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 mod- erate dispersal areas and often show internal stratification. The ignim- brites 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.