Or to Judeu

Fig. 1.3 Locality map of Terceira. Towns are shown as triangles. Numoered localities are: 1. Vale do linhares, 2. Castelinho, 3. Salgueiras, 4. Pico da B~cina, 5. Pico de Pardelas, 6. Porto do Vila Nova. Area within the rectangle is that shown in Figs. 2.9 and 2.16.

37,----_~

36 contours x100m

Fig. 1.4 The bathYmetry of the Azores platfonn, (after U.S. Naval Th Oceanographi c Chaxts) , showing "deeps" as hachured n,'areas. .e Terceira Rift i~ marked by a line of deeps b~tween uraC10sa, ; Terceira and Sao·Miguel. Xro 8 is the island's chief industry.

1.2 The Structure of the Azores and Terceira

Oceanographic studies have proved that a breakdown of the normal magnetic-strip anomaly pattern, parallel to the Mid-Atlantic Ridge, occurs at an E-W zone joining the Azores with Gibraltar (Williams and McKenzie, 1971). Furthermore crustal spreading, as proposed for the North and South Atlantic basins, appears tobe less applicable to the central Atlantic in the region ofthe Azores (Isacks, Oliver and Sykes, 1968). The Azores-Gibraltar ridge, generally known as the East Azores Fracture Zone (Heezen et al., 1959, Laughton et al., 1972) is seismically active and may be a transform fault (Krause, 1965). Krause also describes an aseiamic rift to the west of the Azores, the West Azores Frac- ture Zone. At the point where these two Fracture Zones meet the Mid- Atlantic Rift there is a change in the direction of the Rift from WSW to northerly and the Azores and situated at this bend.

It is generally accepted, from oceanographic work, that the crest of the Mid-Atlantic Ridge passes between Faial and Flores. Yet there may be a zone of very complex offsets of the Ridge in the Azores Platform area, and due to the chaotic anomaly patterns found (Krause and Watkins, 1970) the precise position of the Ridge crest is uncertain. However, some of•the more prominent structures of this area have long been appreciated by geophysicists and Agostinho (1931), described a small ridge on the line of Graciosa, Terceira and Sao Miguel islands. Later Krause and Watkins (1970), interpreted this structure as a small rift, the Terceira Rift, with interconnecting deep basins between the 3 islands (Fig. 1.4). It is oblique to the Mid-Atlantic Rift and trends SW towards the East Azores Fracture Zone (Fig. 1.2, inset). The structure of Terceira is dominated by the subaerial expression of this Rift, namely, a fissure zone marked by the main volcanoes, short enptive fissures, gaping cracks and intense basaltic activity. The Terceira Rift is considered, by Krause and Watkins, to be a secondary spreading centre with a spreading rate in the order of 0.25 cm yr-1. The Terceira Fissure Zone forms the watershed in the central part of the island between Santa Barbara and Pico Alto Volcanoes. Most faulting is subparallel to the trend of the Fissure Zone (Fig. 1.2). One large graben exists in the NE cover and historic earthquakes on the Lajes Fault have twice destroyed the town of Praia da Vitoria in 1614 and 1841. There are minor faults oblique to this general trend, most conspicuous in the area of Angra.

1,3 Previous studies of Terceira The Geological Survey of Portugal has recently published a geological map of Terceira on the scale 1:50,000, accompanied by an explana- tory leaflet (Zbyszewski, et al. 1971). The map presents a good picture of the general geology and divides the lavas into basalt, 'andesites' (a Portuguese term for mugearite-type lavas) and trachyte. Pyroclastic rocks are divided into scoria cones and general Ipyroclasticst.

The essential common factor between most of the previous work is the piecemeal nature in which either various aspects of the island have been investigated without considering the whole, or the island has been described together with the rest of the Azores. Scientists have been visiting the island for more than a century. Darwin, in 1876, compared the trachytes of Terceira with those of Ascension. Fouqug (1883) described a ItridUnic feldspar from Quatro Ribeiras and discussed briefly the petrology of the trachytes. He had also made a visit to the island shortly after the 1867 submarine eruption off the NW coast (FoUque 1873, 10

Zbyszewski 1966-67). Friedlander (1929) listed historic eruptions and earthquakes and first described, indirectly, Pico Alto Volcano as a very complex trachytic area; Esenwein (1929) presented a complementary petrological study. Agostinho (1931) discussed the structure of the Azores and the volcanic landforms on Terceira, and later the form of Monte Brasil tuff ring. Five chemical analyses of trachytic lavas were published by Berthas (1953), together with a report on the 'geyserite' (including opalised plant remains) from near Biscoitos.

Machado (1955) described the fracture pattern on Terceira and suggested that the ignimbrite at Lajeswas possibly associated with Cinquo Picos caldera. At about the same time a general review of the island was published by Krejci Graf (1956a) as part of a general Azores paper. He describes Monte Brasil tuff ring and discusses the tuff thickness and the shape of the deposit on varying slopes (Krejci Graf, 1956b).

More recently there have been useful accounts of historic eruptions in the Azores (Agostinho, 1960; Weston, 1964; Machado, 1967) and the island is listed by Machado in the Catalogue of Active Volcanoes (1967, pt. XXI). The structure of the four volcanoes and a sequence of eruptions from basaltic and landesiticl to trachytic on Santa Barbara, Guilherme Moniz and Cinquo Picos were described by Zbyszewski (1968). The unusual, totally 'trachytic' nature of Pico Alto Volcano was noted by this author, who has contributed much to the recent geological work on the island by the Servicos Geologicos de Portugal. There are many short acmunts, in Portuguese and mainly by Agostinho, on various aspects of Terceira, in the local Azores periodical lAcoreanal.

Recently, Schminke and Weibel (1972) and Schmink (1973) have published ten analyses of Terceira lavas and ignimbrite in a comparative study 11. of the Azores, Madeira and the Canary Islands. They found the Azores to be generally less potassic than the other island groups and described an inter-island variation, Terceira being less potassic than Sao Miguel.

1.4 Objects and Methods There were many reasons for investigating the volcanology of Terceira. Here was a chance to examine the processes at work on a constructive, mid oceanic ridge where new crust is created. On Terceira large volumes of salic lava and pyroclastic rocks are intimately associated with a dominantly basaltic fissure zone, posing interesting problems of petrogenesis in a non-orogenic environment. For a, detailed study of the volcanic products of the youngest formation a quantiative approach was considered essential. The base of this youngest formation is delimited by a well defined stratigraphic horizon, the Lajes Ignimbrite and this formation is the product of many eruptions on Santa Barbara, Pico Alto, the Fissure Zone and a few scattered adventive cones. These recent volcanics are relatively youthful and well exposed,making a volumetric study possible and worthwhile. The Lajes Ignimbrite, moreover, presented a good opportunity to study grain size variations in these problematic pyroClastic rocks on a small island, where access and detailed sampling are perhaps easier than in large ignimbrite sheets. Full volumetric data on a whole sequence of lavas and pyroclastics are invaluable in interpreting the petrological relations on the island, especially in view of the abundance of large volumes of, salic lavas. The following account considers three main aspects of the geology of Terceira; the strdigraphy and volcanology of the youngest formation, a detailed study of the ignimbrites, and the general petrology and geochemistry of the island. 1 2 2. STRATIGRAPHY AND VOLCANOLOGY OF TERCEIRA

2.1 Introduction Terceira shows evidence of a violent volcanic history with infrequent large pyioclastic flow eruptions, which have produced at least five ignim-, brites, alternating with more frequent sub-plinian and strombolian explosive eruptions and the effusion of lavas from basaltic to pantelleritic composition. The latest ignimbrite, the Lajes Ignidbrite, about 20,000 years old, covers a large past of the island and forms a convenient stratigraphic horizon, the base of the most recent volcanic formation on Terceira.

This section describes the products of 116 separate eruptions which have been distinguished in this formation, and the results of the activity of Terceira over the past 23,000 years. It attempts to show the value of quantitative volcanology and tephroohronology in understanding the processes at work on a constructional mid-oceanic ridge.

The produots of one eruption are here referred to as a member, which can be a single lava flow or a lava flow with associated pyroclastics. A further subdivision is made of lavas and ignimbrites into flow units, and of pyroclastic fall deposits into fall units (Nakamura, 1964). The vents of the 116 eruptions are widely scattered and it is not always possible to be certain to which volcano a particular vent belongs. For con- venience of description the vents have been rather arbitrarily placed into four groups: (1) Pico Alto Volcano, (2) Santa Barbara Volcano, (3) the Fissure Zone, and (4) scattered vents which are not obviously related to either a volcano or the fissure zone. 13 2.2 Pyroclastie fall deposits Pyroclastic fall deposits are much less conspiauous on Terceira than they are on some other Azorean islands, notably Sao Miguel and Faial. However, trachytic pumice fall deposits are locally well developed on and around Pico Alto and Santa Barbara, and thick basaltic scoria deposits occur in the fissure zone. Nine particularly prominent pumice deposits have been studied in detail. Those originating from Santa Barbara vents are referred to as A, D, F, G, H (in stratigraphi order) and those from Pico Alto as B, C, E4 I. In addition eight basaltic scoria deposits have been mapped and have proved useful in cor-s relation on a local scale.

The pumice fall deposits are all of polycomponent type with varying proportions of vesiculated juvenile material, (pumice and shards), juvenile crystals derived from the fragmented magma and lithic fragments. The crystals in all these trachytic pumice fall deposits are dominantly anorthoclase. All dense rock fragments found in the deposits are here called lithics. They may be either non-vesiculated juvenile material (obsidian) or non-tuvenile. The latter may be derived from the walls of the source vent.

Measurements were made in the field of the thickness and the MI- mum size of both pumice and lithic °lasts at each locality for each of the nine main pumice fall deposits. From these data isopach and isopleth maps were constructed. From the isopadhmaps the dispersal axis, area of dispersal, volume, maximum thickness (T max.),D (where D is the area enclosed by the 0.01 T max. isopadh, Walker 1973) have been found. The isopleth map of the average maximum diameter of the three largest lithics measured at each locality gives a good indication of the position of the source, enables the range of ballistic fragments to be determined,

_1000m 27°204 2/id` W. E. -SOO M I

SANTA BARBARA FISSURE ZONE PICO ALTO !km 1 06 -21.M isterios I-221761 Ian( 31. Pico Alto 1,I1 .4.1 , 29.Parol I-28.0 outaI,I1 - - - 30.1clga 30 Rossps 212- 27 Farol II 2614' Ca rneiro 29. Lavaca' I, South ',Cobras Ra_chado 25.Negrao 24 Couto III 23G--.--TrK-- 28. Biscoito Rachado 1,1131 2080 19. Algar i2.Serretii 2115' 21. East 18.Misferios 27Pico Alto 111,N 17.Galiarte I 16.x 26. Cavacas I 20. R.da Lapa 19. Ponta da Serra 15.Pico Gordo 25. Lavacca,M 14,Pico 599 24. Azinha 13.Gaspar I23.Biscoito Rachado IV 12.8agacina 1 22. South II 21.Enxofre 26E._ Terra B-ova I it:LcIrei --,,.._._ ._ _ 10. Pico 581 19. South HI 18. Cavacas II ,.. 17. Lagoinha 16. Raminho Aigar II i5.Fajas , 9 North 17.Quinta da Madalena jL9....eg z_.__ . 16.South a z 14. 15. Agualva Zomba 14. East 13. Southwest 13. Ponta Velha I I 21A7.612 as Crayos 6,y_ 12. Ponta Veiha II 5.w 8.GalhadoI,II 4.Cancela II. Mato I 3. Filipe to'g' Pardelas 10. Pico Negro 2. Galiar e II 9. Pico Altc V 9. Doze Ribeiras 8. Biscoito Rachado V 8.A 7 Loircs 6. Boi LIT 7 MatolI 6. P. da Serreta t.Pico da Falta 5.P. dos Dez 5. Terra Brava II 4. P. dos Duos 18.60 4 3.Terrets. 19,68 2.Teles.------

Caldera Flows LAJES IGNIMBRITE CYCLE 2 23)0

Lava KEY 2. Teles 3. Pico Basaltic lavas stk..„„zo Pumice fall deposit l Salic and pyr)clastics, lgnimbrite lateral extent shown by line 15

TABLE 2.1

QUANTITATIVE DATA FOR SOME IPJ:ORTANT ERUPTIONS 'ON TERCEIRA

(A) - Salle Composition (8) - Basaltic " (n) - Minimum Estimate Magnitude of sruPtions on MEMBER VOLUME VOLUME s D P VOLUME OF Tsuyais (1955) shale (lava (actual km3) (km3 D.R.E) (k4 (wt %) ASSOCIAT4D and pyroclastics) LAVA (km') mocusTic ALL 'A' 0:08(m) 0:016 120(m) 5(m) .•.... 5 'B' 0:332 0.092 200 5 0.290 6 pap 0:040 0:012 120 10(m) 0.025 5 ,Dp 0.092 0.018 150 28 0.108 6 0:215 0:054 180 34 0.085 6 I F. 0:033 0.007 55 27 0.060 5 *GP 0.088 0.022 80 .7 0.203 6 .H. 0.089 0:024 40 & 0.292 6 'I' 0.031 0.007 50 16 0.006 5 Monte Brasil (B) 0.280 0.150 9 58 6 PYROCLA$TIC FLOW Lades (A) 0.334 (m) 6 - LAVA FLOW - AREA002i) RAllo(Wv)AMPF61. Pico do Pardelas A) - 0:290 5:1 40 6 ! . w ,Carneiro A 0:292 .4:1 20 6 o Bbi A - 0:430 2:4 4-6 6 Bagacina ' B - 0.107 16.6 >400 6 5-6 Algar do Carvao I(B) - 0.099 18.0 >800

*- (Density of rock 2.5gm cm3 ) Volume =Volume within lcm isopach

TABLE 2.2 CHARACTERISTIC FEATURES OF THE NINE IMPORTANT PYMDCLASTIC FAIL DEPOSITS ON TERCEIRA

Average of 3 samples on dispersal axis and within 0.1 T max.isopach. _ .. Source/Designation Area'within Pumice % Lithics % Crystals % . 'Internal Diagnostic Md Ir.,.-11 a% Estimated &Type icm isopach. & Type & Type _ _Structure. characteristics A_T,max,., . Muzzel Velocity, (km!) , _ (after Wilson 4973) b9...,-PhYri Santa Barbara H 312 white,very 10%.Aphyric 1%.Scarce, Non-stratified, Aphyric pumice, Volcano coarse grained juvenile clear very coarse centre, lithic poor. -2.5 1.8 in places, sttreay,reaky, anorthoclase. finer grained top. Some dense glassy. Panterellite. ., . ,.. _ ...... 'pumice. --_ G 465 76%.whito, 20%.Juvenile 4%.Clear .. Stratified, with Coarse pumice, por- vritic obsidian prismatic 'rainflush' beds. ,rainflushibeds -1.8 1.2 co.enditic streaky, ,, anorthoclase. Angular.fragmentS. . „absence Of non: trachyte ' gla'ssy. juvenile lithics. 368 82%.Millet-- 8 .Scarce 10%.Clear,. Non-stratified, Yellow fine- . • seed, pumice,- --lithics. prismatic" very fine-grained grained pumice, +0.1 1.5 ' rounded, anorthoclase. ; base. few lithics. • yellow,com- ._ '_, enclitic 660 78%.yellow, . 11% Small, 11%.0paque, Fine grained, Opaque crystals fine-grained,. obsidian white anorth-1 millet-seed in,clots, in -1.1 1.8 mainly millet- (juvenile & oclase. pumice. Unstrati- brown soil. seed,porphp- ' rare non- , fied. titic comendite. uvenile. • ce ric.,par Very w i e, >300 87%.very coarse 11%.Lithic- ' 2%.Scarce non-stratified. almost aphyric pumice, almost poor at base. prismatic Vulcanien bed pumice.Strati- -2.3 1.3 aphyric, -Lithics in- anorthoclase. stratified.____ graphic position. trachytic. crease up- Vulcanian deposit wards-until at top. high in -" vulcanian. .- .- deposit. Fine- grained ...... trachyte, -

Pico Alto Coarseness:near- 2%.very Chaotic near source,quickly Volcano I 395 94%.Very coarse 4%.Scarce source, grained near juvenile Scarce becomes- thin bed. - - lithies._ anorthoolase. non-stratified Stratigraphic -_ source.Aphyric oantellerite. --, pOSition.- E 2,200 75%.Cream, 16,A,Winly 9%. Stratified, up to Millet-seed ... . porphyritic juvenile Anorthoclase. 4 fall units, pumice, angular com- obsidian obsidian chips. -1.0 1.5 endite. aphyric, ' - , very small: --- - C .415 66%.Grey,poorly 21%.Trachytic 13%.Small, Finely stratified Stratification, . vesiculated, non-juvenile prismatic into many small grey pumice,h(l - cornenditio and glassy anorthoclase fall units. crystal content. - trachyte. juvenile , . --- lithics. ._.... -, I f 3,000 78%.Yellow 20/..Fine- 'K•,... ,_.,. Coarse grained, Coarse pumice, porphyritic, grained,non- prismatic non-stratified, deep weathering. -2.5 1.4 juvenile aktorthoclas) comenditic trachytic trachyte. lava.

14 27 10'W a. Thickness (m) 23 b. Pumice (cm) 2e' .40 245 /100 35. 1'6 '2.9 90*4 • >365 .380 .110 '22 8.6. =7: 28.2 7 7.0. .70.5 N. • .1.2 %N. 190, 42' (k.11 ?-\>,, 1 56. •40 N 9.0 8 >t . .1.0 *80 .57 14.2.`4d 3.9 N 38"45t-• - 4 6

16 744 >29:21:* .7.1 i7 % >102,, • .,75 .153)2 65'6 \cl• 4 -4. 1 • '24 \ '213 140 1.150 213;2°5 25 9.2. 124' 7° : .85 35 55. 20 .24.o/ . 4°•5 23' IA 28 >&0 235.7°3 '124 .60 13.0 1.2 50' 145:1 45185..17.8114.121 66 1.9 83 AS .65 "14.4 39. A7 .42 40 >,,50 44 AP 40 '17 10 15, 40 A 95- 106 31 • 47 *25 27. 25. '62 5•6 28. .36 25 '42 34 '2.6 .19 1.9. .43 40 5 75.8 f 1p 16 .45 12 .3.1 '33 '22 a.6 '29 2.6 KO 1.0. 0.25 21.8 '23 16 .3.36 2.5 0.125„3

source area • Pico Alto Caldera lava

Fig. 2.2 Member 7B" a. isopach map giving thickness in metres. Locality thicknesses in am. b.average maximm diameter of the three largest pumice fragments in cm. c.average maximum diameter of the three largest lithic fragments in cm. Meps a-c show selected data based on 121 field localities. d.the lava phase of member "B", the Pico de Pardelas comendite flow. The key and scale applies to maps a-d. e4 plot of the average maximum diameter of the three largest lithic fragments against distance from the source. and can show if there has been a directed blast eruption.

Within the framework of the Lajas Ignimbrite and the nine main pumice deposits all volcanic products have been placed in their stratigraphic order (Fig. 2.1). Together the ignimbrite and fall deposits. cover four-fifths of the area of the island. Table 2.1 summarises the quantitative data for most of the important eruptions during the past 23,000 years and Table 2.2 shows the characteristic features of nine important pyroclastic fall deposits.

2.2 (i) The four main pyroclastic fall deposits from Pico Alto Volcano On this volcano four peralkaline trachyte members are known to contain pumice fall deposits. Each pumice layer is closely related in time and place of eruption to a peralkaline trachyte lava extrusion. This is indicated by the closure of the largest lithic °last isopleths around the source of the lava. The following sequence is normal on both Pico Alto and Santa Barbara; a sub-plinian blast ejected vesiculated magma from 'a vent, followed after an unknown but presumably short period by effusion of lava of similar composition from the same or a nearby vent. The surrounding topography is mantled by the pyroclastic fall deposit which has a distribution at least partly controlled by the wind.

Of the pumice fall deposits on Terceira, 1B! (Fig. 2.2), has the largest volume and widest dispersal of all. It covers an area of at 2 least 240km within thelDcm isopach on land with a dispersal axis trending south east of the volcano. It has a small dispersal compared with plinian deposits, but has the internal homogeneity,displayed by plinian deposits, and a low fragmentation index (F) (Walker, 1973).

This eruption was probably a short, but powerful, vertical blast in a

to 19

2710' •

41. 4441' V• • 44 I,. f 1 1. 11 v 0.12 it 1. _. ... •-• - '"1 1 - - .7: 1.;‘,...7-3,42:=7..1:2:75 .,.. A - 0.05:118: 0/ ,Y7A\) ' 3845- , e 4 / • /03- 8. •.045./ • >2 5- ' >2.6, 11 • f • • 0.12 p..20 .or .4,1*—.0.18 — .— . / _0.55• • o.43 0.24 0.07•' 0.15k 0.38 0.50.// li" 4 4 r::%.i. 71 08 .F 4:(77'.,75;:0 ,,....18,...„ .2 036.0•5(10 '" % w70. .12\%.030.. •.0.30.037 —0.85" u ' J \ '0.33 0 •3 • .52/ 1 .<-3:10.30--;0.16 .0.10" • 0 A4 ...... 0.870 51) ■ •--0.125 '0.05 %•• ,...... vv.-025:048 ...... I •—• 0 A°

a. Pumice fall .1. 0 2 m b. Pumice fall 'C' 27.16 p k

Fig. 2.3 a. Isopach map giving thickness in metres of the fall deposit of member III. The map shows selected data based on 46 field localities. b. As a for the fall deposit of member "C". The map shows selected data based on 35 field localities. For key see Fig. 2.2. 2 moderate NW wind. The deposit contains a high proportion of lithic fragments, mostly non-juvenile fine grained trachyte, compared with other Terceira pumice fall deposits. After the initial sub-plinian phase a large endogenous dome grew on the north west caldera rim con- cealing the explosive vent (Fig. 2.2d). This dome ruptured on its downslope side-due to instability, possibly aided by an increased rate of effusion of lava,and a 60m high lava coulee flowed 5km to the sea.

Pumice fall ICI (Fig. 2.3a) is a well-stratified sub-plinian deposit of small volume with a large proportion of non-juvenile ejecta mixed with grey pumice and small, clear feldspar crystals. The percentage of pumice is greater than that of lithics and on the classification of Walker (1973), the deposit is sub-plinian. However it has some of the characteristics of a vulcanian deposit, such as much internal stratification with reverse grading within each fall unit and poor sorting. Instead of one vigorous blast there were probably a number of deep seated explosions, generating a greater proportion of lithics than in other deposits of sirailav/tYpe on Terceira. After degassing of the magma a viscous trachytic lava, the Cravos Coulee, was extruded on the south east side of the volcano.

The most distinctive pyroclastic deposit on Pico Alto is the pumice fall component of member IEI. It was produced by a sub-plinian eruption with 3 or 4 close-spaced pulses each producing a reversely graded fall- unit. The fragmentation index (F) is high, 34 compared with 5 for 'Bt, and the deposit contains a large proportion of small pumice fragments, like millet seed, which in distal localities often comprise the whole deposit. The pumice is distributed evenly around the source area, showing only a slight south easterly dispersal and indicating a weak wind during • • 21

a. vv -...... „ - ..... '■ ' OS' N.. ■ . 1. 5 "--- .. 02. 0 25r) ...... / () _ its __ -... 032. 3 141',,,--;y...--.0.2. 024', . \ 1 0.27. 4- w_i. -.037 -.. 0.15 ' \ 23. N. `\12 / , / • ,.., l'A ..... ". ' il "I' 1,5.4'5.b.w0,4•4.22 I / 2.1 - A 1 *..... -- \ ■ /1".4.3. -11A \ 49.4 , N s \ .- 1.41142-5-.'"N"\.>9i6o5a52%-%0.31 \ \ .10 .-- 1035.10'0/ ''.531P:'•131 / 5" 4 .6.0 ‘-• • . 38.4 5 __ oj i / i 1 v . • % ,.. ..64 \ • I 024 ' 434 >1.:9....") ..2.5211.71 t .0.45 i 4.8 .3:614.;•3;313(i8 f181..:7354 1.911.3. ..j53.3.9;1 \ l.?...., 4 •1 I . 1 I 142.'1 y 3.4::: AI:: 1 097 1 7 1 :2A • 2664..-6o f••4./ I/ si• • 1A• lap x ,i k .6 0:.12 t i \ k \y_55_,.29 .0;2., ....6. 43 i 1 1 i i„ 1 1:2 \ . 0.8 •• 1 \ V' .. ' 6..0 ill * I 1 1 A I 1-1)\ \ •.7 3 .4- ...... 51 1 - \ \ .1.86. 2981.96/ i / 1 1 3.8. ...,. 7.3. 4.5., / I1 1 0.7 \ 2.77...... 31 / ila 42 0.25. ‘ \ . 3A• --67'.-- .3A I 4-32 I 5A. •B 4.6 1023A 043' \ \ 115. :1.8 044• i 9.15 I \ to. \ 1.5 .3.3 •2.2 1 • 0.2. 1 .3.8 '1.8 48 . • 1. 0.25X 0.43 \ (j68....:132;1.71:j4.,1 1 1.1.. Ns. 0A5* ' 32 1 di i '3.5 --- ...... -, 0.15' •20 ./6 1 \ \ VS \ i al5. \ . / i \ .0.9 `\ 1.6. .3.0 \ \ 0.25'0.35`. .213. / 1 1 I \ *. 0.3. / \ \2A. .3.6 1 \ •N• .' N. ... i \ - \ .13 r ..-- .... • ... .- - ... 0.1 -.'. THICKNESS_ PUMICE •. s.... 18015 2 1 31. 2.8 1.0 N. 0.11 • ■ .8

• .• T V1' 5 ••••. 11' 1- •to. 16i. c4= • 2 2.6. .0.6 / .:52 1 'Li y1.2 "P • 0 "1"4 798.44'3?09.251 .1.5t 06' I t' (11.k..44/1" / 1.9 1 33.1• 16.4 -• ...23/2.1* 1.5* 1 9' V 11.1.9 1)5*/ .06 .05 \ .0.6 12. 0.70 8. 0 415 'OR r 0.6 0.4 \ 1.3. '1.2 • / • 1.3• 4.6 3Ia* 0 • ,"•09 .0.7 •• LITH ICS

Fig. 2.4 Pumice fall deposit "E". a.Isopach map giving thickness in metres. b.Average maximum diameter of the three largest pumice fragments in cm. c.As b for lithic fragments. Maps a-c give selected data based on 173 field localities. For key see Fig. 2.2. Plate 2.1 Pyroclastic fall deposit "E", in Pico Alto Caldera, about 1km. SW of source. The deposit is unstratified here and reversely graded. The hammer rests on the scoriacous top of a pantellerite lava dome. 23 the eruption. Fig. 2.4 gives isopach and isopleth maps based on measurements at 173 localities. The elongation of the coarser isopleths north eastward suggests a slight directed blast. Large lithic clasts are rare but small ones, mainly obsidian Ichipst, are common and their presence tends to be diagnostic of 'Et. Samples of the pumice, collected at various points on the dispersal axis, have been sieved and a section from near Cabrita, 1.8ka south of the source area (Fig. 2.5a) illustrates the grain-size characteristics of the deposit. This shows reverse grading within each fall unit and also in the deposit as a whole. There is an increase in median diameter (MO) and an improvement • ' in sorting (a decrease in dfli) upwards through the deposit in all but the finer-grained beds which mark fall unit boundaries. 0 is larger for the finer-grained parts of the deposit which tend to be of polycomponent type compared with the coarser, dominantly pumice beds. The increasing grain-size and complementary better sorting towards the top of tEl are thought to be the result of lower fragmentation of magma in the late stage of the eruption or an increase in vigour of the eruptive blast.

Mao and dO (the Inman sorting coefficient - Inman, 1952) are obtained from cumulative curves of the weight percentage in each size class, plotted on probability paper, for each sample, see Appendix for details. The cumulative curves (Fig. 2.5b), plotted for the 3 components separately show that pumice is the least well sorted and crystals the best sorted of the components. This reflects that the initial population of crystals has a smaller range in grain size than that of pumice or lithics; for instance there is negligible content of crystals greater than 2mm diameter.

MO falls off steadily from the source along the dispersal axis

2 a. b.99 r7 PUMICE 98 4;10,• LITHICS 1014L4 L8 CRYSTALS c 95 wt.% 30 90 S17 20 41) 10

A ae. O 50

> 9caoo es dpO c 4 I E U 10 5- 4 ' 2 - • 1 WP1111 -4 -2 0 2 40 -4 . -2 0 2 4 0 d. m. 1.0- E

0.1- • 0 • Md0 3' . -4 ••••• e. 0..*:16.:Y*1.% • ..... "-2 0.01- . •••••:Z•n• • • ••• • . •• • . • - s•• •. • •• • • T • • o • 2

1 2 3 4 5 6 4 6 8 10 12 DISTANCE ,km DISTANC E, km

Fig. 2.5 Grain size characteristics of pumice fall deposit "E". a.A section 2.6m thick, through the deposit at a locality 1.8km SE of the source on the dispersal axis. Frequency curves of wt% against grain size on phi side (0 -log2,mm) showing the concentrations of pumice, crystals and lithics for three sieved samples are given, together with the vertical variation in sorting (60) and grain size (Md0). b.Cumulative curves of the three samples for pumice (p), lithics (1); and crystals (C). c.00 and Md0 plotted against distance from the source for the 8 coarsest samples collected along the dispersal axis. d.Average maximum diameter, in metres, of the three largest lithic fragments at each loaclity plotted against the distance from the source. 25

(Fig. 6c), but 00 shows a slight maximum between 1.5 and 3 km. This increase in C5 in the distribution is best explained in terms of the different size populations and density contrast of pumice, crystals and lithics. The distance that a given particle falls from an eruptive cloud depends on (1) wind strength, (2) height of release from the eruption column and (3) the fall velocity of the fragments. Large, less dense pumice will fall at the same place as smaller, denser crystals and lithics. In 'Et the size populations of crystals and lithics are similar; both are sparse in the grades coarser than 2mm whereas pumice predominates in. these coarse grades. Near the source samples have greater than 80g by weight of pumice and give a unimodal frequency curve, a steep cumulative curve and a low 00. At the distance where the median terminal fall velocity of tha deposit corresponds to the. of peak terminal fall velocity of the populatioVcrystals or lithics, an enrichment of those components occurs in the deposits. This results in a marked increase in the proportion of denser components in which in turn gives a bimodal frequency distribution and O5 therefore increases.

At distal localities sorting again improves because,as the grain- size decreases the density contrast between pumice and the heavy components also decreases. As an example of the increase in one of the dense components in 'E' towards distal localities, the crystal content in selected samples along the dispersal axis is at 1.8km, 13% at 5.0km, 16% at 4.1km and greater than lag at 5.7km from the source.

Lithic fragments fall-off in size very rapidly from the source (Fig. 2.5d); the maximum range of ballistic lithic clasts can be deduced from the steep part of the graph to be about 3.6km from the source. The range 26 of the largest fragments is still partly influenced by the wind; the inflexion in the slope of the upper part of the graph is thought to be directly related to wind strength at the time of the eruption, which in this case is low (see later discussion). The sub-plinian phase of member fEl was succeeded by a large peralkaline trachyte coul4e, called Terra Brava, which ponded between older domes.

The youngest pumice fall deposit on Pico Alto, designated 'I' (Fig. 2.3), has a very limited distribution but it is extremely coarse grained near the source. Here ballistic pumice clasts are often 1-2 meters in diameter and show bread-crust surfaces with black vesiculated centres indicating that they were still hot when they fell. Pumice fall 'III directly overlies the lava and strombolian cones of the 'Algar do Carvao It member which is on. the Fissure Zone immediately south-west of Pico Alto Caldera. The alkali olivine basalt lava of this member gives a C14 date of 2,115±115y BP (University of Arizona Geochronological Laboratory). Member 'I' is therefore younger than 2,000 years and is postdated on Pico Alto only by two small peralkaline trachyte domes in the northern part of the caldera.

2.2 (ii) The five main pyroclastic fall deposits from Santa Barbara Volcano. Five trachytic pumice fall deposits originating from Santa Barbara Volcano have been mapped. Member 'AI, the oldest, is a coarse- grained, poorly exposed sub-plinian deposit passing up into a vulcanian- type explosion breccia. The two parts of Member lAt have different distributions (Fig. 2.6a and b). Data on the lower pumice rich part are extremely limited due to burial by younger pumice beds and lava, but data on the upper part are more complete; this upper part is composed of 27

Member`A" . Sub-Plinian phase

.0.2 0.85 .Q24 -.... .6735 Pumice fall .F- a 113 v.65 0;471 . -0.05-- / 1.11. ' 1.3' -0.73 '0.53" 6 0.25 // // „-2.47.135 / '055 / *0.06 0.5 _ .015 , / .0281 1 ...41.`",r A 76 / 4.1 A V..4;. .0.78 .0J2 61 '0.33 . .044r0.37 0,56.0.83;0455 ■ I • \ 0..05.0i 2i - 0.54. .:0° :1125 0:95: ....1;j.57. -Q13 _05 03' 0.58/ ,0.28 v0.44 -0.21 .052,.1135 j°1 .V6*0.28 -_, N..025 25 -023 • _0. *0.2 N, •0.23 .0.21 *035 0.125 •0.12 . Pumice fall

Fig. 2.6 a. Isopach map giving thickness, in metres, of the pumice fall phase of member "A". b. Thickness and extent of the lithic-rich vulcanian phase of member "A", including the ground surge beds to the south. Ornamented area around caldera is that covered by lavas younger than "A". C. Isopach map of the pumice fall part of member "D", giving selected data based on 81 localities. Thickness in metres. d. As c for member "F", compiled from data collected at 47 field localities. Symbols as in Fig. 2.2, SB is Santa Barbara Caldera. 2S

a coarse bed rich in ballistic lithic fragments and a well bedded vulcanian-type ash, with incorporated ground surge beds, distributed only on the south flanks of the volcano.

The basal pumice fall deposit, dispersed on the western side of Santa Barbara, consists of coarse, yellow porphyritic pumice with very few lithics. The dispersal of the pumice suggests a strong wind at the time of the eruption. No exposure of the eastern part of the deposit exists.

Near the end or the plinian phase, there was probably a fall of magma level in the vent or a large-scale collapse of vent walls. The .proportion of non-juvenile lithic fragments increased markedly until no juvenile salic material was being ejected. The plinian phase therefore gave way to a vulcanian one, resulting in a stratified fall deposit con- 'mining pulverised material and limited amounts of dense, basic juvenile material with augite and feldspar phenocrysts. Porphyritic gabbro and porphyritic trachyte fragments, with rare blocks of syenite, form the most significant part of the vulcanian deposit.

The change in style of eruption from plinian to vulcanian is taken as indicative of tapping successively lower levels below the volcano. Possibly the second stage caldera collapse of Santa Barbara took place during or after this eruption; grain-size isopleths of the upper lithic- rich part of the deposit are approximately concentric about the new cal-

dera.

Member D (Fig. 2.6c) consists of a pumice fall deposit and lava extrusion in the north of Santa Barbara Caldera. The deposit is of

b. 271 ••••• ‘ ,'1.---- v0.7 .0.5 ...... , •65 6.6. • ‘ 132i-‘,13.61.. 04 • „,... ''''',,-- 12..5 "--4 7 6:5,„40.23 'd, „ 4.r 55. \ ‘ 11.75 \4 025a N ‘‘% ----- ...- -14 - 4.•) ' ..0.1 \ ■ , • - .33 .15 .. •-• ---... -.." •11 X \ .. 021 *0.520. 26. \*02:... \ / ‘-' ,- -0.0 . ss• \ \ \ '.... • -85 st\ r 45.8 5 .S.• , Y .%).' .,5 -■ .4.9 ' \ 10 , •0.26• .04%35' \ \ lt N, 4.5. \ ‘ 1'...?4, ;,".‘,..\ N. .4.. ,.. _ ' ‘ 4513- i 043 \ \ 043' 6.8 .4.6 , . )42 *'' N. ....■. 7b• ,t8 • -1 46...... 1 6.4 • `4/‘ -..s.4 4,1106 N.2'4■05stn , 1 % AY 1,171I" iv.8 10 8: •-- . ) 0.46 .0,31 t6.521- ‘11.45' 348:45:- \ tA\-4 sz,„_ e',21-.4...... zz ,..2 0... i8-'1::4;1.9 -6.51;a626:85?- t..p. :,...... 1--■:1.09:213.1 4,, .; - . , / / *0.35:7 I •4..t, 4.5 .48' 2-64 3 / . „Pt 73 _ ■33 ....1.2. ....„ --- „- -. „ - *0.16 . . 014 021 / ‘...,....3•.2.6.... 2.51.470.:3•2:11_-.6 la LAVA ., .0,11 / ..... 0.12' / ■ .0.13 MUDFLOW 0.08 Pumicei (cm) 0 261 0.5' Thickness (m) writ c. d. 1. 30 20 10 E w

Lir ::'• • A *.% ..*:.. • 2 • "4, - 5. 40.01- .••.• :••• •••• . : . :. • . . • .. • -. : • - 6. p . • rf

f i 2 6 8 10 rf DISTANCE. km rf o 0.0 O. -3 -2 -1 0 0 i 1 i Md 0, A •0 . -• -4 -2 0 2 4 60.0' 3

Fig. 2.7 Pumice Fall deposit 1" a.Isopach map, giving thickness in metres, and the contempo- raneous lava and mudflow. b. Map of the average maximum diameter of the three largest pumice fragments, in cm. Maps a and b give selected data from 119 field localities. c.Graph of the average maximum diameter of the three largest lithic fragments from each locality plotted against the distance from the source. d.A section through pumice fall "F" at a locality 1.81aa SE of the source on the dispersal axis. Frequency curves, with symbols as in Fig. 2.5, are shown for 6 samples. In each frequency diagram the 14 retained by each sieve grade (in 0) is plotted against the sieve aperture. MO and 00 are also shown. Subdivision into pumice crystals and lithics as Pig. 2.5. R indicates a rain-flush bed. 30 sub-plinian type with a volume of 0.034km3 d,r.e. The pumice is highly fragmented, consisting mostly of rounded, millet-seed grains, and the deposit is poor in lithics (12%, average of three samples). The prominent feature of 'D' is the presence of milky-white anorthoclase generally twinned and in clusters. When incorporated in the deep brown soil which has been formed on top of 'D' they are diagnostic of this deposit. Pris- matic, clear anorthoclase crystals are less common and the total content of free crystals can be up to 12%, which is high compared with deposits such as 'B' and IG!. The peralkaline trachyte 'North Caldera Coulgel followed the sub-plinian stage; it is the oldest lava in the new Caldera.

The next member to include a pumice fall deposit is !Ft (Fig. 2.6d), which is remarkably similar to 'D' in composition, content of lithics, dispersal and bedding characteristics. The fall deposit comprises only 1/3 the total volume of the member 'F' and the ensuing lava flow in the south of the Caldera is a large one. Pumice fall 'F' can be distinguished. from 'D' only by the absence of milky anorthoclase phenocrysts.

Member 'G' is larger and more diverse than either 'D' or IF!. It was erupted from a vent on the north west flank of Santa Barbara. The eruption of this bi-lobate, sub-plinian deposit was accompanied by mudflows, and followed by a fissure eruption of five coalescing cumulo-domes and one coul4e of peralkaline trachyte. The pumice of the main sub- plinian phase is distributed southeastwards from the source, (Fig. 27) and that of the other is distributed towards the NW. They have identical, coarse, white pumice, containing clear prismatic anorthoclase crystals. The small crystal content (Fig.2.8) is probably related to the low degree of fragmentation of the magma combined with the small amount of phew- trysts. Plate 2.2 Pyroclastic-fall deposit "G", 2.41 SE of source on dispersal axis. 3 rain-flush beds show as thin, dark layers. 3

During the eruption torrential rain fell,shown by the occurrence of 4

to 7 rain-flush beds (Fig. 2.7, and Plate 2.2), which contain fine grained

and poorly sorted material together with larger pumice clasts, washed

out of the eruption cloud. These beds are found in both lobes of the

dispersal area. Accretionary lapilli have not been recognised. Also a

large amount of air-fall material was mobilised by water and flowed

northwards as a mud-flow to produce poorly sorted, bedded deposits

exposed near the town of Altares. The relation between the two lobes

of the fall deposit and the mud-flow is uncertain. The mud-flows may

have occurred at any-time during or after the eruption. They are mostly

fine grained, and rich in pumice but also contain large blocks of trach-

yte and other lavas picked up during flow. The northwest pumice fall lobe can be seen to interfinger laterally with the mud-flow but the

southeast lobe is cov ered on its west side by the lava domes. The presence of both mud-flow and rain-flushed beds in member 'G' is probably

not a coincidence; no other fall-deposit on Terceira shows either.

The mud-flow and lava extrusions have covered most of the northern

part of the puMice dispersal.area. The isopleths for the largest lithic

fragments indicates that the sub-plinian blasts came from a vent (or

perhaps two vents) now covered by Pico Rachado Dome and its immediate 14 . neighbour. The age of this member is known to be younger than the C

age of 2,040y B.P. given by Carbon in basaltic scoria 'Z', which immedi-

ately underlies tGt on the Fissure Zone.

The gain size parameters of 'G' in an exposure 1.8km SE of the source

on the dispersal axis are shown in Fig. 2.7. Like pumice fall 'E', 'G'

shows an overall gradlial decrease in 00 upwards, although in this case it is not so clear as the deposit has a more pronounced internal strati-

fication. The rain-flush beds give a bimodal grain-size frequency =74'11 w) ea

, ., . .., _ _ . ... . -t•-•"".0....:.::-.- - . . - - 1 ...... ‘ - r1011,6'14.2r44W*-4...... - ..4-■%:,:- ","'', . ''' . ,... -. . • ,es".. ---,...- -- .r . -.-.....e. .,-

• : ' -4:774i..W!".47:4';'... •

.1' ' " •

Plate 2.3 Pyroclastic-fall deposit "H", western flanks of Santa Barbara volcano 1km SSW of source.

•

b. PUMICE LITHICS

•036 •2 7 " ** • —' 70 957.0-4.9 •-.212- :-. N):‘ 13.i•13.5.1:1,\ ‘ e A 77,',,31A--)1 1 I •3. 5'1.>55 I 275 46 770 1'35 • .34 2)0 2;°°(..5.'384705.(i/;.3// 2.56. 113,..>6.0 ic!„ 38.45=-- 3.6 `.8.446 6 / 23. •3i / J..L 44•'). ui %,:.2.1 • 4 597 •49 an... -. 2-1.86 / %41.,Z)10 31•__ - 4 • 4;49.2::8:8.1;6*518,7.640 '114.:.4. 4 4, 20. 058 ?213 • l er 1 • .024 0.15 1/14 •10 "*.2.8.2.6 / — a.8 . *0.6 / - 0.7 '0.8 .015/ AP" 12. \55' 0 2km THICKNESS

Fig. 2.8a Pumice fall deposit 7S". a.Isopach map giving thickness in metres. b.Map of the average diameter of the three largest pumice fragments, in cm. c.As b for lithic fragments. • Maps a-c are based on data from 34 field localities. For key see Fig. 2.2. SB is the Santa Barbara Caldera. '

Fig. 2.8b Palaeowind directions of the 9 important pyroclactio fall deposits on Terceira. distribution.

Each pumice fall may have preceded the growth of one of the domes

at an unknown time interval and in this case two separate vents may be involved. It is less likely that the wind direction changed through nearly 180° during a single sub-plinian blast, which we know from historic examples do not normally last longer than two days.

The youngest pumice fall deposit on Santa Barbara, 'HI, is dis-

persed mainly to the west of the caldera (Plate 2.3). The white, aphyric pumice is possibly the most acidic on Terceira. The source is near the coast but the isopachs of the deposit are close together therefore a

large Part of the total volume fell on land. A slight east wind is

indicated by the isopleth maps (Fig. 2.8a). Although thickness of the

deposit decreases rapidly north and south from the dispersal axis, near-

source localities exhibit the thickest pumice fall deposit on Terceira,

reaching a maximum of more than 14m in the field and an extrapolated T

maximum value of 28m. This compares with pumice fall 'B' which has a

T max. of 21m. but a volume an order of magnitude larger. Exposures of

'HI often show incomplete or false thickness where it fell on steep

slopes; secondary bedding features such as "lens bedding" are indicative

of post-depositional slip. The top is finer grained indicating that a final reduction in vigour of the eruption occurred, coarse material then

falling nearer the vent. The per alkaline trachyte coul4e of Pico de

Carneiro was extruded after the explosive phase of Member /Ht.

Sorting and median grain-size parameters for 28 samples of pumice

fall deposits from Pico Alto and Santa Barbara show that the 60 value is

mainly between 1.0 and 2.0 (see later, Fig. 2.13c). This places the

depositswell within the pyroclastic-fall field of Walker (1971). The :3 fi

Md0 value, as stated above, is controlled by the position of the sample locality relative to the source and dispersal axis of the deposit.

Palaeowind directions, as indicated by the dispersal axes of the nine pyroclastic fall deposits (Fig. 2.8b), show no consistent pattern. There are 4 with a dominantly westerly dispersal, including one part of the bi -lobate deposit 'Gt. 5 axes are dispersed with a dominant easterly trend and one, 'El, in a southerly direction. From this data no meaningful palaeowind directions can be assumed, especially as the data covers such a long time period.

2.2 (iii) Basaltic scoria fall deposits on Terceira. Basaltic scoria deposits are abundant but only the youngest are reasonably well exposed, due to erosion and concealment by later deposits. The best preserved deposits occur on the Fissure Zone where there have been frequent eruptions. The mid-island part of the Fissure is a slightly elevated pile of basaltic lavas and strombolian-type pyroclastics with a prominent line of cones, spatter rings, Igjast and small volcano-tectonic collapse structures along its crest (Plate 2.4). Elsewhere on the island are scattered cones lying either off the fissure or on the older south eastern part of it. Surtseyan tuff rings and associated air-fall tuffs are found along the cliffs and offshore. Seven strombolian deposits of various sizes from the fissure zone (member numbers 22,20, 19, 17, 15,

12 and 9 on Fig. 21) and a surtseyan deposit from the tuff ring of Monte

Brasil (Plate 2.5) were studied in detail (Fig. 2.9 and 10). For the large deposit of Galiarte cone (No. 17; Fig. 2.10a and b) an isopleth map of average maximum diameter of the 3 largest scoria fragments at each Plate 2.4 Pico do Gaspar, a spatter ring on the Fissure Zone, with slopes of up to 520. Such steep slopes are a result of welding of the semi-molten spatter after impact. 3

Plate 2.5 Monte Brasil tuff ring, seen from the NW. 39 3

► 22 2115

.13" 26 ■••.- *6 043•J ••• g" -- --

1 km

Fig. 2.9 Basalt scoria-fall deposita on the Fissure Zone. Isopachs for 5 deposits are shown, giving thickness in cm. Cones are identified by Fissure member numbers 9-22, see Figal. For the deposits of cones 17 and 19 see Fig. 2.10. Arrowed figures give thickness of deposit, the source of which is in the direction indicated. Contours at 550 and 600m are shown. [For area shown, see inset on Pig. 1.3.]

•

--, J . , • a. ,- 27)5, _ .2A* .....,- 22' :24 b. • • ■■ ...... e • , • / / ... ■ '.'26. ... e'.... ;26 / / / • :'" 42* N 8. % • 3 7 ..... • 1.0 \ . 42 ..• • •// 1/4as' ''' \% \ • „56 . / % , , ° .4.8 -... ' 1/4 \ \ • •.16 / ♦. . • . • ‘• • 0.9 /// -17 '1/4 % 8. % .06 , • .2-7 ...... “- --, • % % \ / / / • • 1 ' / / / .6.7.-•165 •,,, • s/I 'it8/11)11:071300:7717i 1 .22 t / / ; / ,45!. * • .2.3 / / / : :1f),S,‘:, ‘ ' ' I t f is t 112' 17 .. t . •1•3 ; / 25. 17.. 1 1 1 I 1 I r v j ,st 121,: il. 4.::11 It IA 1 1 1 il 420: 1 I i I I 1 I t 1 1 1 % tI %26' -,.....”‘::.,1',':... i 1 1 t.), t 1 1 1 1 \ . kr i I 1 1 t.1.1 i 40 1 t \ ‘ •13 1 % \ \ • 3715/ ""7,.11 .3.4 1 • .395 .120 1.96 1 I % % % ...... • ....4.2 - 1 1 ..6 '15 It \ 't 155' 333 •)09/ I I I % / / .54 1 '46q i • % 415\ 615. . \ •.;9's ''..13-•492 I / .421 \ t % "`•9.8 - ... *15 /,' . .3a1 i / / / . t .3.5 '5.4 6 .„... \ % t0 i s• , i , • • / / . ‘ , _ .....,-...... i ‘ .11 \ 's .9 ..., ._ .., 1 \ .0.8 % . / ... n '..0 / /26 I % %, .28 '18 • . ... .1.0 / ‘ % •d. " •as ■ 72 ... .4-3 1..". -.112 / \ ,•27 ' / .26 / N. %. ■. '15 *23 ■ \l b '21 2 \ '-' ' ' \ . . . -,' ' ' . . I ' "...... ■ ■ ■ . \ \ # 0 I kr -3843' \ - - - ...... , :() ...... ■ ■ THICKNESS(cm) .- 18 12/ / GRAIN SIZE (cm) -- 7.i - - - - , I .0-7

.3 . I .., -38'43' • .4 / 2713" ...... C. ' . •• •5 •,.. .i. / . • . . • I ' / • e '" 7,4':, .2 5 .16 % % . % l I I / '55 )6 - s A t / * .20 • I i / „./`.."•24 %\ % t i I ' I 1 ,.,:.0165 ‘ • t /...... ,:4-.V.,.. \ % 1 i i i 1 % I 422 t 1 .31 \ 1 55 t* .:.#4 1 ti t I 1 1 % 156' i 1 / 1 1 i • ‘ \ S.," '>50 I 1 1 t \ % '28...0 • 1 1 .5 % s -61 / 16 / / % \ / / " i / l / 7 •14 / .4 •10 'ea 3. TH 1 CKN ESS (cm)!`-. 27113'

Fig. 2.10 a. Isopach map giving-thickness in cm of the scoria fall deposit from Galiarte cone (member no- 17). b.Isopleth map showing the maximum diameter of the three largest scoria fragments at each locality for the Galiarte deposit, in cm. c.Isopach map of the Algar do Carvao I scoria fall deposit, (member no. 19), giving' thickness in cm. (1'. Thickness in metres of the Monte Brasil surtseyan tuff in the vicinity of Angra along the south coast of Terceira. Maps a-d are all at the same scale. 4 i a. 100

Floc , 50—

1.Z.

\P- <0' /I Z 4\I' "D"E / :1 2 i l \l"( / VN .V.° CtO ICL / 90 15 •C I , *19 0 017 / "G B ATI • 45-qc 012 i "A 0 '20 ' I I 1 f.0 10 D,km2 . .100 1000

increase in volume

1000— Base •r Diam. m • 9 • 12 increase in violence *17 500— • 15 sc ou

t 019 220 of •22 •20 0 3 •15 6. 7 D,km2

Fig. 2.11 a. Plot of F against D (Walker, 1973) for Terceira fall deposits. mb: Monte Brasil surtseyan tuff. Nos. 9-20 are basaltic scoria deposits on the Fissure Zone, see Figs. 2.1 and 2.9. A-I are the main pumice fall deposits. fl and f2 are the Fogo 1563 and Fogo "A" plinian pumice deposits from S. Miguel. k is the Eldfell deposit, Heimaey, 1973. b. Plot of the basal diameter of scoria cones and the dispersal (D) of the associated fall deposits. 9-20 as above. .f is Fasnia cone, Gran Canaria; t is Teneguia cone (1972), La Palma; u is an unnamed cone on S. Miguel; o is CarvAo cone, S. Miguel; g is Serra Gordo cone, S. Miguel; r is Monte Rossi, Etna, Sicily. (Data from Walker, personal communication). k is the Eldfell cone; data is for the end of the first week of the eruption. The arrow indicates the increase of D as the eruption progresses. locality is given as well as an isopach map. The isopachs display a t extension southwards, due to a northerly wind at the time of eruption. The grain, size map does not show this, mainly because of the high density, of basaltic scoria. The grain-size range of the deposit small and typical of strombolian deposits and the upper size limit is at least partly a result of breakage upon impact with the ground, as was observed by the author during the 1973 Heimaey eruption in Iceland.

On a graph of F against D (Fig. 2.11a), the strombolian basaltic fall deposits occupy a distinct field (c.f. Walker 1973), separated from the surtseyan and sub-plinian deposits of Terceira and other volcanoes‘ For comparison the Eldfell deposit on Heimaey is also plotted from data obtained 7 days after the eruption began. The volumes of the strombolian 3 1 deposits on Terceira range in size from 0.1km (tD. R.E.)to less than 0.0001km3. The Heimaey strombolian deposit had a volume of 0.06km3 by the end of February 1973.

The dispersal area 'DI of the deposit of an explosive eruption is related to the vigour of the gas blast because it is dependent on the height of the eruption colwnn. However 1bl also depends on the height of the saria cone associated with strombolian scoria fall deposits (Walker, 1973).• Large volume eruptions will therefore produce larger scoria cones and, usually larger scoria fall deposits than small volume ones. Fig. 2.11b shows this relationship for deposits from Terceira and other areas. It can be seen that some scoria fall deposits have a disproportionately large dispersal compared with other eruptions of similar volume. These are thought to be the results of more violent strombolian eruptions. 'DI will also increase as an eruption progresses, (Self, Sparks, Walker and Booth and this is shown on Fig. 2.11a by an arrow on the Heimaey Plate 2.6 Pico Gordo, showing collapsed side of this cinder cone which is on the north side of the fissure zone, in the centre of the island.

161440111k • Plate 2.7a Base-surge beds within Monte Brasil Surtseyan tuff, at Angra do Heroismo, with cross-bedded middle unit.

Plate 2.7b Mantle bedding over a cast of a tree log, Monte Brasil tuff, Angra harbour. 4-E

Plate 2.8 Monte Brasil tuff, showing slump bedding, overlying a 15m thick compound basalt lava. Baia de Fanal. 4 1--; plot. Many of the scoria cones on Terceira are depleted in size and

profile by more than mere breaching where the lava flow leaves the cone.

The process of cone destruction is thought to be due to scoria falling

back into the vent when it is unable to clear the top of the cone at its maximum development. This may cause blockage forcing the lava

level in the vent to rise, perhaps above the base of the cone, which then becomes unstable due to the density contrast between lava and scoria. Conditions would then be right for collapse of part of the cone. The cones of Pico Gordo (Plate 2.6) and Galiarte on Terceira are partly

missing due to auto-destruction. The recent Icelandic eruption produced a large strombolian cone 200m high before it collapsed; the highest cone

on Terceira is 120m.

Monte Brasil is a surtseyan tuff-ring of partly lithified palagonite

tuff probably standing about 250m above its submarine base. The surto-

. eyan tuff which is exposed along the south coast of Terceiralas a high

fragmentation index, typical of the products of phreatomagmatic eruptions

(Plate 2.7). The value of D is here calculated by doubling the area on

land, as it is thought that approximately half the deposit fell into

the sea. This tuff overlies pumice fall 1Et and the Angra Ignimbrite near

the main town of Angra do Heroismo. A surtseyan tuff older than the with.a Lajes Ignimbrite/maximum thickness of 5.5m is seen below on the coast NE

of Monte Brasil, and a sub-aerial compound lava more than 20m thick is

seen on the west side iroicating that the site of Monte Brasil has been

a long-lasting offshore basalt generation zone (Plate 2.8).

2.3 Ignimbrite

In the following discussion the term "pyroclastic flow" is reserved

for the moving pyroclastic flow and "Ignimbrite" for the resulting rock 47

Fig.. 2.12 Map of the Lajes and Angra Ignimbrite. In the key 1. is outcrop of thick ignidbrite. 2.is area covered by thin ignimbrite and/or ground- - surge beds. • 3.is the air fall deposit associated with ignimbrite. 4.are areas which have no recorded deposits of this eruptive sequence. The westernmost dashed line is the approximate limit of outcrops of Lajes Ignimbrite age. I is the main outcrop of the Angra Ignimbrite. II-IV are the main outcrops of the Lajes Ignimbrite. Localities A-D are referred to in the text. Triangles mark the sites,of Cl4 datable carbon. The star marks the sample locality in Fig. 2.13. PA is Pico Alto Caldera. Arrows mark the main routes used by the pyroclastic flows. The map does not show any volcanics younger than the Ignimbrites. Plate 2.9 Two thin ignimbrites filling shallow valleys in a small quarry near Pumas da Agua. A - the Lajes ignimbrite, here welded but only 1.2m. thick. B - the Angra ignimbrite. body whether it is welded or not. The only ignimbrites on Terceira in the time period covered by this study were erupted from Pico Alto Volcano and are the earliest rocks in the volcanic formation being discussed. There are several different areas of ignimbrite which appear in the field to occupy the same stratigraphio horizon.), it is not possible to prove absolutely that all are the result of one eruption as the ignimbrite forms a discontinuous sheet around the volcano. All areas of ignimbrite have similar lithology, petrology, grainsize distribution, crystal content and chemistry but two pieces of evidence oontradict this: carbonised wood from.the be of the ignimbrite near Angra (Fig. 2.12) gives a C14 age date of 23,000- 350 years B.P., whereas two other samples from the base of the ignimbrite give dates of 18,6001. 650 and 19,680t 330 y..B.P. These latter two can be considered as the same age but the 23,000 year date may be of an older event. At locality A (Fig. 2.12) two thin ignimbrites are exposed (Plate 2.9). The lower is non- welded and is believed to correlate with ignimbrite at locality B, the Angra outcrop. The younger, welded ignimbrite may correlate with the ignimbrite of localities C and D. At locality A the two ignimbrites are separated by a thin bed of basaltic scoria from the nearby fissure zone.

The C14 date and the stratigraphic evidence at locality A, suggest that there may be two ignimbrites. The different outcrops of ignimbriteLl have been referred to as the Lajes Ignimbrite (Self, 1971) and here "Lajes Ignimbrite" is used to refer to all butthe Angra outcrops which are a slightly older ignimbrite, the Angra Ignimbrite. Volumetrically the Lajes Ignimbrite is the largest pyroclastic rock body yet found on Terceira, totalling 0.33km3 on land, but it can only be conjectured how much entered the sea. It is the latest in a long history of pyroclastio flow eruptions from Pico Alto and the two extinct volcanoes, perhaps 50 each one preceding caldera collapse. All the outcrops of the ignimbrite e peralkaline trachyte.

The Angra pyroclastic flow travelled towards the south from Pico Alto to give the ignimbrite exposed at two localities along the flow route and on the coast east of Angra do Beroismo. It is light grey and/I'm-welded through )st of its thickness and probably had a lower heat content than the later ones: welding is confined to less than one metre in the thickest (12m) sections and is only incipient. It is therefore suitable for grain size studies and Fig. 2.13 shows a section through the ignimbrite lkm NE of Angra. Layering of the ignimbrite, marked by grain size variations, (Sparks, Self and Walker, 1973), is present and this layering is equally well displayed by the Lajas Ignimbrites.

Walker (1972 pg. 138-142), has described crystal concentration in a basal layer at Angra now identified as a ground surge deposit, designated Layer 1. This layer contains up to 32% (by weight) of platy, anorthoolase crystals; carbonised wood is also found in this layer here and in other Lajes ignimbrite outcrops. This is evidence of a powerful ground surge (Sparks and Walker, 1973) preceding the main pyroclastic flow, which left deposita,less than lm thick, of crystals and lithics of Inconsistent thickness along and on either side of the flow route. Where the ignimbrite thins against a topographic feature these beds continue and are considered to be the lateral equivalent of the ignimbrite, indicating that the ground surge was more widely dispersed than the following pyroclastic flow.

Cumulate curves of sieved samples from each layer (Fig. 2.13b) show that layers 2a and 2b( sample nos. 2-5), which represent the main bulk of theignimbrite, have grain-size distributions related at the 51.

.. b. 99

n .C 3 c ·2b 0- -~. ~ 'CP- 50 n.o Q ;;: CD --& ..:r Q :l

I . 2 3 l c.

5 ..

it •Yes> 00 3 rI'~··a . : '-a. fl'.e, • A-2-~·.. ~ ... ~. 2 . ,,,,;, 0 8 e -. -. .~ ..;- e . I , - -0 -0 -,. - ~" ;' 0

Fig. 2.13 Layering and grain size characteristics of the Angra Ignimbrite. a. A section through nonwelded ignimbrite 8m thick at Vale do Linhares, lkm NNE of the town of Angra do Heroismo, (see star on Fig. 2.12). Layers as described in Sparks, Self and Walker, (1973) are shown together with the vertical variation .- in grain size Md(iS, sorting 6¢ and crystal content. This data comes from seven mechanically analysed samples, collected at this locality •. 2bP is the pumice concentration layer; 2bL the lithic concentration layer. h. A plot of cUDl'llative l-rt% on pr~babili ty paper of the seven samples. Note that 2, 3, 4, and 5 all converge at the fine grained end.

c.. An Inman parameter plot of a number of samples of the Lajes and Angra Ignimbrites, from layers 1, 2a and 2b. Also plotted are samples of pyroclastic-fall deposits from Terceira. The contoured fields are those for pyro clastic fall (Fa) and flow (n) taken from Walker (1971). Note the intermediate position between the two fields of· layers 2a and 1. fine-grained end. The cumulative curves of the ground surge bed and an air fall deposit (layer 3) cross those of the middle layers and appear to be unrelated. These two deposits are not an integral part of the gnimbrite but are produced by the same eruption (see section 3). Layer 3 mantles the topography and is a very useful stratigraphic marker horizon on Terceira together with the corresponding layer of the Lajes Telimbrite.

After an interval, Pico Alto volcano produced a second ignimbrite, the Lajes Ignimbrite. The wide-ranging pyroclastic flows reached both the north and south. boasts in 5 main areas (Fig. 2.12).

The Lajes area itself is a coastal plain covered with a flat Bur- faced ignimbrite sheet up to 15m. thick. It contains 2/3 of the total volume of, all the outcrops and is strongly welded and dark grey, with fiamme, where it is thickest. Over a wide area southeast of Lajes the ignimbrite thins. laterally and to the northeast terminates abruptly against the Lajes Fault scarp. Towards the source where exposure allows, it can be traced into a coarse, non-welded, lithic and crystal-rich bed up to 7m thick. This is thought to be a thick, proximal ground surge bed which was deposited on a slope too steep for the fluidised pyroclastic flow material to "stick".

A remarkably similar deposit is found at the base of the Quatro Ribeiras ignimbrite outcrops. Above Quatro Ribeiras the slope is steep, falling 700m in 3km from the Caldera rim to the coast, and the ignimbrite is only present in small depressions downslope. It is non-welded with grey to yellow pumice, up to 10 metres thick and layering is well developed. The pyroolastic flow which reached the coast west of the village of Biscoitos formed non-welded ignimbrite which is nowhere more than 2m thick and is poorly exposed.

Carbonised wood from the base of the ignimbrite exposed near Sgo Mateus yields a Cl4 date similar to that of the Lajes outcrop. The distance from source to sea is approximately 14km and most exposures along the flow route are welded ignimbrite. Fiamme are well developed, especially in the welded, finer grained, basal layer (2a). Here the abrupt upper limit of strong welding and elongate fiamme coincides with the top of layer 2a. It is thought that the heat capacity of the ignimbrite was such that only the finer grained and better sorted ignimbrite of the basal layer was able to weld and develop eutaxitic texture. Above this layer the rook shows no more than incipient welding, due partly to the heterogeneity of grain sizes and partly to the high content of non-juvenile lithics which may have been at a lower temperature than the juvenile material. Preferential welding of finer grained material under experimental conditions has been discussed by Guest and Rogers (1967). Compaction by load pressure may play a role in promoting welding; but where ignimbrite is exposed 2.2km south of the Caldera (locality A, Fig. 2.12) it is 1m thick, with little evidence of erosion, and is thoroughly welded.

The Lajes and Angra Ignimbrites at most localities have at leait two flow units; usually there is one main unit and the others are subsidiary. The flow units are best detected by the occurrence of other fine grained basal layers (layer 2a) well above the base of an exposure. The usefulness of the Lajes horizon as a stratigraphic natter is limited in the west of the island partly due to the cover of yaaag lavas and pyroclastics and partly because it does not occur on Santa Barbara Plate 2.10 Misterios Negros comenditic trachyte domes on the Fissure Zone, E. side of Santa Barbara volcar). The wedge of lava described in section 4, is the peak of the right hand (eastern) dome. Volcano. It seems certain that the pyroclastic flows from Pico Alto

did not reach the western extremities of the island because of the unfavourable topography.

To show how the grain size and sorting parameters of the Lajes

and Angra Ignimbrites compare with those published by Mural (1961) and

Walker (1971) a plot of Inman parameters for some samples, is given in Fig. 2.13c. Some Terceira pumice fall deposits are also plotted on

this diagram and the sorting characteristics of the ground surge layer (1),intermediate between those of pumice fall deposits and layer 2b, the main volume of .the ignimbrite, can be seen.

2.4 Lava Flows

In the period since the Lajes Ignimbrite eruption rather less than

80% of the total volume of erupted material is lava. For the whole sub-

aerial pile of Terceira the proportion of lava present is larger than

this probably, partly owing to preferential erosion of pyroclastics and

partly to the dominantly basaltic, early volcano building stages of the

island. The post-Lajes lavas range from alkali olivine basalt to com-

endite and all belong to the alkali olivine basalt suite. A convenient

division into basaltic and trachytic types used here is based on com-

position and morphology of the flow. Lavas up to and including benmor-

eites (56-57% Si02) form thin flows of basaltic type whereas

•trachytic lavas fDrm thick, extrusive domes like the Misterios Negros

(eventno. 21, Fig. 2.1 and Plate 2.10).

Volume estimates of lava flows are based on the area as delineated

in the field and from air photographs. Where several lava flows occur between two pyroclastic-fall deposits, criteria such as overlapping flow - c6 5F

Plate 2.11 The caldera wall of Pico Alto volcano at Juncal. The high walls are comenditic lava and the flows to the right are younger pantelleritic lavas. 5

margins and differences in vegetation cover were used to distinguish the flows.The'lavaextrUsiens on Terceira ean be classified by aspect'; *l ratio U/v).into deMea (A,;;- - ,less than 8) coulees/class is almost . exclusively basaltic. [H is the square root of the area of the lava flow in metres and V is the maximum thickness of the flow in metresj Pico Alto Volcano is notable for its lack of lava of basaltic composition. All the exposed sub-aerial pile consists of comenditie trac] to pantellerite lavas (Plate 2.11) and pyroclastics.

2.4 (i) Trachytic comendite to p ellerite lavas. The salic lavas form endogenous domes and coulges which fill the calderas of Pico Alto and Santa Barbara and adhere to the slopes (Fig. 2.14). On both volcanoes salic lava predominates in the post-Lajes period. Pico Alto has 34 salid lava members totalling 2.534km3 and of these the largest are the Pico Pardelas coulge (No. 10 on Fig. 2), and the Boi Domes and coulge (No. 6 on Fig. 2). The profile of Pico de Pardelas and coulee (Fig. 2.15a) is typical of this type of lava extrusion.

Santa Barbara Volcano has 22 salic lava members, at least 7 preceded by pyroclastic fall deposits. Several of these consist of lines of extrusive domes along fissures (Plate 2.12); in the case of Ponta da Serra (No. 19,11g. 15a) each flow unit is a coulge and all have similar morphology. Pico do Carneiro Caulge (No. 26,11.i. 2 and 15a) has a total • volume of 0.29km3, made up of at least 3 flow units, the largest having thickness of 90-100m at the, flow margins.

The distance flowed by lavas may be controlled by effusion rate as well as the more obvious controls of volume, viscosity and ponding.

(a.r; between 8 and 50) and flows (air. greater than 50); the 'third 5S

Fig. 2.14 Salic lavas'of a) Santa Barbara Volcano and b) Pico Alto Volcano. New calderas are shown by black triangles and old by open triangles. The dotted area around flow no. 25 on map a) is local pyroclastic fall material, with the extent indicated by a dashed line. Parasitic basaltic cones are shown in stipple. The scale is the same for both maps. 250 and 500m contours are shown. The numbers of the lava members correspond to those on Pig. 2.1

m a - 750

500

250

2 . • ar.4 300 • 200 100 3. ar. 5 or. 6-7 700 750 - 600 4. 500 ar.28 500.

250

10- E w 0 A = dome 0. coulee

(i)— 5- C)

e e a • o ********* .....* *** •* *** 0 0 ...... V .0 0 • ..• . ._ ,.. . 0 ... .00 • • • • g A ...... 0 0 0 • ...... • ...... cf a A A ,i,: A AAA A i 0 , 1 1.0 0 001 0.01 0.1 VOLU ME , km3

Fig. 2.15 a. Profiles of viscous lava domes and coulees on Terceira. ar is the aspect ratio. Horizontal scale is the same as vertical scale. 1.Pico da Pardelas (Fig. 14b, no. 10) 2.Pico do Loiras (Fig. 14b, no. 7) 3.Misterios Negros (Fig. 16a, no. 21) 4.Pico Rachado (Fig.. 14b, no. 23) b. Plot of the length of viscous lava flows, or flow units, against the individual volumes for Pico Alto and Santa Barbara Volcanoes. Group A are thinner and have flowed a greater distance than group B.

6 0

m at'. 7 - 750

500

250

2. ar=4 300 200 100

750

500

250

w 0 z = dome • = coulee F- cf) 5 • A

•• • • • ...... 0..*.. .. e. 0 ...... • ... et: ...... 0 • • t...... "o •©•• •° • 0 • • B • • • 0 Az4 A ...... !••• 11 °• 0 A A ...... LiA A AA A A • A 1.0 0 001 0.01 0.1 . VOLUME, krn3

Fig. 2.15 a. Profiles of viscous lava domes and coulees on Terceira. ar is the aspect ratio. Horizontal scale is the same as vertical scale. 1.Pico da Pardelas (Fig. 14b, no. 10) 2.Pico do Loiras (Fig. 14b, no. 7) 3.Misterios Negros (Fig. 16a, no. 21) 4.Pico Rachado (Fig. 14b, no. 23) b. Plot of the length of viscous lava flows, or flow units, against the individual volumes for Pico Alto and Santa Barbara Volcanoes. Group A are thinner and have flowed a greater distance than group B. !•

Plate 2.12 Pico Rachado comendite dome and associated domes aligned along a fissure trending north, looking west from the main Bagacina-Biscoitos road.. It is likely that within a close compositional group like the Terceira salic lavas viscosity effects are small and indeed some of theless acidic lavas form domes of very limited lateral extent, while the more acidic extrusions form coulees which have flowed up to 5.4km. The effect of slope is probably negligible, until it is steep enough to cause break up and avalanching of the lava (Stoiber and Rose, 1969). If the flow remains coherent then there may be an approximately linear relation of volume against distance flowed but in fact some have flowed a greater distance than their volume suggests (Group A, Pig. 2.15b) and this is thought to to due mainly to a higher emission rate. Phenocryst and glass content of all lavas plotted is similar. Where individual flow units can be distinguished they have been plotted. Flows up to 40m thick are usual and sometimes this is exceeded when flow units are auperimposed.

The Bali° lava,s are very uniform in appearance with black, scoria*► ceous, rubbly surfaces showing concentric, convex in the direction of flow due to ramping o lava:during • flowage. When a growing dome becomes unstable because of effusion of lava at a high rate, flow accelerates downslope and a coulde develops.

the lava supply is exhausted lava levees up to 40m high, (Plate 2.13) develop an the coulees by drain-out of the central parts of the flow. This sometimes leaves a jagged, crevassed area at the foot of the dome, analogous to the Ibergschrundi of a glacier (Plate 2.14).

Most bf these lava flows are of peralkaline composition with 63-67% Si02 and peralkalinity indices of 1.0-1.4. Anorthoclase pheno- wysts are ubiquitous with accessory augite, arfvedsonite and olivine, in microcrystalline to glassy groundmass. Rarely In the Pico do Carneiro 62

Plate 2.1 40m high lava levee on the Pardelas comendite coulee, looking north. The levee stretches away from the observer in the centre of the picture. Scale given by figure at bottom left.

Plate 2.14 Lavacal comendite coulee in the caldera of Pico Alto, Showing a jagged area below the source of,the coulee where viscous lava has continued to move after effusion has stopped. Plate 2.15 Flow structures in comendite lava at Serreta (Santa Barbara) shown by glassy (black) and vesiculated (grey) laters. and other flows, obsidian lenses occur, in a vesiculated matrix in.outer parts of the flows, (Plate 2.15).

The extrusion of nine salic lavas was preceded by sub-plinian

eruptions, discussed above. It is not known how many have smaller air- fall deposits accompanying the effusion of lava since the flows usually bury their source area and any pumice formed during a preliminary degassing stage is not now exposed. Two flows on Santa Barbara have a coarse,

dense, pumice deposit underlying the lava and in one, Pico Negrao (No. 5 Fig. 2.14a), the pumice forms a low cone or crater rim around the source of the lava.

Where the fissure zone, reaches lower levels in the centre of the

island, salic lavas give way to basaltic ones. The youngest comenditic

domes on Terceira are the Misterios Negros (No. 21, Fig. 2.14a), which

may possibly be historic. They are located at the lower limit of salic

lava, and are more glassy than other lavas of similar composition on

Terceira. They may have been formed from high level salic magma which

was remelted and mixed with uprising basaltic magma along the fissure.

The two domes at the western end of the Misterios Negros eruptive fissure ae a yellow and grey streaked, mix-lava.

2.4 (ii) Basaltic lavas. Basaltic eruptions during the past 23,000

years have been mainly along the Fissure Zone, especially in the mid-

island region (Fig. 2.16). Here a low elongate ridge has been built up

by basaltic pyroclastics and lavas flowing laterally away from the

fissure. The rocks include olivine basalts, hawiites, feldsparphyric,

mugearites (Plate 2.16) and no differences in morphology can be seen A AA A AA AAA AAAA AAA AAA AA NAA AAA 1AA AAAA .AAA AAAAA AAAA AAAAAA AAAAA, AAAAA ' AAAAA t .. 1A AAAAAA 15 A

VVV VVVVVVj. VVVVV VVVVVV ' VVVVVVV VV .." VVVVVVV VVVIA V*.yVVVVV VVVV ,, ./VVV.UVVv VVVVV , YVV V VVVVVVIPU,kWV/e1

22 6-7 15

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Fig. 2.16 The central part of the Fissure Zone. a.Map of the lavas and scoria cones on the Fissure Zone. Numbers refer to Fig.2.21. Two faults are shown, marked f. b. The traces of eruptive fissures shown in a.; f marks the faults. Dashed line is the present axis of the fissure zone, based on the line passing through the younger fissures. 66 Structures in Fissure Zone invas

Plate 2.16a Lava blister in oiine hawaiite lava lkm. W of Pico da Bagacina.

Plate 2.16b Extremely vesiculated benmareitic lava near Pico de Gaspar. Fig. 2.17 Map of Terceira showing 6 large fissure zone basalt flows, (small nos., see Fig. 2.1 and 16) and 15 post-Lajes basaltic events (large nos.). These latter events are not directly related to a volcano or the central part of the Fissure Zone. 1 is the oldest and 15 the youngest event. Calderas as Fig. 1.2 6g

Plate 2.17 The Algar do Carvao I olivine basalt lava flow (foreground, sparsely vegetated) covering the flat floor of Cinquo Picos Caldera. The northern caldera wall is in the background, a distance of 51m from this viewpoint. 64 6

Plate 2.18 The 1761 eruptive fissure, site of the youngest eruption on Terceira, looking east. 4 small scoria cones are aligned on a curving E-W fissure. The associated hawaiite lavas flowed mainly to the north (left). 70 between them. Several erupions have occurred off the main fibsure e.g. the Porto Martins lava (Fig. 2.17), the youngest lava in eastern Terceira. The largest flow is the Algar do CarvEo I, alkali olivine basalt flows, which totals Oakm3 (Fig. 2.17 and Plate 2.17). It reaches both the north and south coast from a source just south of Pico Alto Caldera, a combined distance for the two main lava tongues of 21.5km.

The fissure zone is 2km wide and consists of short sub-parallel, monogenetic eruptive fissures (Fig. 2.16b and Plate 2.18). The axis of the zone is marked by the most recent eruptive fissures, with the exception of the 1761 fissure. The age tends to increase away from the axis, which is asymmetric. These volcano-tectonio features are believed to be the surface expression of successive injections of magma up the middle of the Terceira Rift, causing lateral spreading from the axis of the fissure zone. The field evidence supports the proposals of Krause;andWatkins (1970), that there is a secondary spreading centre along the Terceira Rift. The asymmetric distribution of indi- vid-ual fissures may indicate that the axis of the Teroeira Rift is migrating northwards with a net result that the south side of the 'ft would have a faster spreatiirg rate than the north. side.

Discussion and Application of Quantitative Volcanology;:

(i) .Volume Relationships. During the past 23,000 years the volcanoes on Terceira have produced not less than 5.25km3 of lava and pyroclastic deposits, converted into dense rock equivalent. The individual volumes produced by the various eruptive centres are given in

Vs. , TABLE 2.3

VOLUMES OF LAVA AND PYROCLASTICS ERUPTED ON TERCEIRA DURING THE PAST 23,000 YEARS A = Salic Composition. B = Basaltic Composition m = Minimum Estimate all volumes in km3 (D.R.E)

TERCEIRA SANTA BARBARA PICO ALTO FISSURE ZONE

TOTAL = 5.255k0 VOLCANO VOLCANO AND OTHERS • A 73 A • B A )1 A B. TOTAL VOLUME 4.516 0.739m 1.483 0.1052 3.033 -. - 0.632m LAVA 3.930 0.402m 1.396 0.089m 2.534 - - 0.313m PYROCLASTIC FALL 0).252 0.337m Oi.087 0.016m 0.165 - - .0.319m; PYROCLASTIC FLOW 04334m - - - 0.334m - - - Table 2-3, where it can be seen that there is a dominance of salic lava and pyroclastics on both Santa Barbara and Pico Alto volcanoes.

From the total volume of material erupted, an average output of approximately 0.023km3 per century is indicated over the last 230 centuries. At this rate the subaerial pile of Terceira could have formed in less than 1 million years. No K/Ar dates are yet available for Terceira but lavas from the eastern part of Sgo Miguel, which is much more deeply dissected tan Terceira, yield dates of up to 3.4 m.y. (Abden Mone,m et al., 196$). This indicates that an age of circa 1 m.y. for Terceira is reasonable. Sao Miguel is the largest Azores island and lies 100km to the SE of Terceira (Fig. 1.1). Both, Croasdale & Walker (fin press) have shown that the 3 active volcanoes on sao Miguel produced 7km3 in the last 5,000 years. This rate is at least 4 times greater than Terceira but Sao Miguel has one more active volcano,.

While 90A of the salic material on Sgo Miguel consists of pyroclastic fall deposits, the position is reversed on Terceira where only 25A is pyroclastic fall material. Sao Miguel has a more explosive history than Terceira; at least two of the pumice fall deposits are of plinian type (Walker and Croasdale, 1971), with much wider dispersal areas than even the largest eruptions on Terceira.

When the volume of lava and pyroclastics erupted on Terceira is plotted against an arbitrary time scale it is apparent that there have been fluctuations in the rate of magma eruption (Fig. 2.18a). No periodicity for eruptions can be given as there are no known dates between 2,000 and 18,000 years. The total of 116 eruptions in 23,000 years gives an average interval of 200 years between eruptions. However there have been 21 eruptions in the

73 .

a. (0.43)

H / Basaltic 0.3_ Salic, Pico Alto mb iSalic, Sta. Barbara

G km3 0.2_

D 0.1_ I F I.

nu ? A I 1 ceJ . 11 I. . 23 Years B.P. x 103

No. SALIC

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BASALTIC

km3 o00ó1. olOol 10

Pig. 2.18 a. The volume of lava and pyroclastics (as D.R.E.) erupted during each event plotted against an arbitrary time scale. - Triangles indicate known dates. A-1 are the nine mnin pumice fall members. mb if the Monte Brasil tuff ring; ? indicates that a minimum volume is given. b. Histograms of the frequency of size of eruption, expressed as volume (km)) for the salic and basaltic groups of lavas and pyroclastics. 74 past 2,000y. since Fissure Member 'Z', dated at 2040y. B.P. by the C14 method, which suggests that this has been a period of more intense activity, shown also by a corresponding peak in volume extruded. The peaks are present even if the 116 events are regularly spaced throughout the time period. Soils at the top of each member where sections show a number of superimposed members, suggest that a considerable time elapsed between eruptions. Sometimes a soil is lm. or more thick, which represents perhaps 1,000 years in a climate like that of the Azores. The spacing of eruptions (Figs. 2.1 and 2.18) is based on the above consider,- ations. When the data on Fig. 2.18a is tested statistically by the moving averages method and a standard deviation test, eruptions of higher than average volume fall into, groups with respect to time. There have been four such periods of high volume erutpions during the last 23,000 years.

The data (Fig. 2.18b) show that the most common basaltic eruptions are an order of magnitude smaller than the most common salic ones. The salic group has"a smaller size range, there being no salic eruptions smaller than 0.001km3 but 12 basaltic ones in this lower size range. The lower volume basaltic events are adventive cones and flows on Santa Barbara and elswhere, whereas the larger volume are all from the central part of the Fissure Zone. Of the basaltic pyroclastic material on Terceira, half is .accounted for by the Monte Brasil tuff ring. Three quarters of the basaltic total (lava and pyroclastics) are accounted for by only 6 out of 51 eruptions.

The main peakson Fig. 2.18 are due to salic eruptions on Pico Alto and Santa Barbara, while the smaller basaltic eruptions continued at more frequent intervals. The median volume of salic lava members on both Pico Alto and Santa Barbara is the same (0.043km3). However, the median volume of salio pyroolastio-fall deposits on Pico Alto is greater than that on

a.- cm 100

10 ______logo A ____ 7E63 ______' ______a l ______0

20 km DISTANCE10 15 m.sec-1 100

1 15 2b km 5 DISTANCE10

Fig. 2.19 a. Average diameter of the 3 largest lithic fragments plotted against distance from the source (Figs. 2.2 and 5) for sub- plinian deposits 7B" and "E". Also plotted are Fogo 1563 (D = 500; volume = 0.15km3) and Fogo A (D = 1500; volume = 0.6kml, both from S. Miguel. (Walker and Croasdale, 1971 . b. The median terminal velocity for samples of depositsnr and "E" against distance of the sampling point from the source along the dispersal axis. Black symbols "B"; open symbols

circles - - whole sample median TV. triangles - lithic sample split median TV. squares - pumice sample-split median TV. 7 (3

Santa Barbara. The median volume of all basaltic members is 0.004km3 and tha median volume of all smile members is one order of magnitude larger, 0.04km3.

2. (ii) Dispersal of the pyroclastio fall deposits. The sub-plinian pumice-fall deposits cover a range in dispersal (D) and volume (Table 2.1). The two largest, 'B' and 'E', are both from Pico Alto Volcano. 'B' was erupted when a considerable NW wind was blowing and 'E' when there was little wind. This conclusion, deduced from the isopleth and isopach maps, is also reflected in the graph of average maximum diameter of the three largest lithics plotted against distance from the source (Figs. 2.2 and 2.5). A comparison of the effects of wind on the eruptive products of these two events is appropriate because of the similarity in dispersal of the pyroclastic deposits and in the density of the lithics in the two.

Fig. 2.19a shows the envelopes of the lithic olast diameter data,plotted for deposits 'B' and 'E'. The range of lithic clast sizes which behave in a ballistio fashion is larger for windless conditions, and decreases as the wind increases, so that with a strong wind only the largest blocks have ballistic trajectories. The steep upper gradient on the graphs is that of near-ballistic clasts and the lower slope that of wind influenced particles; the change in slope (minimum cut-off diameter of near-ballistic clasts) is at a greater diameter for the eruption in a strong wind (B), 19cm, than for the eruption in a weak wind (E), 12.5cm. Pumice fragments are more influenced by the wind than lithics of the same diameter, which can be seen by comparing the circularity around the source area of the lithic isopleths with that of the pumice isopleths (Figs. 2.2 and 2.4). 77

In sub-plinian deposits, fragmented juvenile material with a wide range of grain size is ejected vertically or sub-vertically from the vent. As small particles decouple from the eruption column they are immediately transported sideways by the wind while they begin to fall. Particles of any given size are carried further by a strong wind than by a weak one. In 'B' (Fig. 2.19a) grain sizes at all distances from the .source are larger than in 'E'. This may be partly due to the generally coarser nature of UP; 'E' has a much higher fragmentation (Table 2.1). and this accounts for the fact that the line for 'E' lies lower than that for 1B1. Also part of the difference between 'B' and 'E' is due to wind.

On' the wind influenced slope of the graph, the fall-off in particle diameter per unit distance in 'B' is only slightly less than in 'E'. Considering the above this should not be found. An explanation may be that, although 1E1 has a smaller volume and was erupted when the wind was weaker than 'B',it was a more powerful eruption. In effect, 'E' may have had a higher eruption column, which directly increased the dispersal, even though there was little wind.

Two plinian pumice-fall deposits from Sgo Miguel (Walker & Croasdale, 1971) may be compared with 'B' and 'E'. The Fogo 1563 erqdion was into a strong wind and the Fogo 1 A1 was into a weak one. The graph of Fogo 1563 has a less steep slope than 'B', mainly because of a stronger wind. The similarity of slope between Fogo 1 A15,with a volume of 0.6km3, and 'E', with a volume of 0.05km3/ suggests that wind, or lack of it, may be more important in controlling the fall-off in grain size per unit distance than volume. However,the larger grain sizes evident in Fogo 'A' results only fran.the mph greater power of this eruption (D 1500) oompared with the other three.

LIO 78

Theoretically, the wind-influenced part of the graph should become increasingly parallel to the abcissa as the wind speed increases. Con- versely, the large blocks, which never become accelerated to anything approaching wind speed, still receive a sidewards displacement from the wind, with graphical effect that, for eruptions of similar dispersal i.e. 'B' and 'E', the steep limb moves to the right, away from the ordinate.

For eruptions of increasing power the steep limb also moves to the right, as displayed by the Terceira and go Miguel examples. Wilson (pers. comm.) has tested the relationship between wind speed and the graphical effect on the lithic diameter against distance plot. By com- putation it was found that, for a range of particles falling from a fixed height in winds of two different speeds, the ratio of the ranges of small particles of the same size from two different eruptions, was equal to the ratio of the wind speeds. Also the ratio of the ranges for the large particles was equal to the square of the ratio of the wind speeds The graph of 'B' and IV (Fig. 2.19a) gives values of range ratio tending to 2.2 for small particles (less than 0.5cm) and tending to 4.6 for large particles. As 2.2 squared is 4.84 this relationship may be valid and here is, perhaps, a way of quantifying wind speed from grain size data derived from pumice fall deposits. When eruptions during periods of known wind speed are plotted in the above way it should be possible to calibrate such graphs and work is progressing in this field. One problem with-deposits on Terceira for this type of study is the lack of distal material because of the small size of the island.

The effect of wind on fragments falling from an eruption cloud can be analysed from the data available on pumice deposits 'B' and IV. Iiirer et al. (1973) presented two plots of median T.V. of fragments against 77;19 distance for the Somma-Vesuvius plinian deposits but both appear to have been erupted into winds of similar speed. Median T.V.Is for samples of 'B' and 'E' collected downwind along the dispersal axis were calculated using the curves of computed T.V. (Walker, Wilson & Howell, 1971). The fall-off in T.V. is slower for the deposit erupted into the stronger wind (Fig. 2.19b). This is a simple effect of larger and more dense fragments being carried further in 'B', partly due to wind speed and partly due to the slightly larger dispersal of 'B'. However pumice and lithic splits of individual samples, which should give the same value have differing median T.V.'s. Lithic T.V. Values are consistently higher than pumice,in both deposits. This may be caused by a smaller size range in the initial population of pumice (Walker and Croasdale, 1971, p.34) and also by the breakage of pumice on impact with the ground. This latter process may be very important in forming the final grain size population on the ground in deposits where brittle pumice or scoria predominates (Self et al; 1974).

2.5 (iii) Distribution of Eruptions. The distribution of eruptive sites throughout Terceira in the 23,000 year period being considered (Fig. 2.20) reveals the importance of the Fissure Zone as the controlling volcanic lineament in the western part of the island for both basaltic and salic eruptions. In the Pico Alto area eruptions of salic members show some bias towards sites on the caldera rim. The scattering of basaltic, adventive eruptions appears to have a more or less random distribution except for a few members on the south-eastern extension of the fissure.

The youngest strato-volcano, Santa Barbara, is diagonally bisected Fig. 2.20 Eruption sites on Terceira during the post-Lajes period. The 4 calderas are as in Fig. 1.2. Black circles - Basaltic eruptions. Open: circles - Salic eruptions. by the Fissure Zone and only one trachytic eruption, along the Pico Rachado Fissure (No. 23, Fig. 2.14), which may lie on a radial fracture as described from this volcano by Machado (1955), is oblique to the Fissure Zone. Pico Alto Volcano differs greatly from this pattern of fissure controlled eruption sites. The structure of Pico Alto is a complex pile of domes and flows built on the slopes of the older Guilherme Moniz•Voloano. No basalts have been found in the subaerial pile of Pico Alto; down to sea level on the north coast there is a succession of salic, peralkaline lavas and pyroclastics.

A control on basic magma release may exist, where a viscous body of salic magma resides at high level under Pico Alto, effectively blocking any eruptions of basic magmas in this area. The low-density, salic magma may have ndgrated upwards and away from the Fissure Zone, where more basic magma is uprising and being extruded at frequent intervals; the two species of magma can be extruded contemporaneously by this mechanism.

A similar process may operate under Santa Barbara, where the volume of salic magma may be smaller. As the magma is generated under Santa Barbara it may be extruded quickly, leaving no viscous plug to obstruct the fissure control of eruption sites. At lower levels on the central part of the Fissure Zone, and perhaps also the NW of Santa Barbara where the most recent, submarine eruption occurred in 1867 (Fig. 2.1)1 basaltic eruptions predominate over trachytic. A complementary situation to Pico Alto may be developing under Santa Barbara; a concentration of salic eruption sites is evident in the northern part of the volcano which could indicate that salic magma is migrating north of the axis of the Fissure Zone.

2.5 (iv) Styles of eruption. It appears possible to draw some links between the various types of explosive eruptions occurring on Terceira, 82 8 and on other Azores islands, by consideration of the role of magmatic gas on a number of factors known from field investigation. Below the vent the temperature of magma of one specific type, fragmentation of the magma and the resulting grain size population may be quite uniform yet there is a series of eruptive styles from strombolian though pyroclastic flows to plinian.

It is known that plinian eruptions have very high eruption columns and that they eject large volumes of material in a short period of time (Thorarinsson 1968, Walker 1973). Sub-plinian eruptions are scaled-down versions of the plinian type with lower eruptive columns, resulting from lower gas release rates, and possibly smaller volumes of magma. Sub- plinian deposits on Terceira commonly contain juvenile black, non-vesicular obsidian °lasts, which are not found in the plinian deposits on Sao Miguel (Walker, pers.. comm.). This suggests that in sub-plinian eruptions the magma is incompletely vesiculated due to a lower gas content. Ignimbrites appear to have an even lower gas content and contain many poollyvesiculated, 'dense pumice' clasts and non-vesiculated magma i.e. 'flame'. Although large volumes of material are often involved in pyroclastic flow eruptions the gas blast is weaker, so the whole mass of ejected pyroclastic material is more dense than in plinian eruptions and may slump once above the vent, to form pyroclastic flows. The weakness of the gas blast may also be a feature of pyroclastic flow eruptions. Many ignimbrites in the Azores and Italy have indicators of a low-level origin such as syenite and gabbro xenoliths but sub-plinian deposits on Terceira have less xenolithic material. &plosions at great depths may lose most of their power before the vent is cleared and the low momentum produces a low eruption cloud, which may readily slump and then cascade down the sides of, the volcano as a pyroclastic flow. The controlling factors on eruptive style therefore appear to bs -the volume of magma available, gas content, gas release rate (controlling effusion rate and "muzzle velocity") and the depth at which vesiculation and explosion is initiated. The well-known classification scheme of eruptions of &cher (1933), shows the importance of the viscosity of magma and gas pressure in the control of eruptive style. It appears that the styles of eruption represented on Terceira would fit well into Escherls scheme with a few changes, mainly of terminology only. Escherls "strong vulcano type" may now be replaced by sub-plinian and his "perret type" by plinian eruptions; pyroclastic flows would occupy the "St. Vincent type" snapper- haps,the "Pelee type".

2.5 (v) Conclusions. The volcanism at work on a spreading oceanic rift has been closely analysed and quantified; knowledgeof the sequence and volumes of various lava types enable petrological relations to be examined in a new light (see Part 4). The moderately explosive nature of some activity on this island indicates that energy is released on constructional plate margins both explosively and by fissure-type eruptions.

Volatile-rich, salic magma is responsible for the sub-plinian and pyroclastic flow eruptions. The time periods between extrusions of this magma, gives, in some cases, sufficient time for gas pressures to build up and cause explosive eruptions.

The eruptive products that have been described from the most recent formation on Terceira, which has at the base the latest ignimbrite on the island, are similar in style of volcanic activity and chemistry to earlier formations. Salic lavas, pyroclastic-fall deposits, and ignimbrites are present in the exposed parts of the two extinct volcanoes, although on the oldest mugearitic basalts have the widest exposure. The Fissure Zone has long been a site of basaltic activity, with little or no apparent change in composition of its products. The products of any totally basaltic periods if they exist are now under sea Level; indications from the exposed pile of Santa Barbara are that early volcano-building lavas are of the same mngearitic types as those erupted on the Fissure Zone in recent times.

It is therefore concluded that the products of the past 23,000 years are in no significant way different, either in composition or in volumetric proportions, from the whole of the islands volcanic hi 'tory which is revealed above sea level..

IGNIMBRITES ON TERCEIRA 88/5

3.1 Introduction

Ignimbrites are not common on mid-oceanic islands but they occur on two of the Azores Islands. Besides Tenerife (Booth, 1973) and Gran Canaria (Schminke and Swanson, 1967), Terceira may have a larger percentage of ignimbrite than any other island in the whole of the Atlantic Ocean. Ignimbrite is known to occur on one other island in the Azores, namely, Sao Miguel, but has a smaller volume there than on neighbouring Terceira.

On Terceira ignimbrites are known to occur at 6 distinct stratigraphic horizons but, owing to a lack of deep dissection and the consequent difficulty of correlation, it is not known exactly how many different ignimbrite producing eruptions there have been during the islands history.

The Lajes and Angra Ignimbrites, which are between 23,000 and 19,000 years old (by C14 dating, see previous section), axe the youngest. At least 5 older ignimbrites occur but are exposed only in cliff sections on the north and south coast of the island so that correlation is difficult. Because of the widely scattered nature of the exposures of older ignimbrites, their sources cannot be identified but it is conceivable that some accompanied the formation of calderas on the older volcanoes, Guilherme Moniz and Cinquo Picos. The ages of the older ignimbrites may be from 50,000 to 800,000 years spanning nearly the whole of the island's subaerial history.

All except one of the ignimbrites on Terceira are similar in composition to the Lajes and Angra ones, comenditic trachyte with prominent anorthoolase phenocrysts. The exception is amugearitic ignimbrite restricted to a few outcrops in the south of the island. Most of the ignimbrites are, in the gRibeiras 27/10

•

Fig. 3.1 Map of the Lajes and Angra Ignimbrite, as Fig. 2.12, but showing the location of the sectionsin Fig. 3.2 (A—B) and Fig. 3.3 (c), and other localities discussed in the - text. TABLE 3.1. SCHEMATIC CORRELATION DIAGRAM FOR TWO NORTH AND SOUTH COAST SECTIONS

SOUTH COAST NORTH COAST Known ages. Augra do Heroismo Porto do Vila Nova Present 19,000 y LAJES IGNIMBRITE 23,000 y 1. ANGRA IGNIMBRITE 1. BASALTIC IGNIMBRITE IN CONDENSED SEQUENCE CONDENSED SEQUENCE 2. 2. CONDENSED SEQUENCE CONDENSED SEQUENCE 3. 3. FANAL IGNIMBRITE VILA NOVA IGNIMBRITE 4. 4- CONDENSED SEQUENCE CONDENSED SEQUENCE 5. 5. CASTELINHO IGNIMBRITE CALDEIRA IGNIMBRITE 6. 6. PORTO DAS PIPAS IGNIMBRITE CONDENSED SEQUENCE 7. 7. IGNIMBRITEsi* and IGNIMBRITE*z:

Nos. 1. to 7. are major erosion horizons. large part, non-welded.

The main purpose of this study is the distinction between the various ignimbrites found on Terceira, together with a complementary interpretation of granulometric analyses of the predominant non-welded types. A regularly repeated pattern of layering (Sparks, Self and Walker, 1973) is evident and some physical properties of pyroclastic flows can be deduced from these layers. Sorting of the individual components of the ignimbrites (pumice, crystals and lithic fragments - as previously defined) is also found and the mechanisms involved may be partly defined.

3.2 Stratigraphy

Stratigraphic correlation of the ignimbrites is difficult because of a) their propensity to occur in valleys, b) the ease of erosion of non-welded pumice deposits, c) the lack of distinguishing characteristics between the various ignimbrites. TableAI illustrates a postulated correlation between ignimbrite and erosion horizons exposed at Porto do Vila Nova on the north coast, and at Angra do Heroismo on the south coast. The condensed sequences of thin salic and basaltic air-fall deposits, representing long time periods between the ignimbrites and erosion surfaces on the two sides of the island, are remarkably similar in general character.

The Lajes Ignimbrite eruptive sequence produced pyroclastic flows which reached both the north and south coasts. As the older ignimbrites are erosional remnants of once much larger sheets, it is reasonable to suppose that in other eruptive sequences pyroclastic flows reached both sides of Terceira. Some of the ignimbrites on each side of the island may, therefore, be from the same eruptive sequence and such a possibility is explored in Table32. Here the essential characteristics of four ignimbrites are compared, showing that they may be from 2 eruptive sequences only. All have the same major phenocrysts as the Lajes Ignimbrite but differ in their accessory phenocrysts. In general, each ignimbrite shows a variety of sorting and grain size characteristics (Murai, 1961) and to these are not useful in correlation, as no one parameter can be said/uniquely characterise an individual ignimbrite.

Ignimbrite is only exposed on limited stretches of the north and south coasts. It is well known that ignimbrites tend to fill valleys or depressions (Fenner,1920; Smith and Bailey, 1966) and evidence of re-use of the same valley is seen at Vale de Linhares (Plate 3.1). Flow routes of successive pyroclastic flows may often be similar, as rivers quickly cut down through the unconsolidated pundce of the previous ignimbrite. the Bolsena area of Central Italy, there is evidence of 3 or 4 successive ignimbrites filling a gorge eroded in almost the same position after each eruption.

Two important sections illustrate all the ignimbrites which will be discussed below. The north coast section (Fig. 3.2) has the Lajes Ignim- brite at the top, where itkrms a coastal plain (Plate 3.2). It is obvious that the ignimbrite fills a broad valley and has a flat upper surface,, and that it is absent or very thin on higher ground e.g. at Porto do Vila Nova. Below, in the north coast section, are erosional remnants of several older ignimbrites. The section at Angra Barbour (pig. 3.3) is an old hill partly made of erosional remnants of older ignimbrite sheets around which the Angra pyroclastic flows passed, leaving only'a thin 'smear' of ignimbrite. The location of these sections (Fig. 3.1) marks the seaward end of the most commonly utilised pyroolastio flow, routes on the island. q

Angra Tgnimbrite overlying older igniMbrites and dellf flows in the cliffs east of Angra at the end of Vale d. Cliffs 20m. high.

• •

Ponto. do.. EAST A Co..ldeil'&.. do.s 50m Lo.jes

Po .. to dQ. Vilo.. NoV"tJ.. Ribeiro.. I CIlo..~ P ..oIfO.~ C B I I ~wrrm~~~' ~.. -. SOm o I

Fig. ,.2 The North Coast Section. For map locations of ABC see Fig. ,.1 1. Lajes Ignimbrite. 2. Vila Nova Ignimbrite. ,. Caldeira Ignimbrite. 4. Ignimbrite "i". 5. Ignimbrite "z". "f" and ng" condensed sequences. F = fault. Vertical scale is 2.5 x horizontal.

:~'. ;."

3. TABLE 2 EVIDENCE TO SUPPORT CORRELATION OF FOUR IGNIMBRITES SOUTH COAST. NORTH COAST

FANAL IGNIMBRITE VILA NOVA IGNIMBRITE At least 4 flow units, each 1. Multi-flow unit ignimbrite (at strongly reversely graded. least 12) each strongly reversely graded. 2. Between erosion surfaces 3+4 2. Between erosion surfaces 3 and 4 3. Non-welded 3. Non-welded 4. Complex, cross bedded, ground 4. Complex ground surge at base surge at base 5. Middle flow unit contains 3-4 5. Middle main flow unit contains up small flow units to nine small flow units 6. Large proportion of pumice to 6. Large proportion of pumice to crystals and lithics in lithics and crystals in middle middle of ignimbrite of ignimbrite 7. Heavy minerals: 7. Heavy minerals: Augite 4% of crystal content Augite 2% of crystal content Ilmenite 2% " Ilmenite 1-2% " " Magnetite 0.5%- Magnetite 1% CASTELINHO IGNIMBRITE NORTH COAST IGNIMBRITE • 1. Two main flow units with 1. One main flow unit over debris intraformational debris flow flow in between 2. Between erosion surfaces 5 and6 2. Between erosion surfaces 5 and 6 3. Intraformational debris flow 3. 'Debris flow contains contains rounded clasts of rounded clasts of lower flow unit lower flow unit and vitrophyric clasts 4. "Pipes" common (fossil 4. A few very wide diffuse "pipes" fumarole5 ) 5. High lithic content especially 5. High lithic content especially in upper flow unit P = 57% in upper flow unit P = 547. c 17% C =13% 26% L 33% &. Stratified, cross-bedded 6. Base not exposed surge at base 7. Heavy minerals: Augite • 2% 7. Heavy minerals: Augite 3-4% Magnetite 1% Acicular amphibole 0.5% Acicular amphibole ( negl) Magnetite (negl) Q3

Plate 3.2 The north coast plain, underlain by the Lajes Ignimbrite, seen from the slopes of Pico Alto Volcano, looking NE. The village in the centre of photograph is Agualva. Thick wooded area on right is one of young comendite lava flows.

•

Plate 3.3 Extreme pumice concentration at the top of layer 2b in the Lajes Ignimbrite near Porto do Vila Nova. Horizontal boundary by hammer is the top of moderately welded zone in layer 2b. 5m

Fig. 3.3 The section at Angra Harbour. 1. Angra Ignimbrite. 2. 'Basaltic1 ignimbrite. 3. Penal Ignimbrite. 4. Castelinho Ignimbrite. 4a Mudflow in 4. 5. Porto das Pipas Ignimbrite. mb = Monte Brasil tuff. cc = condensed sequence. For location see Fig. 3.1. The Lajes and Angra Ignimbrites

The extent of these two closely related ignimbrites (Fig. 3.1) an their main features have been outlined earlier as part of the recent -:volcanic history of Terceira. These are the onlrigniMbrites on Terceira; which are seen inland.

The outopp (Fig. 3.1) is not a continuous sheet but occuplisa number of areas of low-lying ground along the flow routes where the pyroclastic flows passed. This is normal for ignimbrites (Minato, 1972; Aramaki, 1956; Yokoyama, 1970), especially those that have been erupted in mountainous areas. The Lajes Ignimbrite is locally as much as 20m thick, for example east of Vila Nova. In places, up to 7 of the thickness is welded especially where it infills depressions. Thin, non-welded ignimbrite is-found on slightly higher areas and in a wider dispersal around the main outcrops. A thin air-fall deposit associated with the Lajes eruption covers a still wider area The lack of near source out 3rops is a result of, the high mobility of pyroclastic flows (Smith, 19601'; Ross and Smitn 1961; McTaggart, 1960; Fenner, 1948; Tazieff, 1970; Fisher, 1966; Ray, 1959). The Angra Ignimbrite is almost totally non-welded and all exposures of the Lajes Ignimbrite show prominent non-welded zones. Because of this lack of welding and the scarcity of data on grain size variation in ignimbrites a granulometric study of these two ignimbrites was made. Grain-size variations have been investigated both horizontally and vertically. Both are useful for examining the eruptive mechanism of pyroclastic flows. As the ignimbrites have distinct layering, vertical grain-size variation will be discussed first and horizontal after; the success of an eerealgrain size variation study rests on measurement of ain-size from one layer only. FINE ASH DEPOSIT P

2 ONE FLOW 0. 0 0 b 0 •0 UNIT 00 0 ,c) o . C °0 o 0 • • $1111 . • • • . • 1:1° • 0 • C o)mo • ' 0 (..)• #c; 156 ,41i 01 alo• Sit 0 --ff. CZ • o I. • • o • • • 0 . -6-' ° • • • 6 • • - ••6 • •4'.. 0 • • • .• 6 • 2 . • • , .• .• . GROUND SURGE 1 DEPOSIT

Fig. 3.4 Schematic section through the products of an ignimbrite eruption, showing one flow unit with layers 2a and 2b, underlying ground surge deposit (layer 1), and fine air fall deposit above (layer 3). P = pumice clast concentration zone, L = lithic clast concentration zone. 97 Welding is one well described zonation within ignimbrites (Smith, 1960b). Welding occurs on, just prior to, or immediately after, the flow comes to rest and it should not, in this case, influence the grain-size layers within the pyroclastic flow as this is attained during flowage. The non-welded parts of igaimbrites containing welded zones are assumed to have formed under the same conditions as similar parts of totally non- welded ignimbrites.

3.3 (i) Vertical grain-size variation. Many ignimbrites show distinctive layers which appear to be present irrespective of composition, welding and size of ignimbrite, (Sparks, Self, and Walker, 1973). These layers are easily recognisable in the Lajes and Angra Ignimbrites. To analyse these layers, series of samples collected from each more or less complete exposure have been used. The samples are not spaced at equal intervals but are intended to be representative of each layer, so that the characteristics of each layer can be compiled.

Samples were analysed by sieving (see Appendixil for details) and cumulative curves of the weight percentage in each grade of grain-size are plotted on probability paper. From these curves the simple statistical , parameters of G'and mayS (Inman, 1952) can be obtained; the grain-size characteristics of the layers are most conveniently described by these parameters. Some samples were separated into their components, namely pumice, crystals and lithics, to examine the behaviour of each component in a pyroclastic flow.

The sample localities were chosen to cover as wide a range as possible from these ignimbrites. Layers consistently found are shown in a schematic section (Pig. 3.4); all layers need not be present at one loodlity but they are always found in the s ame sequence i.e. 1, 2a, 2b, 3, The following 93

-299 Agualva a16163eira 98 , 20 S 235 095 95- s 719

sa 7 rt. 60 0500 50 u 40 SM3 e 3 1 3 2 3.6n, ..:. 2b —2 t 10 y;; 2T '3 5 —3 2 1 -5 -4 -3 -2 -1 0 1 2 3 0 -5 50.5 6 5se Praia 849, 850,851 '

7—ss: —1•31"1.1. —sso 2a 4, — 849 CI.'

•

851

P.Judeu S.Mateus Biscoitos S131 72,74-6,111-114 S199-200 95

2 4:4:7:74 2b 14.11dra 4 Z.1{.7 —199 1- 7.6m n-111 PIPE I ty 1142q 5- i *7113-1- r %112

-5 6 50 - o 50 -5

Fig. 3.5 Grain size data on the Lajes Ignimbrite. Cumulative curves plotted on arithmetic probability paper for samples at 9 localities. Each locality sketch shows from left-right: Thickness of unit in metres. If more than one flow unit present thickness of each is shown Sample positions. Designation of layers present. 99

99~------~ L inha res Quarry Castelinho W, side Castelhino, E side 5215 SI91 S227 95 95 95

50 5 50

521S 191 1- 3::: ~=~ l~~~~~o:.... 1.8 :~:i. -1 2b 1·" "-2 ---- 10r .• "'<1. I2b ~l ~~:::~_~~~ ~=g3 _~~.. r::::-; .;' _... 41 s;~·~!:-3 ~Pl ;I-~.:.:-2 2a 5 '1 ~-4 2<) .• 5 ~-3r

I ·-5 0 5(jJ -5 0 5, -5 0 5; Reguinho Achada Linhares Quarry 5217 5207 S215, 117-120 95 (reca I.cu lated for <2mm fraction) 84

SO SO 50

S207 16 1~-1 .1.( ,g':" 2 2b 2.2m~;::t~~ 2 2a '(;':1 5 !~=~~=5

-5 0 5qS -5 0 5qS -5 0 5tp

Fig. ,.6 Grain size data on the Angra Ignimbrite. As Fig. 5. See also Fig. 2.13, Vale do Linhares ~. Note also curves for samples recalculated to 2mm. Layers are indicated on the curves. 1 (1 0 cumulative curves (Figs. 3.5 and 6), and accompanying sketch sections, characterise the layers present on Terceira. Other ignimbrites examined by the author in ao Niguel and Central Italy show a great similarity of layering and sorting characteristics.

656 and Md0 values plotted on an Inman Diagram show the range obtained within each layer (Fig. 3.7), and may be compared with similar diagrams that have been plotted by Masai (1961) and Walker (1971).

3.3 (i) a. Layer 2b This layer is described first because it const- itutes the main part - typically 8O - of the ignimbrite and is believed to be the least changed part, compared with the initial assemblage of fragmented magma and lithic debris erupted from the volcano.

Coarse grained material in varying proportions is accompanied by fine-grained pumice dust giving alunstratified, pumice-rich rock. In some samples up to 250 by weight is finer than 0.125mm (30) (Fig. 3.8). 2b is therefore poorly sorted with a 60 range of 2.6-6.0 (Fig. 3.7a). The high content of fines is seen on the cumulative curves as a low slope in the grades less than 0.25mm (2/). Pumice, in various states of vesiculation, is the dominant component of layer 2b and this is accompanied by smaller lithics (including some syenitic inclusions), anorthoclase crystals and much pumice shard dust. Some frequency curves show peaks corresponding to each of the major constituents, giving a polymodal distribution (bimodal at the most simple), (Fig. 3.8). The 60 range of 2b is large and depends on the amount of coarse material present; the top and bottom of 2b have the highest (y values, see below. Nd0 appears to be most commonly between Nd0 -• 1 to +1. 1 0 t

a. • 60 b. IN • 5 layer 2b layer 2a 0 • • a 0 Iryt NIin •o • - o -8,;) $• • • .0 • • 0o • • • 0

Mc10 -4 -2 0 2 3 layer 1 AA • - 3 0 • • A OA pipe + • A OA A • A+ •AA a A • + • •• • o A. A• 0.,

Fig. 3.7 Inman parameters, Median diameter (Md0) and sorting (60),for the Angra and Lajes Ignimbrites. Black symbols = Lajes Ignimbrite. Open symbols = Angra Ignimbrite. a) Layer 2b. b) Layer 2a. c) Layer 1, 3 and pipes. 10

A S227-1 S 193 B S212 -1 0

1-5 -5

Fig. 3.8 Frequency curves for samples of layer gb, from the ignimbrites on ceira (see Fig. 3.4 and 5 for sample localities). P = pumice . L = lithics C = crystals I 0 3

Within 2b are two other indistinct layers which are sometimes present. There is an upper zone of large pumice cleat concentration giving an overall reverse grading of pumice (2bP in Fig. 3.4 and Plate 3.3) and a lower zone of lithic concentration giving an overall normal grading of lithics (2bL). The presence of these two zones is thought to depend on the density contrast between the larger pumice, the larger lithics and the matrix of layer 2b (ie. the bulk of the pyroclastio flow). It is supposed that the ability of pumice to 'float' upwards depends on the presence of a fluidised matrix and this in turn depends on the gas/solid ratio of the pyroclastic flow which will affect properties such as viscosity. The viscosity may be variable within a flow, so that in some localities pumice and lithic concentration is more accentuated than in others.

In order to maintain such a concentration of pumice at the top and lithics at the base during flowage, there must be negligible turbulence in layer 2b. The apprent paradox of non-turbulence in a fluidised system will be enlarged upon later.

The high percentage of fine dust in layer 2b is usually maintained in both layers 2bP and 2bL, although some samples show a depletion in fines in the lithic-enriched zone (samples 5131.3, 2147.3, 2164.2, Fig. 3.8), compared to layer 2b above. This may be explained by a transfer of fines upward by elutriation through the flow starting at the base of layer 2b. The very top of layer 2b sometimes shows a depletion in fines, never to a greater depth than 10-15cm, due perhaps to a more vigorous loss of fines from this most active top zone (samples 2210.3, 2215.2, Pigs. 3.8 and 9). The layer is usually soilified, eroded or redeposited, making it impracticable to determine the original pumice' grain size distribution in 104

Whole sample .EL

layer • 2b •2a

pipe CA_ A pumice o 2b top

L P

Individual grades c 2,1,0.5mm

Fig. 3.9, Crystal concentration in the Terceira Ignimbrites.. Pumice (P), Crystal (C), Lithic (L), triangles in w recalcd.to 1000. a) - 0) whole sample triangles a) sm : Lajes Ignimbrite at sgo Matens. lq : Angra,'. Ignimbrite at Linhares Quarry. " " Grota do Vale. b)pj : Lajes Ignimbrite atre t oo.Snden. 1 : tr vn " Vila Nova. c)ca : Castelinho Ignimbrite at Angra harbour. d)- g) individual grades where crystals are most common,2, 1, 0.5mm, plotted as 14 recalculated to 100%. Symbols as a-c. Radial lines are lines of constant component ratios. 105 this layer.

The buoyancy of pumice depends on its density; larger pumice clasts are usually less dense than smaller (as shown by the densities of pumice of different grades measured in the laboratory). Pumice in the Lajes Ignimbrite has a density of 0.6-1.2 gm.cm.-3 depending on the size and degree of vesiculation (see Table 3.3), and lithic fragments have a density of 2.5-2.8 gm.cm. This indicates that the density of the moving pyroclastic flow is between these limits i.e. 1.2-2.5 gm. cm-3. As the density of non-welded samples of layer 2b is 1.3-1.6 gm.cm.-3, it appears that there is very little expansion in the moving flow from the rook in situ. The density of welded Lajes Ignimbrite is 1.5-1.8 gm. cm.-3, which is slightly higher than the non-welded sample, due to partial elimination of pore spaces during welding (Ross and Smith, 1961). In this case there may be'a slight deflation from the pyroclastio flow on cooling 'to form a welded zone.

Some samples from each layer have been separated into the 3 components down to a grain size of 0.125mm (3d) and the cumulative curves of each component have been plotted separately. Crystals are the best sorted of all the components in all the layers, (Fig. 3.8 and 10). Sort- ing of crystals varies little from layer to layer even though there may be only 20% in layer 2b and as much as 48% in layer 1. This similarity of cumulative curves of crystals in all layers is mainly dependent on their limited initial grain-size range (usually <4mm.). Lithics may be as well sorted as crystals but are usually more poorly sorted, while pumice always has the poorest sorting in all layers. All components are better sorted than the whole sample. Fig. 3.9 shows the variation of the threecomponents between the different layers in the Lajes and Angra Ignimbrites. Crystals, 106 -

Fig. 3.10 Cumulative curves of samples from a) Linhares Quarry, Angra Ignithrite and b) Sao Mateus Lajas Ignimbrite, separated into the components pumice (P), lithics (L) and crystals (C). On b) curves of layer (groundsurge) and pipe are shown. 107 dominantly anorthoclase, are concentrated in layer 2b compared to the crystal content of the juvenile material and the main component 'lost' is pumice. The content and grain size population of layer 2b may be greatly different from the initial assemblage due to this loss, and other layers may be even more different.

The crystal content of whole samples and the individual sieve classes where crystals are most plentiful (after Walker, 1972), shows a small variation within layer 2b, (Fig. 3.9a and e). The individual size classes show crystal concentration more clearly but the whole sample trii. angles give a more realistic indication of the kind of component concentration that may be seen in the field. There is a moderately consistent proportion of each component with a slight enrichment in pumice towards the top and in lithics towards the base of layer Oa, showing graphically the concentrations seen in the field.

From crystal concentration studies it may be inferred that layer 2b has lost a large amount of pumice compared with the initial assemblage ejected from the vent. The data indicates that more than a of the initial volume of erupted material has been 'lost' from layer 2b (Fig. 3.9a and d) in the whole samples, but in the individual size classes where crystals are more plentiful, even greater losses are shown. This indicates that the maximum losses are in the fine grades i.e. pumice dust. It is likely that a large amount of dust is lost from the top of the pyro- elastic flow into vast billowing clouds above perhaps similar to those seen above,nudes ardentes (Lacroix, 1904; Taylor, 1958). The majority of the dust may be elutriated out of the flow by escaping gas and dispersed over a large area (see later , layer 3). The crystal concentration in layer 2b may be the result of 2 stages of dust loss; 1) from the 1 0 9

TABLE 3.3

Densities of pumice from the Lajes Ignimbrite.

Grain size diameter Mean density(gm.cm-3) 8mm 0.73 4mm 0.88 2mm 0.93 imm 1.03 0.5mm 1.04

Juvenile material Mean • from different density (gm.cm -3) layers Pumice. Top of layer 2b. (non-welded) 1.0 - 1.25 Dense pumice from incipiently welded layer 2b 1.4. - 1.5 Glassy pumice, base of layer 2b. 2.5 - 1.7

Fiamme 1.9 - 2.1.

Ground Surge- bed pumice 0.6 - 0.7

Density includes 50% allowance for void-- space correction.

Plate 3.4 Basal layer, 2a, in non-welded Angra Ignimbrite, below the Castelinho at Angra Harbour. Note the increasing fineness towards base of layer 2a. Scale shown by rule extended to 10cm. 2b, bulk of pyroclastic flow. T: transition zone. 2a, layer 2a.

5.5 Welded basalt layer, 2a, about 200m to the east of the locality shown in Plate 3.4. Castelinho on left. Below 2a is non- welded layer 1, mostly deposited between the scoriacious 44plo top to the underlying basalt flow. Above 2a is non-welded layer 2b showing coarse, poorly sorted nature with some pumice concentration at the top. eruption column in the initial explosion, 2) by the elutriation of fines from the pyroclastic flow.

3.3

(i) b. Layer 2a The basal part of the Lajes and Angra Ignimbrites show different characteristics from layer 2b. This is recognised in the field as a finer-grained layer, called the basal layer (Walker, 1971) or layer 2a (Plate 3.4). Usually the boundary of 2a with 2b is sharp, for example occupying 15-20cm. within a flow unit 10m. thick. In this transition zone the lithic content and average size of particles drops drasti- cally and there is a relative increase in pumice dust, perhaps from grinding down of pumice fragments in a high-shear environment. Layer 2a may also become increasingly finer towards its base (Fig. 3.8; S217, S191,

849-50) until it becomes a 'powder' of pumice and crystal fragments.

A basal layer occurs at the base of almost every flow unit seen by the author in Terceira and elsewhere. The presence of 2a uniquely defines a flow unit of ignimbrite but the clarity of any such layering may be reduced by welding zones crossing flow unit boundaries. The welded zone at the base of the Lajes Ignimbrite usually includes part of the basal layer. At one locality the Angra Ignimbrite also has a welded layer 2a

(flate 3.5).

Welding usually declines from dense in layer 2a to incipient 1m. or so up into layer 2b, and it tends to die out before the base of layer 2a unless there is a thick ground surge bed underneath, as at Sao Mateus.

At this locality welding appears to be most dense in the basal layer (Plate

3.6) and this is also seen in north coast exposures between Caldeira das

Lajes and Vila Nova. At these localities the basal layer contains L.

Plate 3.6 Welded basal layer at Sao Mateus, showing incipiently welded lithic concentration zone at base oflayer 2 and underlying layer 1, mostly eroded away from the basalt scoria (B) on to which it was deposited. Note the large crack in layer 2a from the intersection of a sharp piece of scoria. This crack dies out above the photograph where layer 2b is non-welded. Many of the lithics in 2bL are basalt from the flow seen below. a.

vector

Fig.3.11 a)Schematic proposed section through a pyroclastic flow. Zone 2b approximates to a Bingham fluid. Zone 2a approximates to a laminar shear zone (A-B). b)The Magnus Effect on a particle in zone 2a - the basal layer. Dotted line is net movement of particle towards higher velocity regime. 1 1 3

Ifiammel (Zavaritsky, 1947), elongate 3enses of non- or poorly vesiculated glass. They are usually the largest fragments found in layer 2a and they only occur where the igmimbrite is densely welded. Their presence per-

sists into laya.c P.b as far as dense welding continues (see laterfsection 3.7).

As the basal layer (2a) is always devoid of the large clasts found in layer 2b it is better sorted than 2b (Fig. 3.7b.; 60 is 1.0 to 3.0). Layer 2a has a smaller median diameter than layer 2b above. When the finer-grained parts of the samples are recalculated to 104 their cumulative curves are nearly identical (Fig. 3.6) showing that 2a and 2b differ only because coarse fragments are absent from 2a. Layer 2a is thought to represent the layer on which the pyroolastio flow moves. Johnson (1970) has discussed the nature of flow in debris flows, which are high density fluids, as are pyroclastic flows. It is suggested that such flows behave as a pseudoplastio or Bingham substances so that there is a zone of laminar shear at the base., In this high shear environment in a pyroclastic flow pumice may be ground down, and in the ignimbrite this zone is seen as layer 2a (Fig. 3.11a).

There may be an alternative, or even complementary, explanation of the fine-grained nature of 2a by the ejection of lithios and, to a lesser extent, large pumice from layer 2a. This may be due to the Magnus Effect (Goldstein, 1965) (Fig. 3.11b); it may even act upon secondary lithios picked up from the ground that the pyroclastic flow passed over, explain- concentrations of basalt scoria at the base of layer 2b (Plate 3.6); Con- versely lithic fragments are stopped from 'sinking' into layer 2a either by material moving upwards out of layer 2a or repulsion of fragmento by 114 the high energy regime underneath. It is possible that fiamme have escaped ejection from the basal layer by their plasticity while in the moving pyroclastic flow (see later discussion). The Magnus Effect will act on rigid bodies but if the fragment is plastic it will deform in the direction of shear i.e. parallel to the margins of the basal layer. Fiamme are in fact, aligned with their long axes parallel to these margins.

Fig. 3.9 (e and f) shows that compared with 2b, 2a shows a crystal concentration. This is best seen on the plots of 2, 1 and 0.5mm, size classes in which crystals are most abundant. Here the crystal/lithic ratios change little from 2b to 2a, indicating an enrichment mainly by pumice loss. Whole samples of 2a show a orystal concentration also because of the absence of large fragments and the corresponding greater contribution of•the finer classes. There is sometimes an increase in lithics over the proportion in layer 2b in the whole samples (Fig. 3.9b). Here the lithic population, which has a higher median diameter than crystals (Fig. 3.10) and therefore influences the whole sample plots more, may be derived partly from a few large olasts picked up by the pyroclastic flow.

It is worth noting here that, probably a low proportion of lithics are lost from the moving flow and soma/gilled from below. Therefore the content of lithics in the ignimbrite may be more constant than either of the other, components. The ratio of pumice to crystals, neglecting the effect of small variations in lithic content, may be obtained by pro- jecting a line from the lithic apex through the point concerned to the pumice/orystal join. (Fig. 3.9 a-c) in s

Plate 3.7 Basal layer (2a) in sub-vertical boundary between 2 ignimbrites, near Pitigliano, Central Italy.

Plate 3.8 Basal, non-welded,part of welded ignimbrite near Piansano, Bolsena district, Central Italy. Underlying soil is dark brown; layers present are indicated.

it / I F Basal layers may persist at the edge of ignimbrite-filled valleys, and in these cases'it is apparent that a laminar shear zone exists even on steep slopes - this is seen in Terceira, near Vila Nova, but a better example is from an ignimbrite near Pitigliano, Central Italy (Plate 3.7).

3.3 (i) c. Layer ; This layer is present under both the Lajes and Angra Ignimbrite and has a wider distribution than the ignimbrite (layers 2a and 2b). The boundary with layer 2a is sharp and the form of this layer is often lenticular. It may exist alone beyond the limits of the ignimbrite and is not always present at the base of the ignimbrite, especially where the pyroclastic flow travelled over 'pinched out' parts of the layer or over ground not covered by it. At Sao Mateus the layer is'up to 80cm. thick but thins to less than 20cm. over a distance of approximately 150m. Layer I also mantles the topography around areas of ignimbrite outcrop (Fig. 2.1).

Layer I varies greatly in grain-size and becomes thicker towards the source (Pico Alto Caldera) where it is seldom overlain by ignimbrite. At the coast it is usually 10-20cm. thick and intermittent in extent due to the lenticular bed-form. Although the size of exposures is not usually sufficient to observe the 'pinch and swell' structure over tens of metres, in two localities (Fig. 3.12) the bed can be traced from a thin pumice bed to a thicker crystal/lithio bed. Here the denser and larger fragments are obviously concentrated in the thicker parts of layer 1. WeWng of layer I is not seen on Terceira unless there is incipient welding imparted from a welded basal layer above (3.12b). •

1. ~[

10m

Fig. ,.12 Field sketches of groundsurge beds from the Lajes Ignimbrite. 1. North of Porto Martins. Pinch and swell bed form is not shown because section is approximately normal to the direction of flow. Note pumice is concentrated in the depressions •. 2. Igreja Velha, Sao Mateus. Pinch and swell bed form shown. Part of surge bed is welded where it is thickest. Note pumice uSually concentrated at base of l~erl. 1 1 Laminated bedding, concentration of individual components in sub- layers (Fig. 3.5 ; S163)and imbricated fragments, especially piety crystals, axe common within layer I. Sometimes plane-parallel bedding is developed, though this may be just an effect of insufficient exposure. The internal structure of layer I is very varied and the best diagnostic features of the layer are its bed-form and crystalilithic concentration. Layer I contains basaltic fragments 'picked up' from the ground. These may be coarser than the initial lithic content of the groundsurge.

The 'pinch and swell' bed form is more typical of sedimentary environments such as river or aeolian deposits and,with cross-beddingthas been described from base surge deposits of basaltic volcanoes (for example, Waters and Fisher, 1971, Fisher and Waters, 1970). Indistinct cross- -bedding is common in layer I on Terceira and this layer is attributed to ground surges preceding the pyroclastic flow (Sparks and Walker, 1973).

Taylor (1958) has described beds which may well be due to ground surges (the 'ash hurricane' of Taylor) deposited nearer to the source, and in a wider apron, than the associated pyroclastic flow. Terceira is such a small island that ground surge beds are found all the way to the coast.

The sorting of layer I is variable (N 3.0 to 1.0; Fig. 70), though it is often as good as for air-fall deposits (6,4 1.0 to 1.5). The ground surge deposits are coarsest and most poorly sorted near the source; they become finer and better sorted farther away. Layer I has little fine material;.most is presumed to have been removed by elutriation within the gas blast of the ground surge. The pumice in layer I is mainly very vesicular but there is also less vesicular, denser juvenile material. Beyond 3km. from Pico Alto the sorting of layer I appears to be consistent C 0 -c

E D

-4 2

Pig. 3.13 Cumulative curves of ground surge beds (layer 1) of Lajes Ignimbrite. A coarse surge bed near AguaIva. B coarse surge bed at Quatro Ribeiras. Other curves are fine grained surge beds from Tgo Mateus and near Lajes. 12i? (60 2.0 to 1.5) in many localities. Layer I is always better sorted than the overlying layer 2b but thick ground surge deposits at Quatro

Ribeiras and Agualva are quite poorly sortedIthough they show very similar grain size characteristics (Fig. 3.13).

The lack of fine material is shown by the steep slope at the finer- grade end of the cumulative curves (Figs. 3.5 and 6). The cumulative curves bear no relation to those of layers 2a and 2b and'usually cross, them in the upper part.

Unusually, coarse-grained pumice layers are found at the base of layer I where it overlies scoriacous, as basalt flows as at sao Mateus (Plate 3.6 and Fig.5, Sample 112). At this locality the base is well sorted and can be traced laterally into a crystal and lithic-rich bed. The pumice is rounded, which is typical of the abraded clasts found in layer 1, and may have become trapped between the rough blocks of the basalt flow surface while finer dust and crystals were winnowed out by turbulent gas.

Crystal concentration is varied in layer I. Nearly everywhere it is greater than in layers 2b and 2a (Fig. 3.9), except where it is particularly rich in lithics, as in the Angra Ignimbrite at Linhares Quarry (Fig. 3.9a and d). Here very little concentration of crystals is shown until the 1 and 0.5mm. plots are considered, because above these sizes the components are dominated by lithics.

The thin crystal and lithio-rich layer at the top of layer 2b, which has ground surge-like sorting characteristics has a similar crystal/lithic ratio to layer 2b (Fig. 3.9a) indicating a concentration of dense components by pumice loss alone. Layer I has a variety of orystal/lithic ratios compared to layer 2b, indicating that the ground surge and the pyroclastic 12! flow have different initial populations and that their origins are probably different. The crystal concentration at the top of layer 2b is not a common feature of other ignimbrites, as would be suggested if it is due to an extra loss of pumice dust. However it may have been eroded from many ignimbrites, yet it should exist in multi-flow unit ignimbrites.

Some of the extra crop of crystals in layer I may be due to breakage of pumice and wearing down by attrition in the high energy environment of the ground surge, with a corresponding release of crystals. Any fine pumice dust created would be instantly winnowed out from the denser fragments being transported.

The ground surge must be a hot, turbulent, gaseous flow from which particles are deposited in much the same way as in aeolian or aqueous deposition. Layer I also contains most of the carbonised remains of plants found on Terceira. Only the Lajes and Angra Ignimbrites contain carbon which is preserved well enough for sampling and C14 dates have been presented earlier (section 2). In places extra concentrations of crystals are found around carbonised twigs, indicating that crystals were 'caught' in the vegetation as the horizontal blast swept past. Thin ignimbrite and ground surge deposits appear to cover a wider area than the ignimbrite and are found on more elevated ground. Ground surge beds have not been recognised between flow units in the younger ignimbrites on Terceira but they occur in this position in the Vila Nova Ignimbrite.

Where the ignimbrite gradually thins against a topographic feature the top of layer 2b can be traced uphill until a crystal enriched bed is left, which is thought to be a ground surge bed (layer I) or is indistinguishable from it. In this way thin ignimbrites may be gradational 1 2 with ground surge deposits and a point is reached where the two become indistinguishable. In fact only where a pyroclastic flow is thick can it transport a dense enough load to produce ignimbrite with a layer 2b present. 3.3. d. Layer 3 There are remnants of what is interpreted as an air-fall deposit above the Lajes and Angra Ignimbrites over a substantial area of the island (Fig. 3.1). It is very fine-grained and well sorted (Fig. 3.7c), and shows mantle-bedding. The layer has been widely preserved possibly owing to the wet climate of the Azores, but it is mostly soilified and unsuitable for granulometric analysis. A few samples have been sieved, however, (Fig. 3.5, 235-1, S210-1) and these show very steep cumulative curves. Besides pumice dust, the layer may contain small, platy anorthoolase crystals.

Over large areas of Terceira the lateral equivalent of the Lajes and Angra Ignimbrites is a thin layer, rarely more than 50cm. thick, composed of the ground surge deposit capped by layer 3. Together these appear as a crystal-rich layer (Self, 1971) with a soil at the top, which is a velum. able stratigraphic marker horizon. In condensed sequences of pre-Lajes air-fall deposits, in the SE of the island, where no ignimbrite occurs, up to 5 crystal-rich layers can be recognised which may be the lateral equiv- alents of earlier ignimbrites, possibly including some of those exposed on the north and south coasts. Layer 3 is interpreted as resulting from dust falling from the dense clouds above the eruption column and the pyroclastio flow.

One notable difference between the ignimbrites of Terceira and Central Italy is that no coarse pumice-fall deposit is found underlying 123 the former, (Sparks, Self and Walker, 1973). Such a deposit is found in Italy but none of the Terceira ignimbrites appear to have been pre-. ceded by a plinian phase. The Italian ignimbrites also have a fine

grained layer 3,underlying layer I at the base, interpreted as air-fall dust from other pyroclastic flows in the same eruption besides those that are found at one particular locality (Plate 3.8).

3.3. (ii) .Areal Grain-Size Distribution The average maximum diameters of the 3 or 5 largest pumice rgollaPmeasured at each locality of the Lajes and Angra Ignimbrites have been plotted on a map (Fig. 3.14 and 15). The measurements have been restricted to layer 2b as this con- , tains the largest fragments, or to the middle of the ignimbrite where it is thin and layering is absent. The maximum-sized pumice °lasts are found at the top of layer 2b and the lithics at the base, but as the pyroclastic flow is thought to move as a dense fluid there should be little lateral difference in the final resting place of the top and base of layer 2b. Some of the largest lithics found are in layer I, the not ground surge bed, and these are/included on Fig. 3.15.

Kuno at al. (1964) plotted average grain-size data from an ignimbrite near Towada Caldera, Japan, and demonstrated a possible source. The maps of grain-size distribution for the Angra and Lajes Ignimbrites show a crudely concentric pattern around Pico Alto Caldera, already deduced to be the source from the morphology, stratigraphic position and outcrop pattern. 'However, the outcrops of ignimbrite are too dispersed to give a coherent pattern for either pumice or lithic clast sizes.

On Terceira no simple pattern emerges relating grain size to distance from source: pumice olasts, which tfloatl in the pyroclastic flow, are

124

Pumice , 0 . . AD° '1/4)- 4'• 11\ 6 .12 9 2i .1 72. . 7. . . ;.!5 _ ::4, 3 6 :287c. 5 ,v.__6 \\ j 80 3:: :. :1, : 2 \ \ / / :28 .55 57 5.5...),P4 \ \„...,/ mr'44 so '3030 . . \ ..71,',26 27 \ ' / "13 \ .18 '/ •8 ■ .16 / /P 1.--- 16.– I5— 1 1 / .9 /./- 'M •26 , 2,5* .14 ti X444 \ % t / .1 I/' . ? 10* • tz.. 1 . /I. .4 j •40 / SO *'••._** ...0' / –ft.– I I / ----? \ i \ / 10" -4....to . /I/ / ''• ii --...,...... S. // -36 4 )-. ‘1 -/ //• 2f //z . / // ///// .'6 •:35'- • .------/ / // /W 5/ / • .3 /,. Kt •. \ / - / II 1 it 3 . I/ 15.0 . / 1 1 8 I el,f1 11, 12 51 45 . .4 1 • V 4)O.. 30h2/ /9 30 51 )9 1 1 / / t 0 5km sz,//,,.)..f..2 4") / / 23 i 1 42 —.,II Lotn 1r),-•

Fig. 3.14 Map of the grain—size distribution of pumice clasts in the Lajes and Angra Ignimbrites, localities show average maximum diameter of the 3 or 5 largest pumice clasts in cm. Calderas as Fig. 1.2.

Lithics 7 4 —6Lri II 4 1 515 8 7 7737 7I29a9 18,.8 '13.6 11. A 7//5 75 11I \ 8 5 .18 '4'1" 7 .17 42 .5.5 "13 .15 .5 .7 .24 17. .30E1

.7 \ 6"•14 •2.0 .4 /• .8 ...... 3 ....„. .7 ...... --.. ..4, N■ N. • •4 .6 •2 5 .3 6. I / •4 4/

1•6 1 4)%; ta 3 .4 / Nat) 15 6 3 8 II I I 1 I • 5km Ln op

Fig. 3.15 Map showing the average maximum diameter of the 3 or 5 largest lithics (in cm.) in the Lajes and Angra ignimbrites. Calderas as in Fig. 1.2. nearly always larger at any given locality than lithics. The lithics show a tendency to increase in size towards the source and pumice °lasts appear to increase in size away from the source; the largest pumices are found in the coastal exposures.

The maximum grain-sizes within the Lajes and Angra Ignimbrites reflect the routes of the pyroclastic flows from source to sea. Maximum pumice and lithic thighs' are along the centre of flow-routes. Sparks (pers. comm.), reports that, in the extensive Bolsena ignimbrites of Central Italy, the maximum pumice sizes are found in parts of the ignimbrite outcrop corresponding to the main flow-routes. This appears to be, so in Terceira, where, in the largest single area of outcrop, the Lajes Plain, the main flow-route appears to lie to the western and and curves eastward, parallel to the sea. 'Pumice sizes decrease irregularly on either side of the maxima along the flow-routes. This is analogous to the maximum load of a river being transported in the centre part where there is maximum velocity. The areas of maximum pumice-size correspond to the areas of the thickest outcrops of ignimbrite. The Angra Ignimbrite appears to have formed from pyroclastic flows (there are a maximum of 3 flow units) in one valley only. Maximum grain-sizes are found along the centre of the valley, where the ignimbrite is thickest.

On Terceira every ignimbrite appears to have entered the sea, as large thicknesses and coarse grain-sizes are found on the coast. A large amount of pyroclastic material may therefore be missing from the island. For the Lajas Ignimbrite, perhaps only half of the material transported as disaussed above, by pyroclastic flows during this eruptive sequence is found on land and" the pumice 'loss' by elutriation during the eruption may be as much as 75g. Considering this, the portion rmwining on land may well be a, very 411111k 4.64.010.1.11mmew

■ ••

, ;1„ 8 Lie

Plate 3,9. The Porto do Vila Nova exposure of the Vila Nova Ignimbrite, showing the 2 lower important flow units (1 and 2). Within flow unit 2 there are 3 smaller flow units. The basal layer of the upper, largest unit (3) is also seen. Each flow unit has 111 good pumice concentration at the top.

PLate 3.10. Plane parallel bedding in the ground surge bed at the base of the Fanal Ignimbrite, Baia da Fanal. S is underlying soil. Dashed lines mar ammo's( rna.i:f?_1 rte' n 1 n1r ay, 1 small percentage of the initial volume of material erupted from Pico Alto Volcano.

3.4 The Older Ignimbrites of Terceira 3.4 (i) The Fanal and Vila Nova Ignimbrites. These ignimbrites, which may be parts of the same eruptive sequence (Tables 3.1 and 3.2), are

light grey, non-welded, pumice-rich rocks with a maximum thickness of 20m. In the two main exposures at Baia do Fanal and Porto do Vila Nova

they have many flow'units; at the second locality there are three main flow units 3.8m, 5.2m and 18.0m. thick (Plate 3.9). The top flow unit

is composed of as many as ten small flow units, some only 50cm. thick.

All flow units show very strong. pumice concentration at the top of layer

2b. At the base, a parallel bedded, ground surge deposit (layer I), of

maximum thickness 70-80cm. is found in both the Fanal and Vila Nova expo-

sures (Plate 3.10).

All the small flow units (Plate 3.11) show layers 2a and 2b. One

•unit (Fig. 3.16; Sample 5234) has a crystal and lithic concentration zone

intermittently at the base, which has been interpreted as layer I. Each

small flow unit is formed from a separate pyroclastic flow, with -flee same

mechanisms at work as in a large pyroclastic flow. The pumice concentr-

ation in the small flow units is very marked. The pumice fragments in a

section (75cm x 80cm.) of the flow unit shown in Plate 3.11 were measured

in situ in the directions normal to the flow unit boundaries (V) and

parallel to them (H). The aspect ratio of the pumice fragments was found

to be H/V = 1.22 (from measurements of 28 clasts at different levels).

Only a slight alignment of long axes of pumice °lasts in the unit is thus 99 Porto do Vila Nova P do V.N. 5212 S 234

50 50

o 0 99 Angra harbour S226

0 5 - scb-

Fig. 3.16 Grain size data on the Fanal and. Vila Nova Ignimbrites. Cumulative curves of greater than size stated plotted on arithmetic probability paper. Details as Fig. 3.5. 130

Plate 3.11 Small flow unit, 79cm thick, within the upper flow unit of the Vila Nova Ignimbrite at Porto do Vila Nova. Layers present are 2b, with extreme pumice concentration, 2a (basal layer) and a very thin layer 1. Plate 3.12. Cliff section east of Angra do Heroismo showing Angra Ignimbrite (1), Fanal Ignimbrite (2), Castelinho Ignimbrite (3), and associated debris flow (3a). 3 flow units within the Fanal Ignimbrite are shown (i)-(iii). The dark layer near the base of the Angra Ignimbrite-is an incipiently welded zone. The dark bed above the Anrga Ignimbrite is the Monte Brasil tuff. Section is 22m high. evident. This may be partly due to the dominantly equidimensional shape of the pumice clasts and partly to the dense medium in which the clasts were contained.

Three thick flow units of the Fanal Ignimbrite are exposed in the cliffs east of Angra (Plate 3.12). Their appearance is very similar to those at Vila Nova but they are largely inaccessible in the high cliffs.

Samples have been collected from Baia de Fanal, Angra Harbour and Vila Nova (Fig. 3.16); Md/3 and 60 parameters fall within the same range as those of the youngerignimbrites (Fig. 3.17), except for some samples from the top of layer 2b which have very high Ed0 values, owing to the concentration of large pumice clasts in them.

3.4 (ii) The'Caldeira and Castelinho Ignimbrites. Two prominent ignimbrites, which may also be part of the same eruptive sequence, (Tables 3.1 and 3.2), are found on the north coast, from near the village of Caldeira to east of Vila Nova, and on the south coast near the Castelinho do Angra

(Fig. 3.1). They are both grey, essentially non-welded ignimbrites, up ' to 25m. thick. In places the Caldeira Ignimbrite is incipiently welded, perhaps due to vapour phase crystallation, (Fenner, 1948; Smith, 1960a).

In general there are two flow units, both with a higher lithic content than the Lajes Ignimbrite, and reverse grading of pumice in layer 2b is not well developed.

The Caldeira Ignimbrite has a well-developed, cross-bedded layer I underlying it (Plates 3.13 and 14), with both coarse and fine-grained pumice beds in it. The wavelength of the 'pinch and swell' bed-form is approximately 15m. and the bed reaches a maximum thickness of 1.7m. This a. c50 • layer 2b. b. 2a. 5- • 5-

47. 4

• • • ..• 3- • 3 •

• 2- 2- •

1 1-

3d0M -5 -4 o 2 -4 0 0 C. 0 debris flow * avalanche vulcanian

Md0

Fig. 3.1 Inman parameters for the older ignimbrites on Terceira. Symbols as Fig. 3.7 except those shown. a layer 2b b layer 2a, basal layer c layer 1, ground surge layer Dotted fields are those of the Lajes and Angra Ignimbrites for these layers. 1 3 g

Plate 3.13 Ground surge bed, layer 1, underlying the Caldeira Ignimbrite on the north coast. Note the coarser-grained lenses, containing mainly pumice. Above is incipiently welded layer 2b, with a very poorly developed layer 2a. •

Plate 3.14 Cross bedding and lens bedding in layer 1, about 40cm thick, underlying the Caldeira Ignimbrite on the north coast. Here there is a sharp boundary with non-welded layers 2a and 2b above. Underlying the layer is the top of a basalt lava flow (B). Scale by rule 12cm long. 136

•

Plate 3.15 Debris flow (mudflow) in the Caldeira Ignin1rite, here seen overlying layer 1. Two large rounded ciasts of ignimbrite are shown near the top of the bed. The extremely heterogeneous nature of the deposit compared with the homogeneous- looking ignimbrite flow unit above can be seen. In left hand corner one of the large blocks of vitrophyre can be seen where it has fallen from the deposit. 1 3 7 is overlain by a homogenous flow unit, 4m. thick, containing a few

quartz syenite xenoliths, up to 24cm. in size, and rare amphibolite xenoliths. This basal flow unit has an irregular, eroded top where

it is overlain by a thick breccia of very poorly sorted ignimbrite material, extremely rich in lithic blocks. This breccia is interpreted as

a mudflow (Plate 3.15). Coating of crystal and lithic fragments by pumice dust occurs and may be indicative of an aqueous medium. However,

accretionary lapilli do not occur. The breccia may have moved as a

dense but fluid debris flow with a low water content (Fisher, 1971).

This debris flow contains huge blocks, up to 10m. diameter and far

larger than any found in the Ignimbrite. Rounded clasts of incipiently

welded ignimbrite, very similar to the lower flow unit, are set in a

dense heterogeneous matrix. The flow is crudely bedded, with imbrication

of the large clasts oblique to the base. Some of thelargest blocks are a

vitrophyric, crystal-rich welded tuff that is not found exposed in situ

anywhere on the island. It was probably derived from the near the source

of the ignimbrite. Tree casts, but without carbon, indicate that the flow may not have been very hot.

Above the debris-flow is a second flow unit of non-welded ignimbrite

with a well developed basal layer (2a) and a strong concentration of lith-

ics at the base of layer 2b. A similar flow unit is found above a smaller debris-flow in the Castelinho Ignimbrite in Angra Harbour, and in the

cliffs to the east (Plate 3.12). Here there are fine examples of fossil fWmarole 'pipes' in the upper flow unit.

Cumulative curves of samples from these 2 ignimbrites (Fig. 3.18) show

that there is less pumice concentration and a greater concentration of

lithics, and that the debris-flow is exceptionally poorly sorted (Pig. 3.17). 13

99 a. C. . 5211 s193,4 . 95

I :37. tp--193 .1. , • 4-194 " isa.:: 1....de 6...... 4 5 •

1 5 99 0 5 - 5 0 d. e. Povoacao f. . 5197 536,7

GP760 1— ib 4° • 1 — 2 b ....,••••-, s197 3— a. Pl'---36 — 1 et.°% ' 4,4.* —37 t -a— cy- 4 , 5 so3 :0 515 - 0

Fig. 3.18 Grain size data of old Terceira ignimbrites and other rocks. Caldeira Ignimbrite, Porto do Vila Nova. Caldeira Ignimbrite, E. of Vila Nova. Castelinho Ignimbrite, Angra Harbour. Porto do Pipas Ignimbrite, Angra Harbour. Povoacao Ignimbrite, S. Miguel (data from G.P.L. Walker). Rock avalanche deposit, nr. Angra, Terceira. i3 In the Caldeira Ignimbrite, where the upper flow unit overlies the mudflow bed, another kind of well-sorted 'pipet is seen intruding into the debris-flow from the base of the unit above (Fig. 3.18; S181). This may be a kind of load cast or flute cast produced. by settling of a dry, dense pyroclastic flow into the aqueous top of the debris-flow below.

Crystal concentration in the Castelinho Ignimbrite (Fig. 3.90 and f) is very well developed in layer I and in layer 2a.

3.4 (iii)The Porto do Pipas Ignimbrite. This poorly sorted and rather rotted, pumice-rich ignimbrite is found only in the Angra area and is best exposed at Porto do Pipas (in the Angra Harbour seotion). It is the oldest ignimbrite exposed along the south coast. At Baia da Fanal and Angra Harbour there are two flow units of grey, incipiently welded igniMbrite, each with a maximum thickness of 4m. Layers 2a and 2b can be distinguished (Fig. 3.18) but layer 2a is poorly developed in the lower flow unit at Angra Harbour.

3.4 (iv)ilataiwIgkrItm. A basic ignimbrite, dark in colour and SUB... :pected to be mugearite or benmoreite in composition, is exposed in the Vale de Linhares Quarry. Here it fills a valley which was re-eroded before being filled by the Angra Ignimbrite. The exact stratigraphic position of this igniMbrite is in doubt but it may correlate with a small remnant of dark coloured ignimbrite exposed at Angra Harbour (Fig. 3.3). The ignimbrite in the Quarry contains very dark scoria, with plagioclase crystals instead, of anorthoolase. The pumice has a different shape, being 140

cuphmek,vesic.ula.r mup.o-r■te lava. f 'ow -94 11 welded bo.so.t. to,ser Fe 1.5 A. upper flaw unit a . . ... •

r • 4.1 V7 3.6 - i-;1; ,• 1-.1.fot I,' welded none 2b1

Fig. 3.1, Section at Vale do Linhares Quarry showing 2 flow units of a basic ignimbrite, believed to be of mugearitic/benmoreitic composition. Vertical shading indicates welded zones. Layers are shown on right. A are outlines of pumice clasts from the basic ignimbrite. B are outlines of pumice clasts from the salic Angra Ignimbrite. 141 much more elongate and twisted than the more rounded 'caluiflower' shaped pumice in the Lajes and Angra Ignimbrites (Fig. 3.19).

Two flow units of total thickness 5.1m. are present; the lower has a welded basal layer and the upper has been eroded down to the welded zone but the welding is not continuous between the two. This ignimbrite is not a simple cooling unit and the upper flow unit may have been emplaced a considerable time after the lower one. Extreme pumice concentration occurs at the top of the lower flow unit. Aphyric basalt scoria clasts are abundant and mad have been incorporated into the flow from the ground or were part of the initial assemblage. An outcrop 1.51am. inland from the Quarry shows that the pyroclastic flows followed a similar route to those of the Angra Ignimbrite (An analysis of the pumice from this ignimbrite is being completed).

On the north coast two ignimbrites locally occur at the base of the cliffs. One, found at Caldeira, has been designated (from the stratigraphic sequence of the north coast, Fig. 3.2). It is a small remnant of greyr densely welded ignimbrite containing ample and =orthoclase crystals in a well-developed eutaxitic groundmass. The other, designated izl ois only found in the cliffs near Vila Nova. It underlies the Caldeira Ignimbrite, which it resembles, being very coarse-grained and pumice-rich. Reverse grading of clasts is seen in an exposure of the top part of layer 2b at Porto do Vila Nova. Further to the west the base is exposed but the locality is inaccessible.

3.4 (v) A possible method for the recognition of ignimbrites. The ignimbrites on Terceira have an assemblage of the 3 components, pumice, 14?

Fig. 3.20 Triangular plot of components (in wt% expressed as 100) from layer 2b of different ignimbrites. Lajes and Angra Ignimbrite, Terceira K Ignimbrite K, Bolsena, Italy A" A, Bolsena, Italy data from S. Sparks ✓Vila Nova Ignimbrite, Terceira C Caldeira Ignimbrite, Terceira P Povoacao Ignimbrite, S. Miguel, data from G.P.L. Walker. 143 crystals and lithics, which, in the middle part of the ignimbrite, is the nearest to the initial assemblage of pyroclastic material erupted from the volcano. In all the ignimbrites, except the basic onelthere is a similar mineralogy. A distinction is shown between the ratio of the components in layer 2b in the older ignimbrites from the ratio shown by the Lajes and Angra Ignimbrites. The latter two have very similar contents of crystal, lithics and pumice which may indicate that they have the same source and were even perhaps derived from the same magma body.

Most ignimbrites, whether on Terceira or elsewhere, have similar sorting characteristics and in a problematic, area grain size parameters may not help to distinguish between ignimbrites. The component ratio may well be more diagnostic but it is a quantity that is not easy to estimate in the field. If samples of layer 2b are separated into the 3 components and plotted on a component triangle (Pig. 3.20), it can be seen that ignimbrites from different areas plot in distinct fields. This method may prove useful in the recognition and correlation of ignimbrites in other areas, if there are no mineralogical differences between them. It must be stressed that only layer 2b would appear to be of use in such an exercise.

3.5 Fossil Fumarole Pipes

Well,sorted, sub-vertical, narrow zones in the ignimbrites, interpreted as fossil fumarele pipes, are found in a few localities on Terceira. In the Lajes Ignimbrite they occur near Caldeira, at SXo Mateus and at Porto Judeu. They are characterised by a lack of fine grained material and an apparent enrichment in crystals over layer 2b, through which the pipes pass. The pipes are usually 10-15cm. wide and

, 14

Plate 3.16 Pipe in welded ignimbrite near Savanna, Central Italy.

Plate 3.17 Pipe in non-welded Castelinho Ignimbrite at Angra Barbour. Rule extended to 20cm. 145 can be traced for distances up to 5m. in the Angra Ignimbrite at Linhares Quarry. Such pipes appear to be restricted to non-welded zones of the Lajes and Angra Ignimbrites but are known to occur in welded ignimbrite from other areas e.g. the Bolsena area, Italy (Plate 3.16).

The best examples of pipes on Terceira are exposed in the Castelinho Ignimbrite at Angra Harbour (Plate 3.17). Here thereare narrow pipes, 5-10cm. with well-defined boundaries and also wider pipes, 25-35cm. across, which have more branches and are generally more diffuse. The pipes are first recognisable just above layer 2a and they become indistinguishable below the top of layer 2b. Coarse Blasts of pumice and lithics loosely packed in a crystal-rich matrix typify the pipes.

Fine material has been removed from these pipes, and the cumulative curves of samples of pipe material (Fig. 3.5 and 6) show a sharp out- of at 0.25-0.125mm. grades (2 to 3 phi). Generally less than 5% of the pipes are of material finer than this size. The pipes are interpreted as fossil fumaroles of primary origin (Walker, 1972),and the removal of fines has been by a winnowing action of gas escaping through the ignimbrite when the pyroclastic flow cameto rest. Material smaller than 0.25-0.125mm. has a terminal velocity smaller than that of the escaping gas and is therefore removed. Where the pipes 'die out' the gas may have be- gun to permeate through the whole ignimbrite and not flow in one specific channel. The gas may be derived mainly from the lower, hotter zones of the ignimbrite and some transport of material byaases must occur before welding in the cases where pipes occur in welded zones. The Valley of 10,000 Smokes, Alaska (Griggs, 1922), was named after the many small fumaroles on the surface of this large non-welded ignimbrite and the features seen on Terceira are thought to be cross-sections of similar 146 fumaroles.

In layer 2b of the ignimbrite there is much pumice dust in the classes finer than 0.25mm. (Fig. 3.8). The median diameter of lithics in the middle of layer 2b is usually 2-4mm. and of crystals is 1-imm. (Fig. 10), and both these components are better sorted than pumice. Most of the fines removed from the pipes are, therefore, pumice and an enrichment in crystals and lithics is seen. This is especially well demonstrated by the pipes at Sgo Mateus and Porto Judeu, (Fig. 3.5 and 9). These pipes also show an increase in median diameter over layer 2b. In fact pipes, when present, have the highest Md0 value of any layer or zone within the ignimbrite, including the ground surge bed (layer 1) due to the complete absence of fines. An exception to this may be the very coarse pumice concentration layer at. the top of. 2b. The sorting parameter of pipes is similar to those of layer 1 (Fig. 3.7 and 17). Crystal concentration is fourfold over layer 2b in the Lajes Ignimbrite at Sgo Mateus, in thegrades where crystals are most common, (Fig. 3.9). In the whole sample (Fig. 3.9a), there is no change in the crystal/lithic ratio from 2b to pipe, demonstrating that pumice only is depleted. A similar relation of pipe to layer 2b is shown by the Porto Judeu samples (Fig. 3.9b).

The pipes sampled from the Castelinho Ignimbrite however, appear to be only slightly enriched in crystals. This may be due to an assemblage of smaller crystals in this ignimbrite, most of which were removed from the pipe. The pipe is less crystal-rich than layer 2b, on the whole sample plot, and only slightly enriched in crystals in the 1 and 0.5mm. grades (Fig. 3.9f). There is little lithic enrichment and the crystal/lithic ratio of the pipes is the same as in 2b. It appears that this is not a normal pipe, as most from the Lajes and other ignimbrites (Walker 1971), show crystal enrichment over layer 2b. 1 4 7 3.6 The Effect of Rising Topography on the Lajes Ignimbrite

Being a hilly island, Terceira presents an opportunity to study the effect of topography on the progress of the Lajes pyroclastic flows. The gradual lateral thinning of ignimbrite against rising ground has been described earlier but in a few localities the main pyroclastic flows met obstructions.

In an exposure west of Lajes village (Plate 3.14 non-welded ignimbrite rests on an old hillside. There are two flow units; the lower is 92cm. thick, fine-grained and has a small basal layer (2a) about 20cm. thick. The upper flow unit is 1.9m. thick and much coarser with a basal layer only 2cm. thick. Lithics are normally graded and so are the large pumice clasts; both are concentrated at the base of the unit where they show imbrication., This concentration of large pumice clasts at the base of layer 2b is not unique in Terceira and is seen near Vila Nova where the ignimbrite thins abruptly against a steep slope. Here there is an upper and a lower pumice concentration, with negligible development of a basal layer in a flow unit only 0.85m. thick. Adverse slope may caase deposition of large clasts to take place at these localities due to a sudden slowing of the pyroclastic flow. Large lithic clasts in the ground surge deposit (layer 1) near Agualva are also imbricated. The slope upon which the pyroclastic flow came to rest in these two exposures is approximately 30° to the horizontal.

Where the Lajes Ignimbrite abuts against the 70m. bigh Lajes Fault scarp, there is an appreciable thickening at the base of the scarp. On the Air Base, below the highest point of the scarp,the Ignimbrite totals 11-12m. thick, where on average it is less than 5m., in nearby loodities. 4-g

1 e)

Plate 3.18 Lajes Ignimbrite west of Lajes village, showing a normal grading of pumice in layer 2b of the upper flow unit. The steep slope on which the ignimbrite rests can be seen by the orange soil of the old hillside. Height of section 3.4m. I4° It is fine-grained, incipiently welded and very dark grey, with the sorting characteristics of layer 2a. Above, on the scarp, only air- fall layer 3 is exposed overlying an older surtseyan tuff. It appears that some parts of the pyroclastic flow came to an abrupt halt against the'fault scarp and a thicker ignimbrite was left, partly piled-up against the scarp. To the NW of the Air Base, at the lowest point on the fault scarp only 15m. above the Lajes Plain, welded ignimbrite is found overlying a ground surge bed (layer 1). Here the dense pyro- elastic flow was able to travel uphill on a small slope, whereas it was unable to mount the high fault scarp 1km. to the south.

r fl

10 cm

10cm Plate 3.19 Three forms of juvenile material from the Lajes and Angra Ignimbrites. A- Large, low density pumice clast from Angra Ignimbrite. B- Smaller pumice clasts from the Angra Ignimbrite, showing the generally equant form. C- Flame in the Lajes Ignimbrite at Sao Mateus.

k 7101.14PPMPIPIrr- 151

•

Plate 3.20 2 dense pumice clasts in incipiently welded Lajes Ignimbrite at Sao Mateus. One clast is flat and the other near spherical although they occur next to each other in the rock. 15 ? 3.7 Pumice in the Lajes and Angra Ignimbrites and a possible origin of

"fiamme" in the former

There is a considerable range of shape, density and vesicularity of juvenile material within these ignimbrites (Plate 3.19). Densities of pumice clasts are known to vary; 1) in clasts of similar size, and

2) in clasts of differing size; smaller pumice being more dense (Table

3.3). Aramaki (1957), found that there was a progressive increase in vesicularity throughout the 1785 eruption of Mt. Asama, which hai plinian, pyroclastic flow and lava effusion phases. The pumice in ignimbrite may well represent a stage between a plinian phase, in which the pumice is highly vesiculated, and a lava flow stage, commonly poorly vesiculated (Sparks, Self and Walker, 1973).

The larger pumice clasts in non-welded ignimbrites of Terceira are generally equant in form, with glassy centres in various states of vesiculation. Some of the largest are highly vesiculated internally, showing perhaps, that vesiculation continued after the pyroclastic flow stopped moving. In the Lajes Ignimbrite the most round . and vesiculated pumice

is found in the non-welded zone. In the incipiently welded zone, more dense clasts are found, often with near-spherical and flat shapes in jux- taposition (Plate 3.20). As seen earlier (in 3.2) the pyroclastic flow may not deflate very much after it comes to rest, so the elongate clasts in layer 2b may not be flattened by compaction but may have an original flat shape. In the densely welded zones pumice is absent, and fiamme are the largest fragments of juvenile material found in these zones.

The origins of flame so far suggested (Smith, 1969 Ross and Smith,

1961; McBirney, 1968; Gibson and Tazieff, 1967) seem to the author to be 153

2b Lin." cob centm-rion zone

Me6.4 f.loitentn8 pumice rortfo';,p umite. I. iititic „1.2: P 'Ectvdi;itl;64;li"ce sj c1Md vesic.wc,t .2.01

sorn'i Litk'Le.lost3 oseboc.- & lt•

f cte-i 0 pouviics... 4 r OF 't'4--1-S E'/V' 22: 1

tt, D -pk.3,41.1

oktlb cuid e. C..o-mme DevLseli -,i-c.-tcLect t y%r lotei_tea. sAAtt. eutix.",cfic.-t5f-xtu 0-t p 014 vesick.rUxtr fickmme h-te_-(-LO.ttefitylci U Flow -o ro_tio • Di FiEr..T to NI 'U

ecz=7 UI O •■=:* 43. 4:Cgitt a -cloal.treti,o)dc.tis...8. ‹.0 10.1.e+r non.- we-tclect &LS e, •4 ,roctvi oo,oad

Fig. 3.21 The •sao Mateus cliff section of the Lajes Ignimbrite, basal 1..0m; photograph approximately parealel -to flow direction. Pine grained., parts not shown on diagram,. / 5C inadequate to account for the fiamme in the Lajes Ignimbrite. Compaction plays a part in forming the resultant flat shape of these fiamme, but it may not be wholly responsible, as suggested by Ragan and Sheridan (1972) for the Bishops Tuff. It is thought that "the flattened pumice" origin of Ross and Smith (1961) is an unnecessary complication, and in fact these fragments were never vesiculated. Examination of thin sections of fiamme reveals few signs of any vesiculation. Moreover, there are no indications of laminar flowage of the ignimbrite to the degree described by Sohminke and Swanson (1967) but a similar, though less dramatic process may account for the fiamme in the Lajes Ignimbrite.

At Sao Mateusla locality typical of the occurrence of fiamme (Pig. 3.2l)„ the basal layer, Zh, is densely welded with an sataxitio texture developed. The rapid transition from equant, dense pumice °lasts in layer 2b to elongated fiaMme of the same material in layer 2a is striking. Layer 2a is thought to be a laminar shear zone (hence the similarity with the laminar flowage of Schminke and Swanson) and some fiamme here have ellipsoidal horizontal sections stretched in the direction of flow. The shape of the fiamme is thought to be due to shearing of plastic magma 'lumps' in this zone, oombined with compaction during welding.

The density increase from non-welded to densely welded ignimbrite amounts to approximately 25%, giving about 20% compaction, calculated from the S.G. of the rocks. The aspect ratio of pumice from the non-welded ignimbrite averages 1.2 and the mean flattening ratio of the fiamme in layer 2a averages 4.2. This is much more 'flattening' than the observed amount of compaction would produce and, perhaps, shearing plays the major part. Compaction may be due to a slight contraction, as discussed above, of the basal layer; evidence for this was stated earlier (Section 3.2) and is 1 5 also seen at Sgo Mateus (Plate 3.6). Here the rigid, welded layer 2a has "cracked" when overlying an 'ea' basalt flow and the crack has widened to lave a gaping fissure, which dies out in the incipiently welded layer 2b. Heat and pressure may inhibit vesiculation in the lower part of the pyroclastic flow, or alternatively, the 'lumps' may be of relatively gas-free magma. Their plasticity may be responsible for their not being forced from the basal layer by the Mvus effect (see 3.2).

Load compaction may not play a great part in this relatively thin ignimbrite; in soma localities fiamme occur in welded zones, where the ignimbrite is only 1 to 1.5m. thick.

3.8 An Eruptive Mechanism Model for the Ignimbrites of Terceira

Any model must explain the observed sequence of a ground surge followed,at a presumably short interval, by one or more pyroclastio flows, and lastly, an air-fall phase (Pig. 3.4). It is now generally acknow- ledged that in order to move the larger volumes of material seen in ,ignimbrites substantial distances, the process of fluidisation must be invoked (Reynolds, 1954). Fluidisation appears to play a large part in eruptive processes in the magma column and in the laterally moving pyroclastic flow. Therefore, a brief attempt to correlate fluidised systems with the observed features in the ignimbrites is included, and a suitable model to fit most of these features is proposed.

In the vent, gas streaming through the salic magma column causes a situation comparable to a vertical up-flow fluidised system (KUnii and Levenspiel, 1969). An up-flow, fluidised bed is one where the gas is 1 t) moving in the same direction as the particle it is transporting. In this system there may be a dense-phase fluidised zone (high proportion of solids to gas) nearest to the magma and a lean-phase fluidised zone (high proportion of gas to solids), extending perhaps a great distance above the vent as an eruption column. In this phase there is pneumatic transport upwards of particles by elutriation in the gaseous medium; fine grained material is carried highest. If the velocity of the upward gas blast is powerful enough to exceed the terminal velocity of almost all the particles produced by fragmentation of the magma, then material is carried to great heights i.e. a plinian-type eruption. Such a vertical transport phase may precede a pyroclastic flow, but has not done so on Terceira.

If the gas emission rate is not powerful enough to fulfill the above conditions, or is reduced after such a phase, conditions in the vent may be right for pyroclastic flow eruptions. Ylost fragmented material isnot clearing the vent and choking may occur (Sheridan, 1973), or gas pressures may build up, leading to a vertical gas blast, containing highly fragmented magma and lithios. The vertical blast may be accompanied by a base surge type horizontal gas-blast (Moore, 1967; Waters and Fisher, 1971). Pumice, crystals and lithics are transported laterally out of the lower dense phase of the eruption column. Several such surges may fol- low repeated explosions. The gas blast has an initial high velocity and speeds down the volcano flanks as a ground surge, with entrained particles which are deposited,,as the surge loses momentum, in a similar way to subaqueous deposition.

In the vent, gas may still be issuing from the magma and the dense - phase part approaches the top of the vent. From here there appear to be

• 0 e: o .0 • ."..-.• 42 • 0 • • 0.6 • ' „,: 00.. • a. .•. - .04.••-•••••-•-•—•", ' 0 0 ••., • • • .e ‹=. •-• • • • •°.-.°. O lean 6. • • • 6 phase

" ,7 1•‘,. /•77:t" slug "•'t€ . • ••• ...... ••••:••••• • ___-..,•;- ...._...... •0. 0'0 , . * 6 -*. .0 • 'gslug 0 •. 0, ..0. •.. •0 • 6.----;-> • • .. Is. b . 0

dense phase

i00% gas solid1°"° t t

Fig. 3.22 a schematic diagram of fluidised phases in a vertical eruption column. b normal-flow lair slide' mechanism. c counter-flow 'air slide' mechanism. Open areas - movement of transported material. Closed arrows- movement of transporting medium i.e. gas (after limn and Levenspeil, 1969 and Sheridan, 1973) 1 5 8

two possibilities for the initiation of a pyroclastic flow: a) dense, fragMented material may be blasted up short distances and then collapse down onto the flanks of the volcano (or back into the vent). This is a process known as 'slugging', which is common in vertical, fluidised systems, especially those containing large grain size solids (XUnii and Levenspiel, 1969) (Fig. 3.22a). The slumped material cascades down as a pyroclastic flow maintaining the high solid/gas ratio (the gas may be mainly air), but the fragments are now cooler and this mechanism may account for non-welded ignimbrites. b) The dense phase may rise above the vent and begins to flow under gravity, analagous to milk boiling over. As the mass of dense material has remained approximately coherent it retains much heat and may result in an ignimbrite with welded zones. A succession of 'slugs' or successive pulsations of magma may giVe a number of pyroclastic flows resulting in a number of flow units of 'ignimbrite.

As the dense pyroclastic flow moves downslope under gravity, a density profile through the flow develops, more dense at the base. Gas to pro- mote fluidisation of this dense phase may come from the juvenile material within the flow (Aramaki and Yamasaki, 1963), but a greater contributor may be engulfed air. Fluidisation is enhanced by the presence of a large amount of fine grained material (fines are added as a 'lubricant' in industrial fluidisation operations), but the whole flow may not be sed. Coarser fragments are transported in the fluidised,finer grained medium and 'float' or 'sink' according to their density. This type of mechanism may be like an lairslidel (Fig. 3.22b), where the gas flow may be normal or counter current. Therefore, the fine particles within the flow are essential to its movement and except for a laminar shear

1 CI zone at the base, the flow may approach that of a Bingham fluid. The larger clasts in the flow are not recirculated unless some obstruction impedes the pyroclastic flow.

Gas, continuously escaping from the dense part of the pyroclastic flow removes much of the fine grained material as a turbulent cloud above the pyroclastic flow. This approximates to a lean-phase fluidised system with mainly pneumatic transport of fine pumice dust. The change to lean-phase at the top of the dense zone may result in the crystal/lithic concentration at the top of layer 2b, due to the preferential loss of pumice from this zone next to the very active, turbulent zone above. Crystal concentration, additional to that from pumice dust 'lost' in the eruption column, is effected by the loss of fine grained pumice dust from the pyroclastic flow.

• The pyroclastic flow finally stops due to, perhaps many factors, including a depletion of the fluidising medium (i.e. gas and fine grained particles), and a loss of momentum. If there is sufficient heat retained near the dense basal part, welding occurs. The velocity of these flows may not be as high as that of ground surges, which at Mt. Pelee and Mt. Lamington were estimated to be in the order of 150- 200m. per sec.

Pine pyroclastic material, suspended in the turbulent cloud over the pyroclastic flow falls slowly to form layer 3, covering a wide area. This layer consists of some proportion of the vast amount of dust lost from the system. 3.9 Debris Flows, Rock Avalanches and other Poorly Sorted Deposits oh Terceira

Ignimbrites are not the only ill-sorted rocks containing a large proportion of fine-grained material on Terceira. Debris flows are not as common as they are in some other volcanio areas but there are a few representatives of 2 types of debris flow.

3.9 (i)Mudflows containing Pyroclastic Material. The intraformational mudflow in the Caldeira Ignimbrite has already been discussed. Near Altares there is a stratified debris flow that has been interpreted as a mudflow, or perhaps a torrent deposit, associated with pumice fall member 'GI (Fig. 2.7). Again, near Altares, there is a poorly exposed debris flow which may be of Lajes Ignimbrite age because it contains occasional pumice fragments and anorthoolase crystals. The flow is • well-bedded and compacted, resembling a fine-grained tuff, with occasional large bloCks 'picked-up' from the feldspar-phyric mugearitio flow below. Leaf impressions have been found on some bedding planes.

3..9 (ii)Debris Flows and Rock Avalanches containing Lava. A very large and old debris flow, containing huge boulders of comendite up to, 20m. x 15m. x 10m. fills an old valley to the east of Biscoitos village. Some of the blocks found nearer to Pico Alto Caldera are so large that they are quarried for roadstone. The debris flow is overlain by thin Lajes Ignimbrite and is therefore part of the old Pico Alta formation but its exact source is unknown.

1 6

eZ)Okla. - . .

10 -m

Fig. 3.23 a) Sketch of slopes of island above Biscoitos, looking NW. The 4 prominences are large blocks, part of the debris flow. b) Sketch of cliff section E of Biscoitos, showing exposure of debris flow containing large blocks of flow-banded comendite. 3 large blocks are shown, set in a poorly exposed breccia- like deposit, looking north. 162 The lava blocks have a marked planar fabric of anorthoclase microphenocrysts, in a trachytic groundmass, with 15-20% anorthoclase phenocrysts. All large blocks In the flow appear to be of the same lava and 2 sampled blocks give identical analyses but. the lava type cannot be correlated with any specific lava extrusion. Near Pico Alto Caldera, above the village of Biscoitos, blocks of lava are found perched on the ground surface, where the finer parts of the debris flow may have been eroded away (Fig. 3.23a). The size of blocks decreases towards Biscoitos and at a small cliff exposure east of the village (Fig. 3.23b) the blocks are 5-6m in diameter. Here the debris flow has a thin fine-grained base, 10-15cm. thick, and the blocks are set in a rubbly, unconsolidated matrix. It appears to be the deposit of a corase-grained, high-concentration flow of the type described by Fisher, (1971). Such debris flows can become mobile with as little as 10% water content. An alternative origin may be a rock-avalanche, transporting a frigmented lava flow under the influence of gravity.

North of Angra there is a deposit which appeared at first to be a non-welded ignimbrite, with a basal layer and a coarser top layer each about lm. thick. Although Inman parameters of these two layers are similar to those of the approprite ignimbrite layers (Fig. 3.17 and 18), the samples are essentially monolithologic. The dominant component is dense but somewhat vesiculated comenditic trachyte lava, with a low per,- centage of loose crystals. Nearby is an old, steep sided coulee extruded onto the flanks of Guilherme Noniz Volcano. This deposit is thought to have originated by avalanching of unstable lava from this coul4e.

1 6 3

WE:571 debris flow- InniT4 goif. normal," eraded cAr5v7.17.1- ii-thic bed.- 000000

ti■0 . . Strati fie& , fine- pooppo grained. debris-

90 0 a

• Teverstj graded titlic. bed .

well str4if led harimen , witk finer 3ra;ned. base

poor W, comrse ro_i ed bed.

Well strati f ;ed basal tic, Fine Brained 0 ash

Fig. 3.24 A section through the vulcanian deposit of member "A", near Santa Barbara village. 3.9 164 (iii) A deposit of a vulcanian eruption in member 'A'. One other pyrocla,stic deposit containing an appreciable amount o f fine-grained material is the air-fall deposit forming the upper part of member 'A' to the south of Santa Barbara Volcano (Fig. 2.7). There are few good exposures, but where it is seen near the coast at Santa Barbara village it is well stratified (Fig. 3.24). FMOand CO of one of the fine- grained beds plot in the same field as ground surge beds (Fig. 3.17), but features of ground surge beds such as cross-bedding are not well developed. The deposit appears to have accumulated mainly from air- fall debris but some ground surges may have accompanied this vulcanian activity.

3.10 Ignimbrite on Sao Fitagl. Three ignimbrites have so far been recognised on go Muguel and were examined for comparative purposes. Two are exposed on the south coast between Villa Franca and Ribeira Chgt. The younger of these two is non-welded and is the result of pyroclastio'flows during one of the plinian eruptions of Ague, de Pau Volcano (Walker and Croasdale, 1971; Plate 3b). No fine-grained, intraformational pyroolastic flows are found in the sub-plinian pumice deposits of Terceira. Their presence in go Miguel may well be due to slumping down-slope of the overwhelming volume of pumice that must have been deposited on the flanks of Agua de Pau Volcano during this large eruption. In the smaller sub-plinian eruptions of Terceira such large volumes of pumice did not accumulate high upon the volcano. The older ignimbrite occurs in a formation below the Fogo 'A' pumice deposit described by Walker and Croasdale. 1 6 5 The only other ignimbrite recognised so far on Sgo Miguel is a partly welded one, over 35m. thick, exposed locally near the town of Povoacgo. It is trachytic, with anorthoclase crystals, and forms part of the sequence of rocks at the older end of the island. The main exposure fills a steep sided valley, which has been cut by a gorge. There are three main flow units, the lower two are densely welded and the upper one is non-welded. This upper flow unit and the non-welded base of the lower one hale been sampled by Walker (1972), and cumulative curves for the prominent layers 2b and 2a are very typical, showing similar characteristics to those of Terceira (Pig. 3.10).

At the base is a layer interpreted as layer 3, an air-fall pumice bed, not seen below the ignimbrites on Terceira. Welding begins only 2cm. above the base of layer 2a and extends through 25m; all three flow units form a simple cooling unit. Welding is so strong in the lower zone of the bottom flow unit that layer 2a is a vitrophyre. There is a remarkable flattening of some fiamme adjacent to other rounded, glassy fragments.

In between the two lower flow units there is a boulder bed which appears'to be included in the welded zone. The bed is so coarse that there is no continuously welded 'matrix' but boulders at the top and base of the bed are welded to each of the flow units. This type of ocarse intraformationa1 bed is not seen in the welded zones of the Lajes

Ignimbrite.

The Povoacgo Ignimbrite is much thicker than any of those occurring on Terceira and the welded zone is proportionally thicker. However, the 166

5.25 Inman parameters for all the Terceira Ignimbrites. Samples from each layer plotted on Figs. 3.7 and 15 are contoured to show the fields of each layer. EA comparison of air-fall data with Lajes and Angra Igoimbrite data is shown on Fig. 2.14.] total volume of ignimbrite on go Miguel is smaller than that-on 1 6 7 Terceira. ao Miguel is twice as large as Terceira and so the proportion of ignimbrite on the island must be much smaller. This is a fundamental difference between the two islands; go Miguel is characterised by highly explosive plinian and sub-plinian eruptions, while Ter- ceira is particularly characterised by pyroclastic flow activity and Belie lava extrusions. Such a relation is a consequence of the con- trasting geochemical nature of the two islands, which will be discussed in the next section.

3.11 Conclusions

There are at least six ignimbrites on Terceira, representing periods of pyroclastic flow activity throughout the history of the island, of which the youngest was between 19,000 and 23,000 y.B.P. A ahem- oteristic sequence of products results, namely a ground surge deposit (layer 1), often overlain by an ignimbrite flow unit or flow units each with a finelgrained basal layer (layer 2a) and the main body of the flow unit (layer 2b). At the top is an air-fall dust layer (layer 3). These layers are recognised in all the ignimbrites on Terceira, although not all are present at each exposure.

The combined granulometric data for all the ignimbrites on Terceira, shows that each layer, except layer 3, falls into a separate distinct field on the Tman diagram (Pig. 3.25); layer 3 plots in the same field

as layer 1.

Ignimbrites are one part of a well-defined sequence,, (Sparks, Self and Walker, 1973), common in explosive eruptions, in which the initial 168

Fig. 3.2'15 Schematic block diagram of the products associated with ignimbrite eruptions. A. Plinian or sub-plinian pyroclastic fall deposit (found ii sEb Miguel and Italy); P Ground surge deposit; C. Ignimbrite, D. Lava flow,in or overflowing caldera. plinian phase produces a pumice fall deposit, followed by ground surges and by a pyroclastic flow producing an ignimbrite. Finally an effusive phase may occur. The products of such a sequence axe shown in a schematic block diagram (Fig. 3.26). There is no evidence that the ignimbrite eruptions on Terceira were preceded by a plinian phase or followed by an effusive one. 1 7 0 4. PETROLOGY AND GEOCHEMISTRY OF TERCEIRA

4.1 Introduction

All four volcanoes on Terceira have produced lava and pyroclastics ranging from basalt to pantel]bite in composition. During the past 23,000 years a minimum volume of 5.2km3 (given as dense rock equivalent) has been erupted from Santa Barbara and Pico Alto Volcanoes and the Fissure Zone.

The wide compositional range shown on Terceira is greater than other Azores islands. This appeared to offer an opportunity to study petrogenetic models that might derive pantelleritic liquids from various parents in an anorogenic environment. Also the time control on petrology can be examined in detail as the stratigraphy is well known.

The rock samples considered in this account were collected from a wide range of ages and types of lava and pyroclastics. All rocks appear to be in the alkali-olivine basalt suite, typical of Atlantic Islands, e.g. St. Helena (Baker, 1969), Tristan da Cunha (Baker, et. al., 1964), and the Canaries (Ridley, 1970). The lavas and pyroclastics range from accumulative olivine/augite basalts through hawaiites, mugearites, ben- morites and comenditic trachytes to pantellerites. In general the more basic lavas have been extruded from the fissure zone; their eruptive sites form a diagonal line across the island (Fig. 2.20). The lower piles, vitible above sea level, of at least three of the main volcanoes are composed of hawaiites and mugearites. Salic lavas and pyroclastics appear to be late-phase products on three of the volcanoes but Pico Alto is wholly composed of salic lavas and pyroclastics. In an alkaline oceanic island environment it is usual to account 1 7 1 ,the foriobserved range of compositions by adopting a model of differentiation of magma from a basanitic or limburgi,tic parental composition by fractionation or partial melting (Gunn, pers. com.). In the absence of evidence of crustal material, such as tentatively proposed for Iceland (Walker, 1966), this is hard to refute. Much work has been done on the phase controlling the lines of descent of these magmas (e.g. Hughes, 1972; Baker, 1969). However, where there is an absence of material of intermediate composition as on Terceira, complexities are introduced which throw doubt on simple models. In the final analysis no simple scheme appears to provide for all the necessary variations seen in the field.

When studying an island such as Terceira, (18km. x 28km., 406km2 in area and approximately 220km.3 in volume above sea level), there is always the limitation of being able to see only "the tip of the iceberg". Terceira at sea level stands about 1500m.-2000m. above the Azores Plat-. form but the average height of the island above sea level is only 550m. This indicates that only about 0.125-0.1 of the total pile of Terceira is exposed above sea level. We have little indication of the nature of formations composing the remaining portion. Plutonic xenoliths in the pyroclastic deposits may well be derived from lower levels still, i.e. the magma chamber or perhaps upper mantle. Syenite nodules from Sgo Miguel, similar to those found on Terceira, have been analysed by Cann (1968) and their relevance will be discussed later.

In the followinglthe field occurrence of the lavas and pyroclastics associated with each of the four volcanoes and the Fissure Zone is first 172 discussed and than the petrography, petrology and geochemistry. This is followed by a discussion of the features shown in relation to volumetic and stratigraphic knowledge and, finally, to current petrogenetic models.

4.2 ,The Eruptive Centres

Cinquo Picos, the oldest volcano, forms the eastern part of the island. It must have been larger than the three younger ones; the caldera is 7km. in diameter, the largest in the Azores. Most exposures in the 150m. high caldera walls and in the cliffs around the perimeter are mugearitic lavas, though a few more salic lavas also occur. The caldera is now floored with basalts from the Fissure Zone. Basaltic activity was so intense in the SE corner of the island that the caldera walls were later swamped by lava flows and cinder cones on the oldest part of the Fissure Zone (possibly contemporary with Guilherme Moniz Volcano). Only small remnants of=clastic-fall deposits are left exposed in the sea cliffs but there is evidence both of pyroclastic- . fall deposits and ignimbrites now largely eroded away.

Guilherme Moniz Volcano has a small, vertical sided caldera exposing comenditic trachyte lavas; coulees of similar lava are also common on the southern flanks of the volcano. The whole of the northern part is covered by the more recent lavas of Pico Alto Volcano. Ignimbrite of comenditic composition exposed in the cliffs in the vicinity of Angra do Heroismo came perhaps from Guilherme Moniz Volcano. The lower levels of the volcano are ill-exposed, even on coast sections.

Pico Alto is a young volcano, lying north of the axis of the. Fissure 1 7; Zone, built on the northern flanks of Guilherme Moniz Volcano. No basaltic rocks at all are exposed; the oldest rocks exposed on the north coast at Quatro Ribeiras are comenditic lavas and an ignimbrite, in 50m. high cliffs. Pico Alto is still considered to be active.

Santa Barbara Volcano has a subaerial structure constituted mainly of feldsparphyric and aphyric mugearites and hawaiites, capped by olivine hawaiites erupted just prior to the first caldera collapse. Later, mainly peralkaline lavas and pyroclastic-fall deposits were erupted. This

volcano has not so far produced an ignimbrite.

The Fissure Zone is most prominent in the central part of the island, and in Cinquo Picos Caldera, where it is marked by a line of basaltic lavas and scoria cones. The last eruption on land was of a hawaiite lava in 1761.

By far the most common eruptive products of the three active sources, are in the hawaiite and peralkaline groups. There is a paucity of rocks in the intermediate range both in the number of eruptions and the volume of material erupted. This relationship is well known from the many studies on oceanic islands (Daly, 1925; Ghayes, 1963; Cann, 1968).

4.3 Petrography

The mineralogy and petrography of the volcanic rocks of Terceira are shown on Table 4.1. This represents all the rock types found on the island to date and summarises evidence based on field examination and

petrographic studies of 135 thin sections.

Petrographic descriptions of Azores Suites have been published 4. TABLE 1. pETROGRAPHY AND MINERALOGY OF TERCEIRA

ROCK TYPES SAMPLE NUMBERS MAJOR PHENOCRYSTS* ACCESSORY AND TEXTURE', SPECIAL ERUPTIVE CENTRE STRATIGRAPHIC (ANALYSED ROCKS) PHASES GROUNDMASS PHASES FEATURES POSITION PRODUCTS CUYON TYPES ALKALI 51,52,55,56 FELDSPAR(PLAGIOCLASE PLAGIOCLASE PHENOCRYST % ' FISSURE ZONE. YOUNG FISSURE EVENTS OLIVINE S10,511,320, An 55-65) (An 45-60) VARIES FROM 1CPA - CINQUO PICOS VOLCANO. IN CENTRE OF ISLAND. BASALT 557,566,T2191, OLIVINE(FAYAIITIC AUGITE 50% +(ANKARAMATIC) SCATTERED ADVENTIVE I ADVENTIVES OF VARIOUS' T2193,T43,T45, (Fa 65-80) OLIVINE (RARE) IN DIFFERENT FLOWS. CONES & FLOWS. " : AGES. T50,T59 CLINOPYROXENE MAGNETITE OLIVINE & PLAGIOCLASE - LATE CINQUO PICOS (AUGITE-PIGEONITE). ILMENITE EQUALLY IMPORTANT LAVAS (BUT EARLY IN MICROPHENOCRYSTS VARIATION IN GROUND- CINDERCONES & THIN ISLAND'S HISTORY). APATITE . MASS FROM MICRO- LAVA FLOWT,SOME OF CRYSTALLINE TO GLASSY GREAT LENGTH. IN DIFFERENT FLOWS, OFF-SHORE TUFF RINGS. MOSTLY UNDERSATURATED.

HAWAIITE S4,524,535,340, FELDSPAR(PLAGIOCLASE PLAGIOCLASE 59,580,5210440, An 45-55, RARE ANORTH- (Alt 40-50) GENERALLY LOW PHENO - SANTA BARBARA VOLCANO, EARLY PILE OF CINQUO OCLASE) AUGITE CRYST CONTENTS, 5-15%. :CENTRAL VENT & ADVENTIVE PICOS,THEREFORE SOME OF T46,T54,T56,T61, CLINOPYROXENE(AUGITE) OLIVINE OFTEN VERY VESICULAR. CONES. OLDEST ROCKS ON MAGNETITE AND SOME APHYRIC TYPES FISSURE ZONE. TERCEIRA. T62,T63,T66,T69, OLIVINE(AS ABOVE) ILMENITE FELDSPAR DOMINANT IN pinup pICOS,VOLOANO,„_ LATER ADVENTIVES. T78,T79,T83, MAGNETITE. MICROPHENOCRYSTS GROUNDMASS. HIGH % Fe ? SASE OF G.M0N12 VOLCANO. EARLY PILE OF SANTA BARBARA. OLD SE PART OF T2098 OXIDE MICROCRYSTS,•UP CINDERCONES & THIN FLOWS. TO 5%.INCLUDES SOME FISSURE &YOUNG FISSURE ' PROMINENT "BIG FELDSPAR ,EVENTS IN CENTRE OF BASALTS". ISLAND.

MUGEARITE S50b,S97,S23, FELDSPAR(An 25-40 PLAGIOCLASE INCLUDES SOME "BIG CINQUO PICOS(PRE-CALDERA?). EARLY CINQUO PICOS T2186,T47,T55, PLAGIOCLASE), SOME Fe OXIDE FELDSPAR BASALTS"; SANTA BARBARA - CENTRAL VOLCANO & EARLY SANTA T57,T58,T64,T65, ANORTHOCLASE RARE MICROPHENOCRYSTS MOSTLYAPHYRIC TYPES. VENT & ADVENTIVE CONES. BARBARA. YOUNG T67,T68rT72,T73, RESORBED OLIVINE GLASS HIGHLY VESICULAR, FISSURE ZONE BETWEEN SANTA FISSURE LAVAS T80,T71,T87,T81, AND AUGITE. "TRACHYTIC" TEXTURE OF BARBARA *PICO ALTO VOL - Silo,s2o5,T70 FELDSPAR PHENOCRYSTS & CANOES. GROUNDMASS LATHS. THIN F1,08 WITH -, ASSOCIATED CINDER. ----- S07,S010,5012, FELDSPAR, OCCASIONAL RELICT GLOMEROPORPHYRITIC CALDERAS & FLANKS OF ALL ALL AGES, INCLUDING S47,S8,358,313' ANORTHOCLASE. OLIVINE,AUGITE/ TEXTURE COMMON IN FOUR VOLCANOES. LATE -STAGE OF SANTA S73,S71, BIOTITE. LAVAS & PYROCLASTICS. BARBARA & CINQUO BIOTITE,HEDEN - ALMOST ARE PYRO- --- PICOS VOLCANOES. 538,588,598, SUBSIDIARY BERGITE, ALL SANIDINE. PHYRITIC. G.MONIZ APPEARS TO BE AENIGMATITE, 1 ALMOST ALL PER - COMENDITIC T49,T51,T52,T75, AMPHIBOLE DOMINANCE OF ANORTHO - THICK COULEES, DOMES, ALKALINE LAVA BUT TRACH/TE T76,T77,T82,T84, (ARFVEDSONITE). CLASE AS PHENOCRYST PYROCLASTIC FALL EXPOSURE IS POOR. T85,S5A,S51, Fe OXIDE MICRO- PHASE (UP TO 15%) DEPOSITS & IGNIMBRITES. TO T2190 PHENOCRYSTS. VARIATION IN CRYST- THE WHOLE OF PICO ALTO (HEMATITE & MAGNE- AILINE STATE OF VOLCANO. PANTELLERITE T2068,5136,5210, TITE AS OCCASION- GROUNDMASS. MICRO - PERAIXALIbE AL.PHENOCRYSTS). PERTHITE FELDSPAR. PUKICE S134,S185,S196, CRYPTOCRYSTALLINE DEVITRIFICATION EVEN & GLASSY GROUND- IN YOUNG LAVAS. S209 ,S 62- ,5208, f MASS MORE COMMON S202f, S13. BUT MICRO-CRYSTAL- LINE OCCURS. TABLE 4.1 (CONTI)

RCCK TYPE :IA AMPLE IM.r.I.BERS MAJOR PHEROCRYST. ACCESSORY AND TEXTURE, SPECIAL ERUPTIVE CENTRE STRATIGRAPHIC POSITION PEASE • GROMODMASS PHASES P ATURES MORPHOLOGY OF PRODUCTS 07-'3R TYPES

ISENMORSITlS . s3, 586,T2067, (IF PRESENT AlE FELDSPAR,(PLAGIOCLASE MAINLY APHYRIC, ' SANTA BARBARA, FISSURE • EARLY ISLAND LAVAS(URDER- ' T86, T42 PLAGIOCLASE ,. Am 18-22) CRYPTO-CRYSTALLINE, ZONE.OINQUOPICOS VOLCANO SATURATED)IN SCANTY CINQUO PICOS EXPOSURE8. An 20-30) Fe OXIDE MICRO-PHENO OFTEN DARK-COLOURED - CRYSTS. REMNANT PYRO- LAVA. (2 types:1.17NDER- DYKE EXPOSURES, CALDERA' ,2 FISSURE ZONE FLOWS EX- SATURATED (CINQUO XENES) WAITS EXPOSURES, SMALL POSED BY SMALL FAULTS. 1 P/COS),2.0VERSAT- THIN FLOWS. FLOW AT BOTTOM OF SANTA URATED.TYPES), MUCH (VERY SMALL VOLUME). BARBARA CALDERA WALL YOUNGER SANTA BARBARA LAVAS AND FISSURE - ZONE).

UNDERSATURATEDi S8, S7, T41, T44 PLAGIOCLASE FELDSPAR, MAINLY FELDSPARPHYRIC, LIGHT CALDERA WALLS OF CiNqUo LIMITED TO OLDEST • TRACHYTES AND (An 15-20) PLAGIOCLASE GREY LAVAS."TRACHYTfC" PICOS VOLCANO VOLCANO, cINquo PICOS PIRALKALINE ANORTHOCLASE TEXTURE OF FELDSPAR TRACHITES LATHS CRYPTOCRYSTALLINE SLIGHTLY PERALKALINE THICK FLOWS, SOME VHS- AMPHIBOLE IN GROUND IN SOME FLOWS ICULAR. MORE.BASALTIC' MASS(AENIGMAT/TE?) IN FORM THAN 'TRACHYTIC' .

XENOLITHS NO ANALYSED SAMPLES

1) GABBRO PLAGIOCLASE EvIGRANULAR AND , . BASALT FLOWS. MOSTLY YOUNG ERUPTIONS (An 65-95) HOLO-CRYSTALLINE. VULCANIAN AIR-FALL BUT NOT CONFINED TO ANY OPITIC AUGITE/PLAGIO- DEPOSIT 'A' ON CENTRE PYROXENT (AUGITE) APATITE CLASH. OFTEN HIGHLY SANTA BARBARA VOLCANO OLIVINE Ti-AMPHIBOLE Fe OXIDE OXIDISED.

SYRNITE I. FELDSPAR CANORTHO- FELDSPAR PORPHYRITIC HOLO- 2) CLASH AND? ALBITIC AENIGMATITE- CRYSTALLINE IGNIMBRITES FROM PICO PLAGIOCLASE). AMPHIBOLE ' ALTO AND ? OTHER' _ ARINEDSON/TIC MINOR BioT/TE VOLCANOES. Fe OXIDE SHOWSS TWO STAGES OF AMPHIBOLE. VULCANIAN DEPOSIT'M IN OLD & YOUNG IGNIMBRITE AMPHIBOLE. QUARTZ OXIDATION

3)AMPHISOLITE e AMPHIBOLE - HOLOCRYSTALLINE , - COGNATE XENOLITHS IN OLD I IGNIMBRITS. VERY RARE

4) OLIVINE. .. OLIVINE - HOLOCRYSTALLINE BUT NODULE FINE GRAINED FISSURE ZONE sioGNAT3XENOLITHS IN 3AS4LT, PRE-19,000 YEARS OLD

•Pbenocryst of major accessory minerals listed in order or abundance

I 7 ( (Assuncao et al., 1970; Zbyszewski et al., 1971) and much of the early petrological data is contained in the works of Esenwein (1929) and Friedlander (1929); Sao Miguel has been the subject of more work in this field than Terceira (Assuncao, 1961; Zbyszewski, 1961). More recently Sao Miguel and Terceira have been compared with Madeira, (Schminke and Weibel, 1972) and the Canaries (Schminke, 1973).

Generally cognate xenoliths found in the lavas consist of augite and plagioclase, olvine-rich nodules are rare. Gabbroic nodules found in a vulcanian deposit on Santa Barbara Volcano are exceptionally rich in clinopyrone. Quartz syenite nodules brought up in the ignimbrites are typical of intrusives associated with alkalis volcanoes, (Chaves, 1963; Cann, 1967).

4.4 Feldspars

Feldspars are the dominant pheaocrystal phase on Terceira in all but the most basic lavas. Plagioclase-phyric hawaiites and mugearites are common; the groundmass of the majority of lavas is composed mainly of plagioblase laths. Of most interest are the anorthoclase crystals which are very common in the peralkaline lavas and pyroclastics. Originally an anorthoclase phenocryst content between 5 to 15% was present in the peralkaline magma but in the ignimbrites this has become concentrated, forming up to 40% of the deposit in some layers.

The anorthoclase crystals (and indeed also some of the plagioclases) are large, up to 1.5cm. and commonly occur as glomeroporphyritic clusters in the lavas and pumice deposits. They are high temperature alkali feldspars of the family: high albite-anorthoclase-low sanidine 177 (Deerl.Howie and Zussman,1966).

"Anorthoclase from Terceira" has been used since the investigations of Fouque (1883), as one of the type examples of this mineral and is now used as a standard in the X-ray Diffraction method, (McKenzie and Smith, 1956). Fouque originally called the crystals 'Albite' and in his paper the exact field locality is very poorly defined. However, the area of Quatro Ribeiras, from which the sample cause has extensive outcrops of comenditio ignimbrite and lava, rich in anorthoclase, which may be the source of Fouquels specimens.

Eighteen XRD determinations of feldspar from the ignimbrites and air-fall pumice deposits all show peaks corresponding to anorthoclase. Variations in peak intensities and splitting of peaks especially the (20i) peak shows the existence of a small composition range and development of perthite. The feldspar compositions plot just on the Albite side of the critical Ab 63 or 37 composition (checked by the 2V method of McKenzie and Smith, 1956 and Tuttle, 1952; results gave Ab An 70 or 30) but vary in their structural states, degree of twinning and development of perthite. Some crystals give an anorthoclase composition but have monoclinic crystallography, simple twinning and a small 2V (noted previously by McKenzie land Smith). Splitting of the (201) peak is generally small in the crystals examined indcating a crypto-perthitic intergrowth. Triclinic anorthoclase, with cross-hatch twinning is also found in the lavas and pyroolastics.

4.5 Chemistry

Rock types outlined in Table 4.1 are all in the sodic series of the 17S alkali-olivine basalt suite, (Yoder and Tilley, 1962; Irving and Barager, 1971). However, Terceira is less potassic than the other Azores islands (Schminke and Weibel, 1972). The division into lava • types is based on the Thornto and Tuttle Differentiation Index (1960), rather; than on normative plagioclase compositions (Thompson et. al., 1972). Alkali-olivine basalts have a D.I. value less than 35, hawaiites 35-45, mugearites 45-65, benmoreites 65-75 and salio types > 75.

Terceira has alkali olivine basalts with greater than 5% normative ne- and transitional-type basalts, low ne or hy- in the norm, (Coombs, 1963). Thd most salio rooks fall into the pantellerite class which have 12.5% normative femic,minerals ( McDonald and Bailey, 1973).

4.5 (i) Analyses of rocks from Terceira. 101 major element analyses and sample localities are given in Appendix I. In all tables and diagrams the following scheme is adhered to: samples prefixed IS1 have been analysed by the author at Imperial College on the Phillips 1212 X-ray spectrometer. 4 samples have been analysed by the author by the wet chemical method and a good comparison with machine analyses was obtained. Samples prefixed IT' have been analysed by the Professor B. Gunn at the Department de Geologie, University de Montreal and were collected by Professor Gunn and the author.

Analyses are given recalculated to 10 0.01 and model values for Fe203 of 2.00% are allotted to basalts, hawaiites and some mugearites, (Thompson, et al., 1972). This value was chosen from consideration of wet chemical analyses by the author, and also Schminke (1973); the low 179

. . • . K2() . v.A P .7 1.x ,.., AA Ay Aaaai • Ara, 10 OCAYNFOrdr 0 0 o 4govvo le 00 00 •

.. Na20 • • • :4 , n'i; 5- • A • • lid)"Allir 1.° 00 .• Nv 0 0 (%)•.,04b 00 00 1'0 , Z' 131 Or I 0.• J •qi0,. 4o9gi b...AA i • 0,a, •, , 0 0 o 00 0 •: 0 iiMMI"e'- ....-y u (8°40% 00 ''I'F°46044 t. TiO2 00 • 00 'IVA 0 V°0 o 0 0 w-Nl*,k ttays4A 0 0 *y0 l- lli • 4, x ) 3"°kW1W o 0 • v Nycleiete 4, A A Y ° 0 0 y 00.

i' - s• CaO e • ) Atisamik i- o 00 V -,•%: 0 o V° 0 0 00 V • k. Y o , • ... w • x , Fe as , • t ,, Fe203 J ' A1203 ° A v V 0 it A. v v■!ag r vy o A Af • •• Oulei vAlliA X • ° • • CP 41 Z A g) 0 0 00 - 08. ifi, X . 1,0 • t-

7 Si 02 6 - xyx5 6 I. • yv • • , • y • 5 ; V AtA 4:11h 5 1. alE A o 114*o o o o CIPY O VO 00 00 -P cc° • "°4,0, • 4

12 11 10 9 8 7 6 5 4 3 2 1 0 MgO

Fig. 4.1 Plot of all major oxides against M& for. Terceira lavas and pyro- elastics. The symbols used are open for porphyritic rocks and closed for aphyric. The symbols divided the rocks into their source volcanoes: oAlo= Fissure Zone lavas, v9v = Cinquo Picos lavas, X = Guilherme Moniz lavas (all aphyric), 1301 = Pico Alto lavas and pyroclastics, A,• . Santa Barbara lavas and pyroclastics. 180 oxidation state of these lavas is compatible with what has been observed in thin section. Samples prefixed IS1 have Fe2 03 determined by wet chemical analysis for rocks with Differentiation Index > 55%.

Samples prefixed ITS use a model value for Fe203 for all compositions; the value of 2.00% appears to be realistic for all but the oldest lavas if fresh samples are used.

4.5 (ii) Geochbmistry. A summary of the geochemistry of the Terceira suite is given on Fig. 4.1, where weight % of the major oxides are plotted against MgO. This choice of ordinate cuts down the multiplying of errors obtained in the more usual Fe ratio; it is not meant to show trends in the peralkaline range but different plots of these rocks will be presented in the appropriate section. Good trends are obtained with an obvious grap in the trachyte range.

As there is detailed knowledge of all eruptions since the Angra Ignimbrite the main sampling programme concentrated on obtaining a representative suite of these lavas. If oversampling did occur it was of the mugearitic oompositions.

The variation in Al205 is a direct reflection of the great range of feldspar content in the rocks. The suite is a sodic one (Fig. 4.2), the 100 Na20/Na20-1-K20 ratio ranging from 65-78. Higher X20 found in the peralkaline rocks and variation in the alkali ratio in this group may reflect a loss of IC and, to a lesser degree Na, during crystallisation of the comendites (McDonald and Bailey 1973; Noble, 1968). Only in this group does the alkali ratio drop below 70%. 1 8 i

CaO

K2O

Fig. 4.2. Ca04Ta20-K20 ternary diagram (in molecular percent) showing the sodic affinity of Terceira volcanics. Symbols on Fig. 4.1.

18 2

A A A A • 75 4 ri o V - xi • 65 Z

A • - A 55 A A o 0 A z „A 410 • • • / 45 .._ • O °VA 0 V z V 1:7 O -35 m 0 0 O 0 , 0 , 0 O 0 -25

is 9 8 7 6 4 3 0 3 •I 6 7 8 9 ol+an Q t Q in by e j of zero

Fig. 4.3. Silica saturation (positive or negative) versus Thornton-Tuttle Index.for Terceira lavas. Symbols as in Pig. 4.1 (after Thompson et al., 1972). Main rock divisions shown by abbrevi- ations on the left. Ne is normative nepheline; Q + Q in by is normative quartz plus the quartz used to make normative hypersthene. Note that normative plagioclase and normative olivine zero values plot off the diagram. Plagioclase plots along the ordinate. The main peraJkaline group of rocks lie off the diagram, top right. Solid line is Cinquo Picos undersaturated trend. Dashed line is Fissure Zone and Santa Barbara trend. 1 83 4.5 (ii) a) Basaltic and intermediate lavas and pyroclastics.

Mugearitic (hy-normative) lava appears to be derived from ne- normative basalt by fractionation of a crystal phase or phases (Fig. 4.3). On this diagram rocks of Thornton-Tuttle D.I 75 are plotted against normative ne- and an indicator of saturation. Two trends appear; the first is confined to rocks of the oldest volcano, Cinquo Picos, and it shows that they are undersaturated except for the most salic sample. The other trend crosses the thermal barrier and con- stitutes a series through to benmoreite compositions.

In the alkali basalt class the phenocryst phases are feldspar, olivine and clinopyroxene. However Fig. 4.3 shows that the differentiation trend crosses the critical plane of silica undersaturation in the direction opposite to that which would result from crystal fractionation of olivine and feldspar. Cawthorne et. al. (1973) have proposed a differentiation trend from ne-normative alkali basalt through hy- normative andesite to Q-normative dacite for a series of lavas in Grenada. Lesser Antilles and this trend also :drosses the thermal barrier. Some low pressure phase may crystallise out, probably under H2O vapour pressure, thought it may not be present in the rocks seen on the island. The trend may also be examined on a ternary projection, of the base of the ne - of Q - di tetrahe&ron of Yoder and Tilley (1962), (Pig. 4.4).

This trend may be produced if amphibole (kaersutite) or Fe-oxide (magnetite) are the phases separating from the liquid. Amphibole is 184

Pig. 4.4 Ternary projection of Cpx - 01 - Ne - Q tetrahedron for Terceira lavas with Thornton-Tuttle Tnaices less than 55. Normative values are CIPW (wt%). Symbols as in Fig. 4.1. Olt = ol + 0.75 hy. Q' q + 0.4 ab 0.25 loky. Net = ne + 0.6 ab Line AA' is the Yoder andTilley Critical Plane of Silica Undersaturation.

1 8 5

16- , 14- A 0 Na2 u 12- • IT rkX ti e, v t v it , , • K20 8_ A '-

. 4 4A A4 4 6- J4 A ,/ / W 444 41 1" / 4 - ?Aiovtrg, / -' 2- -9- CJ " 40 45 50 55 60 65 70 Si 02 wt%

Fig. 4.5 Total alkalis plotted against Si02 (wt%) for the Terceira lavas and pyroclastics. Symbols as in Fig. 4.1. The dotted line is McDonald's (1964) dividing line. 8 6 not common on Terceira, even in gabbroio nodules, but opaque Fe ores are common in the hawaiites and mugearitic lavas, especially in the aphyric ones where they occur as phenocryst and microphenoyrst phases. Besides the arpedsonitic amphiboles found in the peralkaline lavas and syenite nodules, amphibole is found only in the older ignimbrites. One amphibolite nodule from these pyroclastics may be kaersutitic but the mineral is very oxidised. Kaersutite in nodules is much more common in other Azores islands e.g. Faial, Graciosa and Sgo Miguel. Borley et al. (1971) have analysed Kaersutite from Faial (sample 470); the mineral is extremely no-normative (15f) and may represent the type of composition that could be subtracted from the IqUid to give the observed trend. Magnetite and ilmentite are present in large amounts in the xeniithic rocks from vulcanian deposit W. Specularite is common, especially in spatter and scoria, and hematite phenocrysts from comendite flows at Serreta have been studied by Sunagava (1960).

A plot of total alkalis against silica (Fig. 4.5) shows that a few lavas are on the border of the alkali and sub-alkali fields. The amount of total alkalis is variable throughout the series, e.g. 3-6% in basalts. This is due to the varying phenocrysts content in the lavas analaysed. Lavas rich in olivine and augite will have smaller percentage volumes re,grOnndmass - where the alkalis are concentrated. Some lavas, as previously mentioned, have a large content (up to 45%) of plagioclase phenoorysts which will increase the alkali content above the average for that composition.

An AFM (mol. p.c.) plot for ailTerceira analyses (Fig. 4.6) reveals a single well defined trend with greatest iron enrichment in the hawaiite/ mugearite class. As mentioned above, it is in these,compositions that 187

Fig. 4.6 AFM triangle (in mol p.c.) for all Terceira lavas and pyroclastics. Symbols as in Fig. 4.1 18-8

Cinquo Picos

Fig. 4.7 a) AFM triangle for Cinquo Picos volcano lavas. b) AFM triangle for the lavas and pyroclastics of the last 23,000 years on Terceira. Symbols as in Fig. 4.1. .18g most iron oxides are found. Two distinct groupings of analyses are seen, one in the basalt-mugearite range and the other in the comendite-i pantellerite compositions. Only a few analyses fill the 'gap'; some are ne-normative, mildly peralkaline trachytes from Cinque Picos Volcano (Pig. 4.1). The rocks of Cinquo Picos may show a continuous trend, but there are insufficient analyses to be certain (Fig. 4.7a). The others are hy-normative benmoreitic rocks from the Fissure Zone and Santa Barbara, represented by only 3 analyses of lava from flows of insignificant volume. On Fig. 4.1 these lavas are part of a trend with a higher Si02'content, which continues into the pantelleritic group. Analyses of the volcanic products of the past 23 000 years (Fig. 4.7b) show less rocks in the Itrachytel composition range, as none of the Cinquo Picos rocks fall into this time period. One such lava (s86) is a black, aphyric, vesicular flow 1.2m. thick with the morphology of a basalt flow; a possible indication of higher viscosity (Si02 56%) is that the vesicles are stretched out. It is exposed in the upthrown side of a small fault. Another (T2067, Si02 = 57$) is the bottom-most flow in the SW wall of Santa Barbara Caldera. It is a light grey, non- vesciular flow, thick but with a very small outcrop. It is notable and probably significant that both these flows would not be exposed unless it was for faulting. A "trachytic" sample (s54) would also not be exposed without its unusual occurrence as a plug-like mass (see next section).

Santa Barbara volcano shows more restricted compositions and a large 'gap', (Fig. 4.8a). Although the pre-caldera pile is quite well exposed in the cliffs, the basaltic compositions are closely grouped in the hawaiite/mugearitio range. The volcano is still active, erupting 1 9 0.

1 AA SAA: A A • A

AAA

. Santa Barbara b.

204caldera. _A

A A A • a) E •

8 6 4 6. 8 ne q +q by

Fig. 4.8 a) AFM triangle for Santa Barbara Volcano lavas and pyroclastics. b)Silica saturation (positive or negative, as in. Fig. 4.3) plotted against an arbitrary time scale for Santa Barbara lavas of hawaiite benmoreite compositions. c)Part of CaO-Na20-K20 (mol. percent.) ternary diagram showing a possible plagioclase fractionation trend on Santa Barbara lavas. A = plagioclase microprobe analysis from lava B. = 2 samples of plagioclase - phyric mugearite with C slightly different phenocryst content. D aphyric mugearite underlying the plagioclase- pbyric lava. 191 comenditic lavas with mugearite eruptions from adventive cones, the axis of the latter lying along the centre of the Fissure Zone. There are, no primitive basalts at all on this volcano. Ages plotted against the increase in differentiation of the mugearitic suite show only an indistinct trend in increased normative by-with time (Fig. 4.8b). Immediately after the first caldera collapse, comenditic trachytes began to erupt and have continued as the major type volumetrically since that time (estimated at approximately 30,000 y.B.P.). For this volcano it may be possible to demonstrate feldspar fractionation in the upper feldspar - phyric and closely related aphyric members of the pre-caldera mugearites. One analysed plagioclase (G. Witty on Geoscan) from a feldsparphyric flow, 2 analyses of the flow (with different phenocryst contents) and one underlying aphyrio flow all lie on the same trend when plotted on.a CaO-Na20, - K20 (mol. p.c.) triangle (Fig. 4.8c). Feldspar fractionation may be expected to operate only in the late stage develop-. ment of these lavas and may have no bearing on the derivation of the Q or hy-normative peralkaline liquids.

4.5 (ii) b) Peralkaline lavas and pyroclastics. In this section as the group is compositionally close, lavas are not ascribed to their source volcano but are divided into comenditic and pantelleritic groups. The peralkaline rocks of Terceira are voluminously important on three of the islands, 4 volcanoes.

Guilherme Mbniz Volcano,represented by only five analyses (Fig. 4.1),is ill exposed as the northern half is obscured by viscous Pico Alto lavas and the southern part by thick pyroclastics. All lavas 192

30- norm. Q0/ 20 COMENDITE PANTELLERITE 0 0 0 o 00 0 0 0 COMENDITIC • • • • 0 o 0 PANT. TRACHYTE --f. 00 0 o TRACHYTE IIVO. 0" , • 0

0 10 20 30 40 Z norm. Femics %

Pig. 4.9 Normative Quartz plotted against normative ramie minerals for the salic group of lavas and pyroclastics of Terceira. Lavas (after Macdonald and Bailey, 1973) of Pico Alto and Santa Barbara plotted by same symbol. Black circles are comenditic lavas and pyroclastics. Open circles are pantelleritic lavas andpyroclastics. The group within the dotted line are Lajes and Ana= Ignimbrites samples. 193 exposed in the steep, caldera walls are thick domes and coul4es of comenditic trachyte. It is likely therefore that unless there was more than one caldera collapse, a large amount of peralkaline material is present in this volcano. Similarly Pico Alto lavas and pyroclastics are all peralkaline but here it is almost certain that there are no: basaltic rocks. The few old basaltic lavas exposed on the central parts of the north and south coasts are probably from the older Guilherme Moniz Volcano, though this cannot be proved. Santa Barbara has late- stage peralkaline lavas and pumice fall deposits. One Santa Barbara lava of comenditic trachytic composition (854) (peralkalinity index 0.93), is a crystalline lavaland if K and Na have been preferentially lost on cryStallisation, the low peralkaline index may be attributed to this. This particular lava has the same petrography as the comenditic trachyte group and is a late, plug-like injection through the centre of a comenditic trachyte dome of very recent age (estimated at 5-700 y.B.P.). * The peralkaline group are classified here using the scheme of

McDonald and Bailey (1973) (Fig. 4.9), where total normative femics are plotted against normative quartz. All classes of peralkaline lava are found but a concentration in the lower Q-normative range is obvious. This suite of lavas forms an indistinct trend (Pig. 4.10,, after Noble, 1968 and Fig. 4.10b). Both Si02 and A1203 are depleted in rocks with the highest peralkalinity indices, due to an increase in alkalis at the expense of several other elements, particularly alumina. This

accounts for, the sudden fall off in Al203 on Pig. 4.1, where A1203 decreases (and alkalis increase antipathetically) with little or no variation in MgO content. Si02 is only slightly higher in the comendites 1 9 4

171 • • • I6 • •• •• • 15 • • • IA • , 0 000 • O 13 • 0 0 CV 0 0 < 12 8 0 0 II 0 0

10 F 60 65 70 Si02 wt. pc.

jta 0. ea* 0 0 0 0 65 0 0 0 * • 0CO 0 0

Pig. 4.10 a. A1203 against Si02 (wt. %) for the salic group from Terceira, show a scattered trend, with the panteller- its. group lying on a slightly silica-poor line compared to the comendites. b. Al203 (wt.%) plotted against Peralkalinty Index and SiO2 plotted against the same Index. 195 than in the pantellerites. This exchange process between alumina and alkalis may be caused by fractional crystallisation of anorthoclase which takes selectively more Na from the remaining melt than K, so that the final lavas should be more K-rich (Nicholls and Carmichael, 1969; Thompson and McKenzie, 1967; Bailey and Scheirer, 1964). In fact little K-enrichment,is seen (see Fig. 4.2) and this may indicate that the process of differentiation is not yet complete under Santa Barbara and Pico Alto or is not operating.

To examine the peralkaline trend we must first consider various • factors about this type of lava. The most detailed work to date has been done on glassy obsidians because these are believed to approach closest to the liquidus compositions. X20 and Na20 may be lost during crystallisation and devitrification (McDonald, Bailey and Sutherland, 1970), as are the ,hologens F and Cl (Noble et. al., 1967; Noble and Haffty, 1969). In Terceira the lavas have a range from micro-crystalline to glassy groundmass, often varying within one flow; pure glassy obsidians are not usual. Zr203 may account for well over IA in peraikaline glasses (Noble et. al., 1972), but this element has not been determined for Terceira lavas. Oceanic comendites show more scatter than continental types (Bailey and McDonald, 1970) and so an indistinct trend may be expected for Terceira. A phenocryst phase (<15,$) does not affect the loss of alkalis and hologens (McDonald and Bailey, 1973).

When the peralkaline lavas and pyroclastics are plotted on a triangle (Fig. 4.11) (Bailey Si02 - Al203 35.02 - 20 + Na2O. 35.02 and McDonald, 1969) the trend is along a line radiating from the alkali feldspar join. The alkali/silica index for this line is given by the intercept on the SiOralkali side of the t angle. Any liquid 1 9 6

SiO2

Fig. 4.11 The top part of the Si02 - A1203 - K20 + Kap (mol. percent.) ternary diagram (after Bailey and Macdonald, 1971) showing a trend from the alkali feldspar join. The alkali/silica index for this trend is also indicated (10.90). 197

too Na/Na;K 0

20 (Albite) K/Na Ratio (Orthoclase

Fig. 4.12 Quadrilateral plot to show the variation in alkali ratio and Al203 (mol. p.c.), representing the plane of alkali silica index (10.90). The Quartz/feldspar cotectic is shown; all the Terceira peralkaline rocks lie on the feldspar side of the cotectic. 199 fractionating by alkali feldspar crystallisation is constrained to move along one of these radial lines. Glassy lavas lie closer to the mean trend which can be drawn through the scatter though this may be purely fortuitous. An alkali/silica ratio of 10:90 is used to plot a quadrilateral representing the feldspar composition plane along which differentiation proceeds and this quadrilateral is bisected by the qpnrtz-feldspar cotectic (Fig. 4.12). Here all the lavas lie on the feldspar side of the cotectic, and not across it, indicating that fractionation of feldspar, only was responsible for driving the liquids towards an increasingly peralkaline composition. Pantellerites appear to be on the same trend as the comendites.

It is probable that this trend starts with a sub-peralkaline trachytic magma and the possibility of the existence of this is discussed below. Noble et. al. (1969) noted that the extremely low MgO contents of peralkaline lavas are the result of being at the end of a protracted fractionation sequence. Terceira peralkaline lavas have MgO values between 0.8-0.005 wt% which are typical of very evolved liquids of increasing peralkalinity (Carmichael and McKenzie, 1963).

4.6 Volumetric and Time Considerations 4.6 (i) Proportions of Basalt to Salic types. For Terceira there is evidence of olivine fractionation in the Cinquo Picos lavas with amphibole and/or iron controlled fractionation in the lavas and pyroclastics of Santa Barbara and the Fissure Zone. A minor amount of plagioclase fractionation in the Santa Barbara basalts may accompany and accentuate the amphibole/Fe fractionation at a late stage. Alkali feldspar fractionation in the peralkaline suite is indicated but

Plate 4.1 Cliff section on NW side of Santa Barbara looking ENE, showing many flows of mugearite and hawaiite, which it is believed make up most of the pile of this volcano. The town, far right, is Altares. 200 presents many difficulties. One problem is the derivation of sufficient quantities of parental "trachytic" magma and another is why few extrusions of composition between mugearite and comenditic traohyte have taken place. To examine how much material is involved careful volume measurements have been made on the products of all 116 eruptions that haveoccurred during the past 23,000 years.

There have been 48 basaltic eruptions (D.I. < 65) and 68 salic (D.1.>65) onesin the last 23,000 years and the total volume of new rock is a minimum of 5.25km3 (D.R.E.). Of this 0.75km3 is basaltic and 4.5km3 is salic. This volume represents only 2.5% of the 220km3 volume of the island above sea-level.

3km3 of the 5.25km3 erupted in this period have been produced by Pico Alto Volcano, which has a total volume of only 7-8km3 indicating that Pico Alto Volcano, is approximately 50-60,000 years old, if a consistent rate of salic activity is assumed.

There is evidence that the above proportions during the past 23,000 years are not markedly different from the previous few tens of thousands of years of activity (Section 2), certainly for Pico Alto Volcano and the adjacent part of the Fissure Zone. Santa Barbara Volcano over the same period may have had a higher proportion of basaltic eruptions than it did in the last 23,000 year period.

When considering volumes of ill exposed parts of the island it must be remembered that a coast section exposing a pile of basaltic lava, such as that around Santa Barbara Volcano (Plate 4.1), may be composed of lava from adventive cones and does not necessarily mean that the 201 whole sub-structure of the volcano is basaltic. Consequently, in the centre of the island salic rocks may go down to great depths in feeder dykes or perhaps even large plugs. This means that the proportions of salic to basaltic material may be larger than is obvious from the field and in fact it may be possible to maintain proportions in the order of 50:50 beneath a volcano.

In the case of Santa Barbara, however, many of the basaltic flows exposed in the cliffs can be traced from the top of the volcano and it is thought that Santa Barbara has an almost totally basaltic pile. This is supported by the gravity work of MacFarlane (1968).

The Fissure Zone must be presumed to have always been basaltic. Guilherme Moniz Volcano may have been similar to Santa Barbara but the basaltic sub-structure is not exposed. used on crude estimates 10-12.5% of the total pile of Terceira from the sea bed is exposed above sea level and the accumulations of the last 23,000 years represent no more than 0.25% of this amount. Considering this it is necessary to bear in mind the relevance of any petrologenetic schemes that are indicated by the portions of volcanoes for which there are detailed data.

If all the pile of Pico Alto is considered there is probably a 70:30 ratio of salic to basaltic in the past 50-60,000 years. Putting this in to perspective as the proportion of the whole volcano the salic volume may be late-stage differentiates of a very large volume of basic magmas. This possibility cannot be checked without borehole data. There is still, however, the problematic lack of trachytic material.

The gravity anomaly pattern on Terceira (Macfarlane, 1968) is not 202 indicative of a large volume of subjacent low density magma.. A positive Bouguer anomaly crosses the middle of the island, obviously dominated by dense basic magma under the Fissure Zone. No area of low gravity is detected under Santa Barbara but there is a low area under Guilherme Moniz caldera which may extend northwards to include Pico Alto (there were no survey stations on the latter). This may represent a magma chamber or it could just be an expression of near-surface low density, salic rocks. There are insufficient grounds to propose one source of salic magma under each of Pico Alto and Santa Barbara Volcanoes or one source for both.

, The plutonic inclusions brought up by volcanic activity are not particularly helpful since they are, on the whole, rare and include both syenites and gabbro, the former mainly from Pico Alto and the latter mainly from Santa Barbara. The syenites, similar to those from Sgo Miguel, analysed by Cann (1967), are peralkaline like the lava extrusions.

The periodicity of volume of basaltic and sane types (Fig. 2.16) shows peaks, controlled by larger volume salic eruptions occurring at less frequent intervals than basaltic eruptions. These appear to have been periods (of largely unknown lengths) when only basaltic eruptions occurred. Salic magma may therefore require a long period to build up enough concentration in a high level position to break through to the surface. The volatile content may increase during such repose periods with perhaps an increase in peralkalinity of the magma. The most frequent size of basaltic eruptions in an order of magntidue less than the most frequent size of salic eruptions (Fig. 4.13). Furthermore, the frequency of eruptions (Fig. 4.14a) plotted against a composition

203

No. SALIC

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10 8 BASALTIC 6 4 2

T r ► km3 0.0001 0001 001 1.0

3N Fig. 4.13 The volume (1m3)of salic and basaltic lavas and pyroclastios plotted against the frequency of eruptions of each size on Terceira during the past 23,000 years. No.

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20 30 40 50 60 70 8 0 90 100 T.T. Index

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T.T. Index

Fig. 4.14 a. Frequency of eruptions plotted against the Thnrnton, Tuttle Differetiation Index for Terceira during the last 23,000 years. b. Volume (km3) plotted against the Thornton-Tuttle Index, as 14a. 205

indicator (D. I.) shows a peak in the basaltic range and a larger peak in the salic range, with a negligible number in the intermediate range. When volume is plotted against the D.I. (Fig. 4.14b) the great excess of salic material erupted during the past 23,000 years becomes evident.

Considering that if 45-50% of the island above sea level (a crude maximum estimate taking into account eroded salic pyroclastics) is salic, this is still only 4-5% of the whole complex above the sea bed. Some evidence on land suggests that most of the salic material so far produced is exposed, this figure is not unreasonable for an end- differentiate suite. Chayes 0963) indicates that not more than 10% by volume of a basaltic parent is capable of differentiation (by crystal fractionation) to salic compositions. The volumes on Terceira are well within this'proportion. If, however, the 80% salic to 20% basaltic pro- poritions of the last 23,000 years have been maintained throughout the whole complex of Terceira, or even the 70% to 30% of the last 50,000 years, it becomes extremely difficult to generate this amount of salic magma by known petrological models. Such a process may be possible by extremes of partial melting in the upper mantle or even of oceanic crust. If there was such a prepondenance of salic magma throughout substantial time periods there may well be no basalts at all exposed on the surface, as an upper viscous, low, density magma would obstruct basalt extrusion. /77----

4.6 (ii) The composition gap. The real problem concerns the scarcity of rocks in the benmoreite trachyte range. While basaltic lavas were 2 6 erupted along the Fissure Zone, salic eruptions occurred contemporaneously on Santa Barbara and Pico Alto. During the past 23,000 years only 2 small volume eruptions fill the gap between the two main types (Fig. 4.14b) and this gap appears to have been maintained for a long period of the islands history. Such a gap has been recognised for many years from other oceanic islands (Daly, 1925).

One possibility is that the gap is due to a sampling bias but, as explained earlier, the past 23,000 years of activity has been sufficiently well documented to say that the gap actually exists during this period at least. It appears that only during the time of Cinquo Picos volcano, early in the island's history, there may not have been a gap (Fig. 4.7a). Perhaps the missing benmoreite-trachyte rocks are in unexposed partsof the island or at depth beneath the island. The evidence of the fault exposures of benmoreite may be compatible with this. However, there can be no comment beyond stating that xenoliths found in the various pyroclastics do not have compositions which fall into this gap.

The gap may also be due to a lack of extruded benmoreite-trachyte material, due to the higher viscosity of these compositions compared to the surrounding basalt magma, which moves more easily up available fractures. Such a gap may be the result of an efficient differentiation leaving no intermediate products (Holmes, 1936; Chayes, 1963). Or there may be a long gapbetween different groups of salic eruptions allowing differentiation to continue through to peralkaline liquids without any trachyte being produced. This last mechanism may have some foundation in the distribution of volume plotted against time (Fig. 2.16). Not all the above ideas need be independent of each other in explaining the gap. 207

1.7 1.6- • PA 1.5 SB 14 13 1. 1.1 1.0 A O Time 20,000y

Fig. 4.15 Peraikalinity Index plotted against an arbitrary time scale for the lavas and pyroclastics of Santa Barbara Volcano and Pico Alto VOlcano. 20

4.6 (iii) Composition changes with time. In the basaltic parts of Santa Barbara Volcano and along the fissure zone there appears to be no correlation of saturation state with age (Fig. 4.8b). For example, the two youngest basaltic flows along the Fissure Zone are the 1761 hawailte , which has both ne-normative and hy-normative flow units, and the Algar do Carvao basalt flow, which has 15% normative nepheline.

As discussed above, there have been peak periods of salic magma generation and during each period the volatile content and peralkalinity of the magma may increase. In fact there is a correlation of increasing peralkalinity with time (Fig. 4.15), operating on both the young volcanoes. However, sampling has not been rigorous enough to determine whether the increase is step-like, being approximately constant within a specific salic eruption 'peak', or gradual.

4.6 (iv) Discussion. Olivine and pyroxene controlled fractionation is a feasible explanation for the petrology exhibited by the oldest volcano Cinquo Picos. Despite the fact that sampling has been piecemeal a possibly continuous trend from dkali basalt through undersaturated hamaiites and mugearites to undersaturated trachytes is shown (Fig. 4.7a). One peralkaline, by-normative trachyte has been found at sea level on the SW flanks of the volcano. Olivine, feldspar and pyroxene are common in all lavas except the aphyric mugearites. It would be reasonable to propose olivine fractionation from a basanitic parent, giving the undersaturated trend seen in Fig. 4.3. This trend has been described from Skye lavas by Thompson et. al. (1972). Modal nepheline has not been observed in the Cinquo Picos lavas. A steady rate of basaltic generation may be explained by low hydrous pressure, fractionation of magma at about 5-6km. under the island, approximately the thickness of oceanic crust, which also fits the low pressure requirements of the saturated fractionation trend discussed earlier. This basalti terial is essentially being extruded up the Fissure Zone (the Terceira Rift) in various states of differentiation. A separate source must be sought for the peralkaline lavas and pyroclastics which have been 5 times more voluminous than the basaltic lavas during the past 23,000 years.

• It is theoretically possible to derive peralkaline magma from an alkali basalt parent by fractional crystallisation of feldspar (Bailey and Scheirer, 1964; Thompson and Mackenzie, 1967), but the theory has been invoked to fit models for derivation of acid rocks in but a few areas (Nash et Suswa), 1970; Ridley, 1970). However, it is a useful concept to use as a basis for examining the rocks of Terceira_ and, considering the anorogenic environment of the island, would appear logical. Coombs (1963) has suggested that peralkaline rocks in oceanic environments are associated with 'transitional' basalts, and low pressure differentiation across the thermal barrier (Yoder and Tilley's Critical Plane of Silica Undersaturation) has been invoked as an explanation. Bowen (1945) had earlier proposed an origin for peralkaline liquids by the 'plagioclase effect' of fractionation from residual liquids of intermediate compositions.

The possibility of anorthoclase controlled fractionation from a trachytic parent giving comendites and pantellerites must be thought of with some reservations. Most important is the ability of feldspar crystals to 'float' or to even move at all, in magmas of high viscosity 211, and density very similar to the crystals. Second is the restricted ability of any type of fractionation system, as we know it, to develop in high viscosity magmas. Peralkaline trachytes, such as those on 8 Terceira, may have viscosities of 10 poises, which is comparable to rhyolitic magmas. The form of the Terceira flows is comparable to those of rhyolitic lavas. Such magmas may have viscosities of 106 poises under the volcano (Flurase and McBirney, 1970) (basalts are 103-4 poises at the point of eruption).

The key to these problems may lie with the volatile content of these peralkaline magmas., Under the volcano the magma maybe more fluid because of the presence of a substantial volatile phase, most of which is lost as gas blasts in an explosive phase, often preceding lava extrusion pn Terceira. This would involve a considerable viscosity and density increase immediately prior to the eruption. If the magma is fluid enough to reach the surface it may be fluid enough to allow fractionation to take place but very little is known about such systems.

4.7 Possible Models for the Petrogeneis of Terceira

The following possibilities appear to be worthy of consideration as models to explain the observed 'gap' in volume of the lavas of trachytic composition in the late stages of Terceira.

L Differentiation from one basaltic parent [from amphibole, Fe and plagioclasejwith a change of fractionated phasesl■ to alkali feldspar at approximately a benmoreitic composition where SiO2' Al203 and alkalis predominate. Little material of trachytic composition was produced as the alkali feldspar controlled trend was efficient or had sufficient 2 1 1 time to differenting to a peralkaline composition. This model depends on the acceptance of the salic rocks of Terceira as an end-differentiate suite, but the volumetric data are not decisive upon this fact.

2. As 1., but due to the increased viscosity of the magma from benmoreitic to trachytic composition, the magma could not rise to the surface. Differentiation continued along the feldspar trend, and with increasing peralkalinity, a build-up of volatiles and a decrease in viscosity, there were explosive eruptions, often followed by lava extrusions. A decrease in viscosity from trachyte to pantellerite may be inferred from the decrease In Al203 (and to a lesser extent SiO2) with increasing petalkalinity (Fig. 4.10b). The change in viscosity between benmoreitic lavas and salic lavas is sudden and meTked, as seen by the form of the flows of each type, i.e. thin flows and short, stubby flows respectively.

3. Two parent magmas are involved, one basaltic and the other trachytic. A basalt to mugearite and benmoreite trend, controlled by fractionation of amphibole, Fe and, perhaps plagioclase, is unrelated to another trend producing peralkaline lavas from a trachytic parent magma. The basalt parent comes from the upper mantle under the Mid-Oceanic Rift (in this case a branch Rift - the Terceira Rift), while the origin of the trachytic parent may be from partial mailing of an alkali basanitic parent at upper mantle levels in the very early stages of the island's growth. By this method large volumes of trachytic may be accounted for. The lower density trachytic material was slow to rise to a high level and slow to fractionate (possibly owing to the greater viscosity of the magma). It has, during each episode in the island's history, reached a level where it was able to erupt. An extended fractionation time or () .1, (.4 I n an efficient fractionation system gave a peralkaline magma with little or no trachytic liquid left. This model would explain large amounts of peralkaline lavas with a volume not related directly to the basaltic parent magma.

4. From partial melting, or fractionation (as in 1 or 2 and 3), a high level trachytic magma chamber develops under the volcanoes. As it col- lects, and before it is able to erupt, it is moved off the Terceira Rift by crustal spreading. This effectively cuts off any source of supply of new magma and the unit fractionates until it has a high enough volatile content to erupt explosively. Pico Alto Volcano lies to the north of the main axis of the Rift and this displacement may be due to such a process considering the spreading rate of less than 0.5cm. 1 year (Krause and Watkins, 1971). Santa Barbara, which is in the early stages of a peralkaline episode, is already showing a bias of eruptive centres to the north of the axis of the Fissure Zone, which may indicate a migration of the magma chamber. Indeed Pico Alto may be analogous with the late stage of Guilherme Noniz Volcano and likewise a wholly peraikaline volcano may develop on the north side of Santa Barbara. Basalts derived from the upper mantle can still be erupted through feeding fractures along the axis of the Fissure Zone whilst peralkaline eruptions continue.

The regularity with which a Igapt appears in other oceanic islands may indicate that there must be a common petrogentic process at work which does not produce the 'intermediate' type in equal volumes to the basic and salic magmas (Holmes, 1936). The controlling factors of this process can only be determined by full-scale research into the common factors on all islands where this phenomena occurs. 0 (.1 1 :3

Of these alternatives only 3. does not comply with Coombs' (1963) contention that peralkaline lavas can be derived by fractionation of an alkali or 'transitional' basalt. However there is no direct proof of this mechanism being the controlling one, due to the 'gap' in the composition range. Alternative 4. would seem likely to produce mixed- magmas at the junction between the basalt fissure eruptions and the salic magma bodies. Mixed-magma is found on'Tercdra, e.g. in the Misterios Negros Domes, where a black and cream lava with a vesicular 'crust' occurs. Any of the above alternatives appears feasible given the data from the•major element analyses presented. Strontium isotope studies may be able to throw more light on the problem.

4A? A Brief Comparison with other Azores Islands and other Provinces.

Santa Maria, Pico, Sao Jorge, Corvo and Flores have a range of volcanics from basalts to benmoreites with no salic differentiates. Faial is mainly basaltic but has produced trachytic pumice fall deposits very recently in its history. The other three islands Graciosa, Sio Miguel and Terceira all lie on the Terceira Rift, a secondary spreading centre, transverse to the Mid-Atlantic Rift. Terceira and Graciosa are the least potassic of the Azores so far investigated.

Graciosa, which lies closest to the Mid-Atlantic Rift, has had only the youngest rocks sampled (by G.P.L. Walker and the author). They range fram basalts to comenditic trachytes with no apparent 'gap' (Fig. 4.16a). A1203 is higher than in Terceira but alkalis remain similar thus reducing the peralkalinity of the suite. K contents are the same or slightly lower than Terceira. On the island there, are 1

• • • e • • •• • • •

• • • • es •

Graciosa

Fig. 4.16 a. AFM.triangle (mpl. p.c.) for Graciosa lavas and pyroclastics (data from G.P.L. Walker and G. Borley).

Fig. 4.16 b. AFM triangle for Sgo Miguel lavas and pyroclastics (data from samples collected by G.P,L. Walker and analysed by B. Gunn). 215

abundant mugearitic and benmoreitic lavas forming thick light-grey flows. The youngest volcano, Caldeira, has produced plinian and vulcanian deposits as well as trachytic lavas and hawaiites. The vulcanian deposits contain pumice of an 'intermediate* type, which fall, compositionally, into the 'gap' area of Terceira. This pumice contains a phenocryst assemblage of feldspar, olivine, hornblende, augite and biotite; some lavas also contain this assemblage. Amphibole is a much more common mineral than in Terceira and there is an absence of anorthoclase.. Xenoliths'incIude rocks rich in amphibole, plagioclase, apatite and Fe-oxide which would indicate that the fractionation trend suggested for Terceira, may also occur on Graciosa. Essexite and syenite have also been brought up during explosive eruptions.

Sao Miguel is a large and complex island. Most samples collected have been young trachytic pumice and lava (Schminke and Weibel, 1972;

Booth, Croasdale and Walker, in press). Although there is a composition

'gap' within the youngest eruptions, it is not known whether the island as a whole shows one. Sampling is not yet adequate to draw any detailed conclusions. Volumetrically the largest amounts are in the basaltic and trachytic range.

Trachytes do occur, unlike Terceira; some are mildly peralkaline but differentiation to comendites and pantellerites is halted by a high in the most differentiated rocks, c.f. 12-14% on % of Al2 (3 (16-18% Terceira). Sao Miguel is also highly potassic, with Na/K ratios less than 50,0 in some cases and rarely above 65%. The trend (Fig. 16b) is parallel to that of Terceira but there may be less Fe enrichment in the mugearitic lavas. The suite as a whole appears tobe slightly less 216 saturated than Terceira but hy-normative mugearites and hawaiites do occur.

One important feature of the SgO Miguel salic lavas is that they may have greater viscosities than the Terceira lavas, due to an excess of A1203 and Si02. This may be reflected in the greater explosivity of trachytic eruptions from Agua da Pau volcano (Walker and Croasdale, 1971) compared with the comenditic eruptions on Terceira.

Terceira is the most peralkaline of the three islands on the Terceira Rift, and it has less numerous lavas of composition between mugearite and comendj.te than either of its neighbours. Although detailed petrological investigations have not been made on Graciosa and Sio Miguel it is reasonable to suppose that a,similar differentiation sequence exists on all three islands.

Among other islands and areas with similar petrology are St. Helena (Uker,1969), Easter Island (Bandy, 1939; P.E. Baker pers. comm.), the. Ethiopian Rift (Cole, 1968; Gibson, 1972), AscenSion (Bell, 1967; Atkins 1964) and Nandewar Volcano, Australia (Abbott, 1969).

4.9 PYroclastic Rock Analyses,

In the last 23,000 years on Terceira 1510 of the total of salic material and 45f of the basaltic total are pyroclastic rocks. There is no reason to think that the salic proportion has ever been larger and so they are not very important. 4% of the basaltic total is rather a misleading figure as it includes a large offshore tuff ring. In fact probably less than 15% of the basaltic total from eruptions on land is pyroclastic. As each basaltic eruption, with the exception of Monte Brasil, X 17

Si02 FeO Na20

AT 7e M/P • •

AT 7d • • •

!(.5;

AT 7c • • .o.• 0b0 . • AT7 • •

63 64 8 66 6 65 4 4.5 m=matr:ix P = pumice

Fig. 4.17 A section through the Lajes Ignimbrite near Lajes, Major Element variation through the flow is shown; very little changes are evident. (Data from Schminke and Weibel, 1972). I 0) is intimately associated with a lava flow, it appeared more sensible to analyse the lava. Ignimbrite forms just less than 50 of the comenditic pyroclastic total during the past 23,000 years, so analyses were considered essential and have proved wa±hwhile.

Seven anlyses from the Lajes and Angra Ignimbrites, all from different parts of the island, show only minor fluctuations within a group (Fig. 4.9). All have similar peralkinity indices and only slight variation in Na/K ratio (perhaps due to K loss). Four analyses were of the centre parts of large pumice °lasts, one from a glassy 'fiamme' and Abe other from the welded ignimbrite around it.

Schminke and Weibel (1972) have analysed five Lajes ignimbrite samples from a vertical section (Fig. 4.17). They list only minor changes in chemistry through the unit, no zonation and a great similarity between pumice and matrix. They also noted that the small size of the igmimbrites probably precludes zoning.

Two analyses of pumice from air fall deposits give mixed results. One (8185) is from a very young, fresh pumice fall deposit on Santa Barbara Volcano (member till); the composition is very close to that of the ensuing Pico do Carneiro Coul4e (S88). The other pumice from member 'B1 , is much older, more yellow and, presumably more oxidised. The analysis (8134) is the same rock type as the associated Pico da Pardelas Coul4e (S10) but compositionally they are not very similar, perhaps due to alteration of the pumice. 0 1 0 5. THE EARLY HISTORY OF TERCEIRA

The only exposures showing sequences of volcanics in the pre—Lajes Ignimbrite periods are around the coast of Terceira. The north coast section from Lajes to Vila Nova (Fig. 3.2) has already been shown to contain a number of ignimbritm which indicate that there were explosive salic eruptions early in the island's history. Likewise the section at Angra harbour gives evidence of similar activity. The many and varied compositions and styles of eruption are best shown by detailed cliff sections where the products of many eruptions are exposed (Fig. 5.1). Between these exposures are ones with only the most recent products exposed, making correlation of the old volcanics around the island more difficult. In some localities dykes are exposed cutting old pyroclastic deposits and feeding basaltic lava flows (Plate 5.1).

r------Santa' Barbara------, H I J C\..~ 1MIk3"~ ,..,.,J .. ~I'CIIJ..A ~vIo~le fr- d st C\2 ~1't....u.r.covlCe .. VWCJU\w.... w.faA.' ~J:. -,~ ,if:j~' ' '~fc.U.'G' ~: _. L 11_"'"-r ·s' ? st CI"~ I'I"-ol sc.t'io.. Lc.J<1.$ Iot";, ~.~~ ..i(. M\l5W~ lAVA. '3 f-c..~ v';'.-J;s POtP"'~~~ . ~ra.t, 4.V~ v:;.. ~a...,wb... f- nuli/<&. UO\A • fe.lIUp.o.r I~<{].x... 2...... ·d~. {.(.olo,) v ... .n p()r~n<.~wt..

Fig. 5.1. Ten seotions representing the older formations of Terceira. fu;.$,.....~nC:. For locality of sections see map in Appendix II. The sections hILI..:1O..i.itA. • are divided into groupS according to which volcano has .2.f~..,vll\.as F-~';)p.u-pl...t)';' \..~it:;.. contributed the majority of the ro~ seen in each section. 2 f-l..oW\l~.h

A-,,~ ... jc:.~v, ~fl..o""u~~

Ap~..\(. h~ti4.JAvo.. ~~ 'P~~t!.. :l..f<..o",,~

SCt>r~ <.or...e Ioidt. IwlJ.U . ~bc1la.x SOt"' ~ At..». fl.ol4. At~"",vai~ b(u~~~ 222

Plate 5.1 Olivine hawaiite dyke cutting old surtseyan tuffs, Baia da Mina. Dyke is 45cm. wide; the lava flow it feeds can be seen above. This dyke is on the old SE part of the Fissure Zone. 6. Summary:

The important conclusions to be drawn from this study of

Terceira axe contained at the end of the appropriate sections. The

tripartite approach to the volcanology of this island shows some of

the great variations in composition and eruptive style to be found on

the Mid-Atlantic Ridge. The following points are also worth noting

from the results presented above.

1. Terceira, lying.on a branch rift, some 250km. from the supposed position of the axis of the Mid-Atlantic Rift has had several, styles of eruption during its history. For the most part these eruptions of

different styles have occurred contemporaneously and have produced a

wide variety of volcanic rocks at all times. The most prominent styles

are: a) strombolian basaltic eruptions, producing scoria cones, scoria

fall deposits and lava flows; b) salic lava effusion producing coulees

and domes; c) sub-plinian, explosive salic eruptions; d) salic pyro-

clastic-flow eruptions.

Other islands in the Azores, notably Faial, Graciosa and Sao Miguel

also show this pattern of varying eruptive style, but none has a com-

parable range of volcanic rocks and abundance of each type of Terceira.

The other islands tend to have only one or two prominent eruptive styles;

for instance Sao Miguel has mainly strombolian and plinian eruptions with

some salic lava extrusions and Fatal has strombolian and plinian

eruptions. All islands have a number of surtseyan basaltic eruptions

from offshore eruption sites.

2. The nine sub-plinian pumice deposits have helped to characterise this type about which there have been little quantitative data published. It 22 4 is now known that sub-plinian eruptions in the Azores are smaller than plinian eruptions in volume as well as in dispersal. The deposits are often internally stratified and have more dense juvenile fragments than plinian deposits. They represent a regime of lower-power explosive eruptions probably with a laer gas content in the magma than plinian types.

Only by a combination of chemical and volumetric data can the bi- mcidality of composition which exists in the rocks .of Terceiralle eatab lished. A knowledge of the age relations of the rocks also shows that such a bimodality has existed during a large part of the island's history. While basaltic material was being extruded along the line of the Terceira Rift, salic material was erupted from the central vent volcanoes. three of the'volcanoes salic eruptions appear to be a late-stage features but one volcanic edifice is completely salic.

Perhaps a totally basaltic volcano first appeared above seao.level to initiate Terceira, or perhaps some of the first sub-aerial eruptions were salic. The weight of evidence presented by the. Azores group as a whole indicates that the former is the more likely.

The chemical data presented shows that this Fad-Oceanic environment is capable of producing peralkaline magma, just as continental environs ments are, and, in substantial volmnes. However, the proportion of a submarine volcanic pile exposed above sea-level is very small and may not be representative of the whole.

4. Cinquo Picos, the first volcano which formed Terceira grew as a basaltic stratavolcano then entered a period of explosive salio eruptions and caldera collapse. Guilherme Moniz Volcano grew adjacent to Cinquo

Picos, as, probably another basaltic volcano, while Cinquo Picos was producing late-stage salic eruptions. Atthe same time, the Fissure Zone was erupting fissure-type basalt flows across the floor of Cinquo Picos

Volcano. Guilherme Moniz also underwent a prominent salic phase and caldera collapse. This picture has now been repeated with the growth of Santa Barbara Volcano, although it is not as close to Guilherme Moniz as that volcano is to Cinquo Picos. The active part of the Fissure Zone has shifted from Cinquo Picos to occupy what is now the centre of the island.

Santa Barbara grew into a large, basaltic stato-volcano before caldera collapse and the start of salic eruptions. Pico Alto Volcano grew at the same time as Santa Barbara in a position on the northern flanks of Guilherme Moniz and may, in fact, be a unique type of parasitic, salic volcano complete with its own Caldera.

5. The basaltic erutpions along the Fissure Zone on Terceira appear to be intimately associated with crustal spreading on a small scale. It can be envisaged that crustal spreading along the Mid-Atlantic Rift would have a similar surface expression to that in the Fissure Zone, but on a larger scale. The presence of salic magma and the associated explosive eruptions may not play a part in crustal spreading but may be merely the manifest- ation of an uppermost zone of new magma generation in the upper mantle and crust.

Acknowledgements 11 "4

This study of Terceira was initiated by Dr. G.P.L. Walker and I am pleased to acknowledge his invaluable help, supervision, criticisms during the preparation of this manuscript and the loan of samples and photographs. I would also like to thank the following for helpful discussion, criticism and for reading pasts of the manuscript: Dr. B. Booth, Barbara Dickinson, R.S.J. Sparks; Dr. R. Thompson and Dr. L. Wilson.

During analytical work at Imperial College, I received valuable assistance from Dr. Gloria Borley, G. Bullen, P. Watkins and Mr. R. Curtis assisted duringX-ray diffraction determations of alkali feldspars. G. Witty performed an analysis of a feldspar on the electron-probe microanalyser.

I would like to thank Professor B. Gunn for 45 major element analyses completed at the University' de Montfsal using samples collected by Professor Gunn and the author, and for his useful comments in the field.

C14 dates an carbon from Terceira were kindly made by Professor F.W. 14 Shotton at the C dating unit at the University of Birmingham.

The Servicos Geologicos de Portugal gave permission to proceed with work in the Azores and I would like to thank Eng. Dr. P. Moitinho de Almeida and. Dr. G. Zbwysewski for help in Lisbon and for procuring aerial photographs from the Forces Areas de Portugal. In Lisbon I have also received much help from Dr. V.H. Forjaz and from Sr. J. CalState at the British Embassy.

On Terceira, I received assistance and hospitality from Ten. Col. J. Agostinho, Eng. Corainiche, Sr. AntOnio Carve°, Naroiso M. Mesquite,

Roberto da Silva Lourenco, Pedro Bettencourt, M. Guiomar Azevedo Lima, (...,22 7 Henrique Braz and many others.

Other acknowledgements must go to Mr. A. Hill, Mrs. Laura Barker and her assistants in the Watts Library at'Imperial College, Mrs. V. Self, Mr. T. Mulane, and my wife, Lesley. Christine McGrath typed the manuscript.

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Addendum. Aramaki, S. and Yamasaki, M. "Pyroclastic flows in Japan", Bull. 1963, Volcanol., 26, Series II,p 89-100. Tsuya, H., 1955, Geological and petrological studies of volcano Fuji,5. On the 17Q7 eruption of volcano Fuji," Bull. Earthauake Res. Inst. Tokyo, 33,D 341-383. APPENDIX I

MAJOR ELEMENT ANALYSES, CIPW NORMS,THORNTON TuTTLE DIFFERENTIATION INDICES (D.I.) AND PERALKALINITY INDICES (P.I.) OF TERCEIRA LAVAS AND PYROCLASTICS

Map: Sample localities of analysed specimens and localities of sections in Fig. 5.1. (A-J).

Table: Major element analyses.

Key: Description of analysed samples and localities. 159 S 3 T58)T2191 NI 520A • T2190 T*82 T2186 S13 S?3 •S8 8 T81 5196 TS8 S08 SpI0 .17 7 oS134 SI 2 T 76 •S185 •T 57 528 S,;1 T75 .1-56. SV6 S97 1.2068 - 12098 330 S242S54 T80. 1-2 06 7 T8;*".-S218 50.7 • .T71 86•,..T84 ;5 5 79 S2 05* .T 72 •T52 87 k V 8 .T73 Tu3 •S506 557 S61 150 .T6 9 586 S38 Tgl T70 •• 5.4

12193 .T54 T .T53 66 S? .44 51 65 T4 s' B T6 562•• 538,149 55 T45 63 5110 62 11 61 94 T41 142 53 S 09 S 35 47 8 208 510 .56 208F 9 5131 52

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580. Aphyric basalt. Cinquo Picos Volcano. Serra do Santiago, near Air Base. T55. Aphyric mugearite. Fissure Zone. Pico da Bagacina. T78. Hawaiite. Cinquo Picos Volcano. E rim of caldera near Radar station. T47.Hawaiite. Cinquo Picos Volcano. E rim of caldera at Pendinhos. S66.Basalt. Fissure Zone. Algar do Carvito lava in G. Moniz Caldera. T50.Basalt. Fissure Zone. Algar do Carvao lava in G. Moniz,Caldera, B of S66, S57aBasalt. Fissure Zone. Vareiras lava flow, E of Bagacina. Sl. Basalt. Fissure Zone. Porto Martins lava at Ponta4egra. E20.Basalt, Fissure Zone. Pico Gordo lava at Porto do Biscoitos. T59, as 520. . 510. Basalt. Fissure Zone. Pico doliefugio, old SE part of Fissure. 54. Hawaiite. Cinquo Picas Volcano. Pico das Favas lava. S11. Basalt. Cinquo Picos Volcano. S of Baia do Castelo. T56. Hawaiite. Fissure Zone. Galiarte lava by side of road at Pico Gordo. T79. Hawaiite. Cinquo Picos Volcano. Fault scarp at Praia da Vitoria. T54. Basalt. Fissure Zone. Vareiras lava flow, near Matela. S5. Ankaramite. Cinquo Picos Volcano. Cruzes lava at Salgueiros. 56. Basalt. Cinquo Picos Volcano. Dyke at Baia da Mina. 59. Hawaiite. Cinquo Picas Volcano. Pico da Faleiras lava, near F. Bastardo. S2. Basalt. Fissure Zone. Pico das Contendas lava, at Baia das Caninas. T83...Basalt. Fissure ZOne. Lava flow on Bagacina- Sta,13arbara road. T48. Basalt. Fissure Zone. Floor of Cinquo, Picas Caldera, old SE parpigure. S50. Hawaiite. Fissure Zone. Pica da Bagacina lava, 2 km SW of Pico. T43. Basalt. Fissure Zone. Ribeira da Testa, flow from. old. SE part of. Fissure. T45. Ag S5. T2191. Basalt. Fissure Zone. Algar do Carvao lava at Achada. T2191. As S20. T66. Hawaiite. Santa Barbara Volcano. Sta. Barbara village, near parto,2nd T61.Hawaiite. Santa Barbara Volcano. Porto do Cinquo Ribeiras. flow from base T62.As T61, one lava flow above in cliff section. S24.Apbyric hawaiite. Cinquo Picos Volcano. E side of Grota do Vale. T68. Hawaiite. Sta. Barbara Volcano. Near Queimada, small quarry. T40. As S24, one lava flow below. S110.Aphyric mugearite. S.Barbara Volcano.Near Sao Mateus, canada do Pombal. 540. Hawaiite. Fissure Zone. Eastern flow unit of 1761 lava flow, near Pags. T58. As T68, from Porto do Biscoitos. T81. Hawaiite. Fissure Zone ? Porto do Vila Nova, below Lajas Ignimbrite. T57. Mugearite. Fissure Zone. Western flow unit of 1761 lava flow, near source T46. Hawaiite. Cinquo Picos. Pico do Martini lava at Salgueiros. 535. Basalt. Fissure Zone. Lava flow on S coast, W, of Vila Maria. T63. As T62, 3rd lava flow up in cliff section. T67. As T66, 3rd lava flow from base of cliff.. T2098. As T57, central flow unit. 5210. Hawaiite. S. Barbara Volcano. Road cut by Casa da Agua. T2069. As S210. T65. As T67, lowest lava flow in cliff section. T64. As T63, top lava flow in cliff section. T73. Mugearite. S. Barbara Volcano. 300m above Pico Negro on summit road.' 597. Mugearite. S. Barbara volcano. lava beneath oldest pumice deposit,Serreta T71.Mugearite. S. Barbara Volcano. 100m below summit of volcano, on roadside. T2186. Hawaiite. S. Barbara Volcano. Cliff top below Ponta do Raminho. T70. hugearite. S.Barbara Volcano. Below S210 at Casa da Agua. T87. Benmoreite. Fissure Zone. Lava of event 'w', 2 of Pico da Canseia. T80. Mugearite. Oinouo Picos Volcano. One lava flow above T79. KEY (contd).

S205. Mugearite. S.Barbara Volcano. Summit road, by JUnta building.

T72. Mugearite. As 5205 but 300m higher summit road. • T42. Trachyte. Cinquo Picos Volcano. Feteira, 6.3 km E of Angra on Main road. S3. Benmoreite. Cinquo Picos Volcano.Pico dos Canos on E coast. T2067. Trachyte. S.Barbara Volcano. Base of caldera wall in SW side. S86. Benmoreite. Fissure Zone. Pico da Falta, on Bagacina- S.Barbara road. T86. Benthoreite: Fissure Zone. Fault scarp SW of Pico da Cansela. 58. Trachyte. Cinquo Picos Volcano. Baia da Mina, N side. $54. Trachyte. Fissure Zone/S.Barbara. 'Plug' in Misterios..Negros camendite domes. T44. Trachyte. Cinquo Picos Volcano. Ribiera Seca, 100m below road. T41: irachyte. .Cdnquo Picos Volcano. Road cut at Ladeira Grande. S7. As T44, from quarry above Ribeira Seca village. 5134. Comenditic trachyte. Pico Alto VolcanO. Pumice clast from pumice deposit T49. Comenditic trachyte. G.Moniz Volcano. Vale do Linhares quarry. 'B'. T51. Pantelleritic trachyte. G.Moniz Volcano. Caldera wall at Cabrita. S47.COmenditic trachyte. Cinquo Picos Volcano. Cliffs E of Angra. 5131.Comenditic trachyte. Pico Alto Volcano. Pumice from Lajas Ignimbrite, „ Porto Judeu. S13. Comenditic trachyte. Pico Alto Volcano. " " Quatro Ribeiras. T76.Pantelleritic trachyte. S.Barbara Volcano. Faro' da Serreta. 5209. Comenditic trachyte. Pico Alto Volcano. Pumice from Lajas Ignimbrite, Sgp MateUs. S62. Comenditic trachyte. Pico Alto Volcano. Pumice from Angra ignimbrite, Linhares Quarry. $196. Comenditic trachyte. Pico Alto Volcano. Pumice from Lajas ign., Lajes.

T84. Pantelleritic trachyte. S.Barbara Volcano. Misterios Negros domes, W side

T85. As T84, easternmost dome. T82.Pantelleritic trachyte. Pico Alto Volcano. Pico Alto II flow at Ponta Velha. -T77. Pantelleritic trachyte. Santa Barbara Volcano; Pico do Carneiro flow.

T2190. As T77, different flow unit. S208. Comenditic trachyte. Pico Alto Volcano. Matrix of Lajes Ign.,S. Mateus.

S208f. ' Fiamme enclosed by 5208. T53. Comenditic trachyte., G.Moniz Volcano. Matela Quarry. S51. Comenditic trachyte. S.Barbara Volcano. Ponta Velha flow , near Queimada. S93. Comenditic trachyte. Pico Alto Volcano. One of large blocks in debris flow near Biscoitos. S38. As T 49. T75.Pantelleritic trachyte. S.Barbara Volcano. Serreta dome.

S58.- As T51, caldera wall at Rosto.

508. As 393, different block. S010. Comenditic trachyte. Pico Alto Volcano. Pico da Pardelas, by roadside.

5136. Pantellerite. Pico Alto Volcano. Pico das Rossas. S012. Pantellerite. Pico Alto Volcano. Lavalcal coulee.

T52. Pantellerite. Pico Alto Volcano. Enxofre coulee. 5230. Pantellerite Pico Alto Volcano. Dome Ni; of Algar do Carve.° scoria cone.

$73,. Pantellerite. Pico Alto Volcano. Terra. Brava coulee. S07. As T52. P2068. Comendite. Santa Barbara. Volcano.. Coulee in south of caldera. S185. Comendite. Santa Barbara Volcano. Pumice from pumice deposit 'HI Serreta S88. Comendite. Santa Barbara Volcano. Obsidian from Pico do Carneiro coulee. 398. Comendite. Santa Barbara Volcano. Serreta dome. APPENDIX II

SAMPLES INVESTIGATED BY GRANDIOMETBIC ANALYSIS.

A.Ignimbrites, ground surge deposits and air fall-deposits associated with the ignimbrites. B.Pumice-fall deposits. C.Others including surtseyan tuffs, debris flows, avalanche deposits.

Samples not prefixed IS1 were collected by G.P.L.Vialker, 1967-70.

Grain-sizes are given following the phi (4) scale of Krumbein, where 0 is a logarithmic transformation of grain- size based on -log2mm, i.e. lmm = 04; sizes less than lmm are positive yl values and those greater than lmm are negative values. Samaes were hand-seived, to avoid breakage of delicate pumice using Endecotts Test Sieves covering,therange 32mm (-50) to less than 1/16mm (+5$) by 10 intervals. 'Incipiently welded samples were dissaggregated by hand before sieving. The material held by each sieve,and by the less than 1/16mm pan, was weighed on a Mettler automatic balance (error + - 1g0). The weight in gm in each grade was added and the weight percentage of the total in each grade was calculated. Frequency curves of the weight percent retained by each sieve grade and standard cumulative curves were plotted for each sample. Two parameters, those of Inman (1952), were chosen to summarise the features of each cumulative curve; median diameter Md0 (= phi valueetwhich the cumulative curve crosses the 50,g'line) and a sorting coefficient 00 (= 084 - 016 / 2) which is an expression of the slope of the cumulative curve and therefore of the degree of sorting. Pumice, crystal and lithic components of the samples were separated by hand-picking in the grades coarser than 4mm. Grades between 4 and 1/8mm (-2 to 3$) were water-separated (most pumice floats leaving the 2 dense components). Grades finer than 1/8mm are not separated into components. If a sample. is large a 1/2, 1/4 or 1/8 split is taken, using a sample splitter. Frequency and cumulative curves for each component can also be plotted.

The estimated error for a 1/8 split is +5% and larger splits have less error. The dense components are separated by hand- picking of a split under a binocular microscope. Separation intouthe 3 components is difficult.and inaccurate,by these _ran-mechanised methods, below 1/8mm (3$) and errors are estimated to be greater than 10. Usually, except in some ignimbrite samples, the grades coarser than 1/8mm comprise more than 84% of the total sample. A . N°. SAMPLE. N°. DESc.-KIPT10N LA YDS DA P0511164 N DEPOSIT Md... crO REMARKS A4-31) CHARAcreAls-rics - I. 84.6 I , -- o.9 I • 5 2. 847 Lajes /16,tz.,b:,G, Za ' -1. 3 2.25 3. 848 nem.- Whx. nova- 2b 0.3 3.';< - 4, $57 Les 1.0 ,,...:..-.1,:st... 2.a . -1.s 1. .3 5. 858 cot Ci4,caro kitatifas, 2.b o.3 3 - 2- 6. I el) 21 P 0.1+ 5. 0 1.1;34 % ert (.40.,se, ` t_.0..der PI...lt--a' - 7 S131 ® II Kt...., 11,;(r..d. 2. 1, 0.5 3.1 ii,3 S. o.i. ptpe a. tub Paler' 21:. -ca. 17 1- 4t. q • Poch) LAW., . C 0 2-o. o.3 2.9 i 0. 2•-•J;1/4- Z . 1 7-.5 u. 6.) 1.s 2.1 Jz. 60 L-ASe.% 7-onti..,Alvte, 21,- i3. giv, 3 0_3 ZS S235 do 21, 0.0 3 .8 14.. 0..t. Alvah &. 2.a -0.3- 1 - b )5. (Y> 1 . 0.3 I. b 16. 8 55 Lies Tzp,,,....,z,t.',,t.... , vaw il• vet 2.o.. --0.8 1.7. I?. 7.2. 2b -0.2. 4-9 ii. 74- 2b 0.4. 3.7 II i 19. Laies Ityliwtbi..t. 0...t 2b 0.3 3.8 incipient we-Latta, 26. 113 2.0. 0.2 21. Sao M oreus. 3.4. 4.5 2..a I • b 2.1. )14. / :7- P; pe im. tater 2o... -o.6 L $ 23, 112. 1. 2.5 2.8 24. 6 Pipe. r'rt 14.ye...- 230 -D. 2 2.2.. - g..9. S 75 '"Chi..- Lades -ZT.,:...,104te-, 0 . 2 1- i CI,Z4 v....0 Ncos CAckeva., . 26. S45 I. Laies .1.-.14,....,d,;,- ..t... c.. 1 '- 0.4 l.6 $514. 2?. Czazt.P-i ea. oLas /..z.ies. 2.0. 1.5 2.? 2S. S2206) I 1.-•xes Il om+.04.■;,r... 0.,t. 2 b P -0.7- 4-3 1...Z5u %, A tooate Poosice, z9. (i0 0. OD CIO Gi..t.:..-1-ro 12...,be;r.a.s . 1 2.0 30. 1 -2.4 2.4. Cvounal 1011Z 61443 vali/OL 1 31. S233 -2. b 2.4. 32. 5210 GS 3 1.0 1. 1 • 35_ 6i) Lades igva.46%.ta., 26 -0-5 3.7. (iii) 070,.;3, „ $4- nom- /...o..e$. 2i.P 0.-f I. 6 i Lut.g. puslictA e4ot A4c&tdeol. 35. S166 co 1 0.2 1.6 i - 3E, c) 2o. (i)i) 0. 0 3.0 37. 2b -0.1 2..4 sg, 532- i-aie.4 iii..;..,6:4,1;-,6"-"itkvo 2.,,, Pico 1 G.X.Le.1.0.. 2.3 . 2.0

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