Later Stages of Volcanic Evolution of La Palma, Canary Islands: Rift Evolution, Giant Landslides, and the Genesis of the Caldera De Taburiente

Home , El Hierro

Later stages of volcanic evolution of La Palma, Canary Islands: Rift evolution, giant landslides, and the genesis of the Caldera de Taburiente

Juan Carlos Carracedo* Estación Volcanológica de Canarias, IPNA-CSIC, P.O. Box 195, 38206, La Laguna, Tenerife, Spain, and Department of Geography and Geology, Cheltenham and Gloucester College of Higher Education, United Kingdom Simon J. Day Department of Geography and Geology, Cheltenham and Gloucester College of Higher Education, United Kingdom, and Greig Fester Centre for Hazards Research, University College London, WCIE GBT United Kingdom Hervé Guillou† Laboratoire des Sciences du Climat et de l’Environnement, CNRS-CEA, F-91198 Gif-sur-Yvette, France Philip Gravestock Department of Geography and Geology, Cheltenham and Gloucester College of Higher Education, United Kingdom

ABSTRACT bre Vieja volcano. Interactions between erup- lowed by a period of quiescence and deep ero- tion rates, coastal erosion, and glacio-eustatic sion (erosional gap), which in turn is followed by The islands of La Palma and El Hierro form sea level change allow stratigraphic subdivi- a posterosional stage of activity (Fig. 1A). the western end of the hotspot-induced Canary sion of the edifice. The structure of the volcano In the Hawaiian Islands, Hawaii, the last is- Islands chain. Both islands are at present in the has been controlled for most of its history by a land to be formed and currently the most active, earliest and fastest shield-building stage of rift system with a clearly dominant north- is the most intensely studied. In contrast, little ge- growth and show many similarities with the south and less-distinct north-east and north- ological information has been available for the Hawaiian Islands. La Palma shows two very west volcanic rift zones. The rift reorganiza- youngest and most active of the Canary Islands, distinct phases of volcanic construction: (1) a tion of the volcano to a single north-south rift La Palma and El Hierro, which are commonly re- Pliocene submarine volcanic and intrusive se- since 7 ka and the opening of faults during the ferred to as the lesser Canaries. However, it is ries, interpreted as an uplifted seamount at 1949 eruption probably reflect increasing in- precisely in these youngest islands where it is least 1500 m above present sea level, and (2) a stability of the west flank of the volcano. possible to observe features critical to the under- subaerial volcanic series erupted in the past 2 standing of the genesis and development of the m.y. Taburiente volcano initially formed the INTRODUCTION AND archipelago, which are difficult to identify in the northern part of the island and then extended GEOLOGICAL FRAMEWORK older, posterosional islands. to the south, forming a ridge (Cumbre Nueva This trend has changed in the past few years. Ridge) that was partially destroyed about The Canary Island archipelago is one of the There has been a spectacular increase in the 560 ka by a giant lateral collapse (the Cumbre most extensively studied groups of oceanic is- number of published radiometric ages for the Nueva collapse), possibly involving 180–200 lands in the world. This archipelago is close to western islands (Fig. 1B): from 4, for El Hierro, km3 of subaerial volcanic material. The north- the passive continental margin of northwest and 8, for La Palma (Abdel-Monem et al., 1972), west boundary of the Cumbre Nueva collapse Africa and has developed over the past 30 m.y. to 30 and 70, respectively, at present (Staudigel formed a vertical scarp that, enlarged by head- as a result of the slow east to west movement of et al., 1986; Ancochea et al., 1994; Guillou et al., ward erosion, formed the spectacular depres- a mantle hotspot (Holik et al., 1991; Hoernle 1996, 1997). The precision of these ages has sion of Caldera de Taburiente. Cumbre Vieja et al., 1991; Canas et al., 1995; Carracedo, 1996; been greatly improved (Guillou et al., 1996, volcano, a north-south elongated rift, forms Carracedo et al., 1997a, 1998). The stages of de- 1997) by using samples collected in stratigraphic the southern half of La Palma and constitutes velopment of the Canary Islands show many sequences, separating out microcrystalline its last stage of growth, including all eruptive similarities to other hotspot-related island groundmass for K and Ar analysis and using an activity in the past 125 k.y. Detailed field ob- groups, such as the Hawaiian Islands and the unspiked K-Ar technique (Cassignol et al., 1978) servations, mapping, and high-precision radio- Réunion group (Carracedo et al., 1997a). to determine the isotopic composition of Ar and metric dating have allowed reconstruction of Quaternary igneous activity in the Canary Is- Ar content with a precision of 0.4% ( ± 2σ). The the growth and structural changes of the Cum- lands is concentrated at the western end of the results are then cross-correlated with the magne- archipelago, close to the present-day location of tostratigraphy defined by field and laboratory measurements and with stratigraphic constraints *E-mail: [email protected]. the inferred hotspot. Individual islands in the Ca- †Corresponding author; e-mail: herve.guillou@cfr naries, as in Hawaii, are characterized by initial from onshore and offshore mapping. .cnrs-gif.fr. rapid growth (the shield-building stage), fol- Recent sidescan sonar (GLORIA and TOBI)

GSA Bulletin; May 1999; v. 111; no. 5; p. 755–768; 11 figures.

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Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/111/5/755/3383233/i0016-7606-111-5-755.pdf by guest on 28 September 2021 Figure 1. Age relationships in the Canary Islands. (A) Oldest dated subaerial volcanism (in Ma). The youngest islands (in black) are in the shield-building stage of growth. (B) Radiometric ages obtained from the different Canarian Islands. Geochrono- logical data for the western islands have improved dramati- cally; about 100 of the 450 ages determined from volcanic rocks of the Canary Islands correspond to La Palma and El Hierro.

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and swath bathymetric mapping around the western Canary Islands (Holcomb and Searle, 1991; Masson and Watts, 1995; Masson, 1996) has demonstrated the importance of giant lateral collapses in the evolution of these islands, corrobo- rating evidence obtained in the contemporary onshore studies. In this work we present the results of detailed volcanological and structural field observations of the last stages of development of the Taburi- ente volcano (the Cumbre Nueva Ridge) and the Cumbre Vieja volcano. These observations and 31 new (Guillou et al., 1997) high-precision radiometric ages (29 K-Ar and 2 14C) allow us to reconstruct the relevant events in the most recent stages of evolution of the island of La Palma. The Caldera de Taburiente is the most spectacular topographic feature of La Palma, a 15-km- long and 6-km-wide depression, to 2 km deep, with precipitous bounding cliffs on most sides and a deeply dissected floor. Charles Lyell (1855) considered Caldera de Taburiente as the prototypical erosion caldera. The caldera, as it is named, supplied Lyell with the term “caldera,” to which he gave a general application. However, the genetic meaning of the word “caldera” has changed so completely in more recent years, we consider the claim of the Caldera de Taburiente to be the type caldera to have lapsed. Caldera de Taburiente is not an erosion caldera but a large depression initiated by a giant gravitational collapse later enlarged by retrogressive erosion. Herein, we refer to it by its proper name and not as “the caldera,” which is a practice that causes confusion.

GEOLOGY OF LA PALMA

The island of La Palma is the second-youngest and westernmost island of the Canary Island Archipelago (Fig. 1, A and B). La Palma is currently in the shield-building stage of growth and is the fastest-growing island in the archipelago. Recent activity is concentrated on the southern part of the island, at Cumbre Vieja volcano, where six historic (post A.D. 1500) eruptions, including the last one in 1971, were located. Staudigel (1981) and Staudigel and Schmincke (1984) defined two main geologic units: (1) a Pliocene submarine and plutonic basement complex, exposed in the walls of the Caldera de Figure 2. Sketch of the island of La Palma showing the main geologic units and features re- Taburiente (to 1500 m above present sea level) and ferred to in the text. For latitude and longitude, see Figure 1. interpreted by these authors as an uplifted seamount; and (2) subaerial lavas that uncon- formably overlie the seamount. The subaerial vol- of La Palma. More specifically, we examine the GEOCHRONOLOGY canic rocks compose three successive volcanoes period from the last collapse of the Taburiente (Fig. 2): Taburiente volcano, Bejenado volcano, volcano to the growth of Cumbre Vieja volcano. There have been 29 new K-Ar ages deter- and Cumbre Vieja volcano. The most recent events, which indicate incipient mined for lavas of Taburiente volcano (853 ± 10 In this work we consider the subaerial stage of instability of the western flank of Cumbre Vieja, ka to 566 ± 8 ka) and for basic lavas and phono- growth and partial destruction by lateral collapse will be considered in a separate paper. lite domes of Cumbre Vieja volcano (123 ± 3 ka

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to 1.04 ± 09 ka). These ages were obtained using concerned with, however, was largely assigned top of the sequence in the Cumbre Nueva escarp- an unspiked technique on crystalline groundmass by Ancochea et al. (1994) to a separate volcano ment, implying that this thickness of lavas accu- separates (Guillou et al., 1996, 1998). One of the that they considered to have developed on the mulated rapidly over a period of 220 k.y. youngest ages (4 ka) was determined for a lava southern flank of Taburiente volcano between 0.8 A second feature in favor of abandoning the flow from Volcán Fuego in the Cumbre Vieja by and 0.7 Ma. The rocks of this volcano, as defined two series proposed by Ancochea et al. (1994) is K/Ar techniques. This age was tested by compar- by these authors, are particularly well exposed on that there is no volcano-wide unconformity or ing it to 14C determinations in two different labo- the Cumbre Nueva Ridge (named after the pro- structural discordance that may be used to sepa- ratories for a pine tree burned and engulfed by posed volcanic edifice). However, our work indi- rate them: such a discordance might be expected the same flow, and was found to be consistent cates that the division of this sequence into even if they were contemporaneous, because with these independently determined ages. De- Taburiente and Cumbre Nueva series may be an lavas would be flowing in different directions tails and discussion of these ages may be found artificial one, although a definitive conclusion from the two volcanic centers. Dikes exposed in in Guillou et al.(1998). However, the ages of all awaits detailed lithologic and structural mapping the Cumbre Nueva scarp, discussed in the fol- samples are consistent with their stratigraphic of the rest of Taburiente volcano. lowing, and mapped by Ancochea et al. (1994), positions as determined during the course of Our radiometric dating work and field map- trend northward toward the structural center of mapping discussed here, and, most notably in the ping of magnetic reversals provides evidence in Taburiente volcano, as defined by dike swarms in Taburiente samples that straddle the position of favor of abandoning the two series proposed by the volcano that are of similar age (Ancochea the Matuyama-Brunhes magnetic reversal, with Ancochea et al. (1994). These data indicate that et al., 1994). There is therefore no evidence for their magnetic polarities. rocks younger than the Matuyama-Brunhes the existence of structurally distinct but contem- These unspiked groundmass separate ages are boundary are widespread, although not of great poraneously active volcanoes in this stage of de- in clear conflict with previously published thickness, in the northern, western, and eastern velopment of La Palma comparable to the coex- whole-rock determinations (Abdel-Monem sectors of Taburiente volcano. Ancochea et al. isting Mauna Loa and Kilauea volcanoes active et al., 1972; Ancochea et al., 1994). The reason (1994) did not employ magnetostratigraphic on Hawaii today. for this is that spiked K-Ar dating of whole-rock methods in their work, but it appears that the base We therefore recommend that the term “Cum- samples from young lavas (<1 Ma) shows errors of their Cumbre Nueva series is close to, or even bre Nueva Volcano” be discontinued, although that often give older ages. Spiked ages from the below, the Matuyama-Brunhes boundary. The when detailed mapping of Taburiente volcano is young volcanic rocks of La Palma have similar southern region of Taburiente volcano was most sufficiently complete, allowing other volcanic rift problems to those determined for rocks from El active in the latter stages of its development, but zones to be defined, it will be appropriate to dis- Hierro (Guillou et al, 1996; Day et al., 1997): contemporaneous volcanic activity occurred in tinguish a southern Cumbre Nueva volcanic rift they lack sufficient precision and reproducibility other parts of the volcano. zone. As noted here, the “Cumbre Nueva rift for stratigraphic determinations and correlations Nine new ages, from 853 ± 10 ka to 566 ± 8 ka, zone” was the most active area of the volcano in and, therefore, will not be used here. were determined in lavas of the upper Taburiente the later stages of its development and was the re- Magnetic polarities were also used to make volcano (Guillou et al., 1998). These samples are gion affected by the subsequent collapse of its stratigraphic correlations. Paleomagnetic rever- from stratigraphic sequences in the western sec- southern flank, which we define below as the sals characterizing the upper Matuyama and tion of the wall of the Caldera de Taburiente near Cumbre Nueva collapse. A prominent product of Brunhes epochs have been identified in the El Time and in the west-facing scarp of the Cum- this collapse is the topographic feature of the Taburiente edifice using portable fluxgate mag- bre Nueva topographic ridge (Fig. 3A). The Cumbre Nueva escarpment. netometers. Oriented samples from polarity units Matuyama-Brunhes reversal was also mapped in were analyzed and tested in the laboratory. The these areas and other suitable sections using Cumbre Nueva Giant Lateral Collapse Blake (117 to 111 ka, Zhu et al., 1994) and portable magnetometers. The topmost reversed and the Sequence Filling the Collapse Laschamp (42 ka, Jacobs, 1994) events of re- polarity (Matuyama) lavas give consistent ages of Embayment verse polarity were not detected in the younger 853 ± 10 ka, 834 ± 12 ka and 833 ± 11 ka, lava outcrops (<130 ka) of Cumbre Vieja vol- whereas the oldest normal-polarity (Brunhes) Several debris avalanche units have been iden- cano, probably because the durations of these lavas are 770 ± 11 ka and 734 ± 8 ka (see tified at Taburiente volcano by Ancochea et al. geomagnetic excursions were too brief to be Fig. 3A). The age of the Matuyama-Brunhes (1994), some of which formed, according to these recorded by the episodic Cumbre Vieja volcan- boundary has been established as 778.7 ± 1.9 ka authors, during the destruction of the edifice de- ism. However, the evolution of the coast in rela- (Singer and Pringle, 1996); therefore, although veloped by early-stage volcanism. Most of these tion to the glacio-eustatic sea-level change in the the reversal may coincide with a brief hiatus in structures are buried within the Taburiente edifice, last glacial maximum makes it possible to define activity in the areas from which samples were but the most recent debris-avalanche unit forms a stratigraphic units by their relationship to the collected for radiometric dating, there is no evi- prominent topographic embayment in the center coastal cliffs that have developed around the vol- dence for a significant break in activity coincident of La Palma. The walls of this embayment have cano (discussed in the following). with the reversal. Thus, although the paleomag- undergone subsequent erosion but are still readily netic feature of the Matuyama-Brunhes reversal, identifiable as components of a lateral collapse LATE-STAGE SEQUENCES OF readily identified in the field by the use of structure by their steep inclinations and arcuate or TABURIENTE VOLCANO portable magnetometers, is a useful tool in map- linear geometries. The southern part of the struc- ping Taburiente volcano, it does not have general ture is buried by the younger Cumbre Vieja vol- Volcano Stratigraphy structural or volcanostratigraphic significance. cano (Fig. 2), and its full extent is unknown. From However, it is particularly useful in measuring the the northern end of the summit ridge of this vol- Taburiente volcano forms the northern part of rates of growth of different parts of the volcano in cano to the rim of Caldera de Taburiente at Pico La Palma and may have started to grow about the period between the reversal and the end of ac- del Cedro (Fig. 3A), the headwall of the collapse 2 Ma (Ancochea et al., 1994). The region we are tivity: for example, it is about 400 m below the embayment forms the steep arcuate western es-

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Figure 3. (A) Simplified geologic map of the Cumbre Nueva giant collapse area. (B) Geologic cross section along the collapse embayment and scarp.

carpment of the Cumbre Nueva Ridge (Fig. 2), The position of the floor of the Cumbre Nueva primarily of lavas from Bejenado volcano and the which rises 500 to 800 m above the top of the col- collapse structure is less well known. The south- thick sedimentary sequence of El Time (Figs. 3 lapse scar fill sequence. The Caldera de Taburi- dipping contact between the much older Sea- and 4). Information on the base of the postcol- ente has been incised in the northern part of the mount Series and the postcollapse Bejenado vol- lapse sequence and the subsurface geometry of collapse structure (discussed as follows), but the cano (Figs. 2 and 3, A and B) is exposed in the the collapse scar is provided by data from bore- northwestern lateral wall of the collapse structure south wall of Caldera de Taburiente below Pico holes drilled near Los Llanos (Figs. 3 and 4). must have been close to the nearly vertical and re- Bejenado, and probably represents the position of The top of borehole S-01 is located at 395 m markably linear cliff on the north side of the Bar- the collapse boundary in this area, albeit modified above present sea level; the borehole crosses ranco de las Angustias (Fig. 3A). We consider that by erosion: no slide surfaces or debris-avalanche 320 m of Bejenado lavas and pyroclasts (Fig. 5). although important retrogressive erosion has deposits have been described from these out- The lavas yield consistent normal polarities. At taken place, this cliff inherited its unusual geom- crops. The entire surface outcrop to the south is 320 m the borehole cuts into a homogeneous unit etry from the collapse. formed by the postcollapse sequence, consisting of fragmented basalt in a loose sandy matrix. The

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Figure 4. Oblique aerial view of Caldera de Taburiente and Bejenado volcano. Locations of the boreholes referred to in the text are shown. The approximate locations of the Matuyama-Brunhes limit and outcrop of the seamount series submarine volcanics are also shown.

fine gravel to large blocks of basalt have random Boreholes S-02 and S-05, drilled in the Bar- (Carracedo et al., 1997b). Deposition of the polarities that show they are not lava flows, but ranco de Teniscas at 340 and 270 m above sea overlying sedimentary breccias took place after part of a clastic unit. This unit is very similar in level (Figs. 3 and 4), cut a similar clastic unit a short period of erosion of the collapse scar, appearance and polarity behavior to breccias ob- (Fig. 6). The thickness of the unit, 100 m in this since the overlying Bejenado volcano has been served in boreholes located in the El Golfo lateral part of the collapse embayment, can be deter- dated as 549 ± 12 ka (Fig. 5). collapse embayment on the island of El Hierro mined by using borehole S-02, which cuts into Three main units compose the overlying vol- (Carracedo et al., 1995). The recovered clasts are the pillow lavas of the uplifted Seamount Series. canic sequence and, in addition to the northern unaltered subaerial basic lavas and do not include These pillow lavas must underlie the basal de- lava flows of the much younger Cumbre Vieja the distinctive lithologies of the Seamount Series, tachment of the Cumbre Nueva collapse struc- volcano, occur within the Cumbre Nueva col- such as metamorphosed pillow basalts and gab- ture because they show no evidence of deforma- lapse scar. The stratigraphically lowest unit is a bros (Staudigel and Schmincke, 1984). This sug- tion postdating submarine alteration. Recovery south-dipping sequence of volcanic rocks, gests the clastic unit is not a debris-avalanche de- of core from the overlying, weakly indurated se- mainly lavas, which forms the Bejenado volcano. posit, because the basal layer of such a unit in this quence is poor (around 10%); the core shows no The preserved Pico Bejenado represents only the part of the collapse structure would be largely de- evidence of cohesive fault rocks such as in the southern flank of this volcano: the summit and rived from the Seamount Series. It is more likely aborted San Andrés collapse structure on the ad- feeder complex appear to have been located far- a postcollapse sedimentary unit produced by jacent island of El Hierro (Day et al., 1997) that ther to the north, in what is now the Caldera de rockfalls and rock avalanches from higher parts of might be expected to be recovered from a basal Taburiente. Ancochea et al. (1994) obtained ra- the adjacent collapse scar wall that exposes sub- detachment zone. Therefore, it seems that the diometric ages for Bejenado rocks of 710, 700, aerial lavas from Taburiente volcano, as can be detachment surface of the collapse structure in and 650 ka. However, the whole-rock ages of this seen in the cliffs above Barranco de Las Angus- this area has not been preserved, as is the case in postcollapse unit are not consistent with other ra- tias today. the El Golfo collapse structure of El Hierro diometric ages; they are older than upper lavas

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and young dikes of the precollapse Cumbre Nueva Ridge, dated as 700, 590, 580, and 530 ka by Staudigel et al. (1986), and dated as 770, 659, 647, 621, and 566 ka by Guillou et al. (1997). The age of 549 ± 12 ka we obtained for a lava flow of the Bejenado volcano (Figs. 3A and 5) is consistent with this volcano developing inside the collapse embayment immediately after the collapse. Post-Bejenado volcano sediments consist of two main units. The first is a 300-m-thick sequence of sediments exposed in the walls of the present-day Barranco de Las Angustias, and is called the El Time sediments (Figs. 2 and 3A). This unit mainly consists of coarse alluvial deposits, ranging from pebbly sandstones to boul- der conglomerates. The clast population includes subaerial lavas and scoria, and various extrusive and intrusive rocks of the Seamount Series. However, the lowest units exposed near the mouth of Barranco de Las Angustias at Tazacorte (Fig. 3A) are dominated by well-sorted, low- angle cross-stratified sandstones with a high car- bonate content: we infer that these are beach sandstones. Also exposed at the base of the El Time sediments are a few basaltic lavas that have not yet been dated. The El Time sediments have subvertical contacts with lavas of the Taburiente edifice to the northwest, and the Seamount Series and Bejenado volcano to the south. We infer the El Time sediments to have been deposited in an ancestral Barranco de Las Angustias that is now being reincised. The second sedimentary unit consists of more laterally extensive, finer grained alluvial sediments that are exposed around Los Llanos and in the mouth of a deep, steep-sided valley that has been incised between Pico Beje- nado and the Cumbre Nueva collapse scarp (Fig. 3A). This latter valley is of particular inter- est because it has been truncated at its northern end by the retrogressive erosion that enlarged the Caldera de Taburiente, and thus helps constrain the valley’s age.

CUMBRE VIEJA VOLCANO

Recent activity on La Palma is concentrated in the south of the island. This activity has produced the large polygenetic, Cumbre Vieja volcano, which has a subaerial area of 220 km2 and a subaerial volume of about 125 km3, and forms a ridge with a height of 2000 m above sea level (Fig. 7). The volcano is mostly built of sequences of alkaline lavas (alkali basalts, basanites, trachy- basalts, and tephrites) and Strombolian pyroclasts, but also contains a number of phonolite Figure 5. Geologic reconstruction of the Bejenado stratigraphy using the cores of borehole domes that are scattered over the volcano. S-01 (see location in Figs. 3 and 4). The Bejenado lavas are all of normal (Brunhes) polarity. The There is no central vent complex. Instead, the volcano is mainly composed of two thick aa lava formations (Aa1 and Aa2) and two pahoehoe summit of the volcano is formed by a long north- lava units (Ph1 and Ph2), overlying the sedimentary sequence filling the Cumbre Nueva giant south–trending ridge, composed of a high con- collapse embayment.

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Figure 6. Disposition of the Bejenado lavas filling the Cum- bre Nueva collapse embayment, as shown by the cores from boreholes S-05 and S-02 (locations shown in Figs. 3 and 4). The Be- jenado lavas overlie 100 m of sediments, probably a postcollapse sedimentary unit produced by rockfalls and rock avalanches from higher parts of the adjacent and unstable collapse scar wall. Borehole S-02 intersects the Seamount Series. m.a.s.l.—me- ters above sea level.

centration of largely monogenetic volcanic fis- trend with a superimposed radial dike swarm. tially define the stratigraphy of the volcano and to sures and vents forming a volcanic rift zone. In Such an alternative interpretation would predict reconstruct its volcanic and structural evolution. addition to this main north-south rift zone, which west- or southwest–trending dikes in the cliffs bisects the volcano, concentrations of vents are near the coastal platform formed by the 1712 Sea-Level Changes and Stratigraphy also present in the northwest and northeast sec- lavas and south-west–trending dikes in the high tors of the volcano. In these areas, the elongation cliffs farther south, where there are no dikes. The The western flank of Cumbre Vieja volcano directions of the vents, indicative of the orienta- structure of Cumbre Vieja volcano will be dis- has undergone significant marine erosion, result- tion of underlying dikes (Tibaldi, 1995), trend cussed in detail elsewhere. ing in subvertical coastal cliffs that are as high as generally, northwest and northeast, respectively. 700 m in some places. This recent period of rela- The distribution and geometry of the vents, Volcanic Stratigraphy tive inactivity and strong erosion makes it possi- therefore, suggest that weaker northwest and ble to define stratigraphic units. We define a cliff- northeast volcanic rift zones also exist, extension Detailed mapping (1:5000 in the zones of high forming series that is exposed in the coastal cliffs being accommodated by dike emplacement. vent concentration and 1:10000 in the lavas drap- and a scree-forming series that is draped over the Their topographic expression is weak, implying ing the flanks of the volcano) has been carried out cliffs and forms coastal lava platforms at their lower average effusion rates than the north- and 20 new high-precision K-Ar and 14C radio- bases (see Fig. 7). south–trending rift zone. A further reason for the metric dates for stratigraphically well-defined Radiometric dating (Guillou et al. 1997, 1998) indistinct nature of the northwest and northeast units within the volcanic sequence have been ob- shows that these units have chronostratigraphic rift zones is that they are currently inactive and tained (Guillou et al., 1997) to aid in the assess- significance: cliff-forming lavas are older than have no historic (<500 yr) or prehistoric erup- ment of Cumbre Vieja volcanic stratigraphy. about 20 ka, whereas scree- and platform-forming tions along them. Instead, a number of historic The sequences in Cumbre Vieja volcano are lavas are younger. The boundary between the two and prehistoric eruptions have occurred along the composed of many small eruptive units, and no series coincides with the sea-level lowstand in the north-south rift zone and from broadly east- consistent compositional variation with time can last glacial maximum (Imbrie et al., 1984). As west–trending fissures on its west flank (Fig. 8). be mapped in the field. Because the age of the vol- shown in Figure 9, we propose that the cliff devel- The pattern of vents in these zones and rare cano is less than 125 ka in outcrop, the separation oped in part as a result of intense marine erosion exposures of north-west–trending dikes within of magnetostratigraphic units is not possible using during the last fall to low sea level. Erosion would the Cumbre Vieja sequence on the west coast of the Brunhes-Matuyama boundary correlation. The have been particularly intense at this time because the volcano near the 1712 lava flows (Fig. 8) are absence of lithostratigraphic or magnetostrati- the fall in sea level would have caused waves to at- inconsistent with an alternative interpretation in graphic elements to differentiate and correlate dif- tack soft hyaloclastites, and thus undermine any which the volcano has a dominant north-south ferent volcanic units made it very difficult to ini- older screes and coastal lava platforms that may

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Figure 7. Oblique aerial view of the Cumbre Vieja volcano. The length of the view is 30 km.

have developed. The reduced volcanic activity in tribution of the vents for these eruptions is Although most of these eruptions have in- the period 80 to 20 ka (Figs. 8 and 11) also en- markedly different from that of the earlier activ- volved vents along the north-south–trending hanced coastal erosion. Bench collapse, common ity, as discussed in the following. rift, the bulk of products is from three eruptions in the Hawaiian Islands, may also have been an The stratigraphic and geochronological infor- (the 1585, 1712, and 1949 eruptions) located important erosion factor. mation mentioned here is summarized in the ge- along elongate volcanic fissures and fissure In addition to these two main units, we also ologic map and sections of Figure 8. The ages of swarms on the western flank of the rift. Al- distinguish two subunits within the scree-forming the cliff-forming lavas decrease continuously though most historic eruptions have involved series. The first subunit is composed of a few along the coast from northwest to south and from the extrusion of basanitic to tephritic lavas with lavas between 20 and 15 k.y. in age, which enter south to northeast. Coastal cliffs parallel this associated Strombolian activity, potentially the sea directly and appear to have submerged trend; they become progressively lower from more hazardous activity has also occurred. lava deltas. This unit was emplaced during the south to northeast and finally disappear at the Phreatic and phreatomagmatic explosions, par- postglacial rapid rise in sea level when coastal northeast coast, where they are buried by recent ticularly associated with drainback of magma erosion rates would actually have fallen because lavas from the northeast rift. down near-surface fissure systems, occurred in the sea would now be attacking resistant lavas that the 1585 and 1949 eruptions. In addition, de- had been emplaced subaerially, rather than hyalo- Historic Eruptions tailed mapping of the vent system for the 1585 clastites. eruption at Jedey (Fig. 9) has shown emplace- The second subunit is composed of prehistoric In the 500 yr since the Spanish conquest, half of ment of a number of phonolite cryptodomes and historic lavas and pyroclastic rocks. These all eruptions in the Canary Islands archipelago that barely breached the surface and underwent are identified in the field by lack of vegetation have taken place on Cumbre Vieja volcano. There significant downhill sliding on the steep flank cover as compared to older adjacent rocks and by may have been a recent increase in the rate of of the volcano. Examination of older phonolite reference to historic accounts, although the latter eruptive activity at the volcano, as can be observed domes on the volcano has shown that most un- are not always reliable, as shown by several mis- from the map in Figure 8, where as much as 15% derwent small-scale lateral collapse during their located historic eruptions (Hernández-Pacheco of the volcano has been resurfaced in the past emplacement because of steep slopes: an exam- and Vals, 1982; Carracedo et al., 1996). The dis- 500 yr, compared to about 40% in the past 7 k.y. ple is the pumiceous block and ash-flow deposit

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associated with the prehistoric Nambroque dome (Fig. 9).

DISCUSSION

Late-Stage Evolution of Taburiente Volcano

The last stages of evolution of Taburiente volcano prior to the Cumbre Nueva collapse are summarized in Figure 10. Comparison with similar studies of the initial growth of El Hierro (Guillou et al., 1996) and Cumbre Vieja volcano suggests the presence of radiating volcanic rift zones and underlying dike swarms. This early geometry, if it existed, was replaced by a highly active north-south–trending rift forming a southward prolongation of the volcano, the steep-sided Cumbre Nueva Ridge (Fig. 10A), while the other rift zones may have waned or become extinct. A sequence of lavas more than 400 m thick accu- mulated on the Cumbre Nueva Ridge in less than 200 k.y., and the volcano may have extended tens of kilometers to the south under the region currently covered by Cumbre Vieja volcano. There may be some similarities between this development and the recent dominance of the north- south–trending volcanic rift zone at Cumbre Vieja. However, there is no evidence at present for northward propagation of the Cumbre Nueva rift zone through the center of Taburiente volcano, as is the case for Cumbre Vieja volcano. The intense activity and overgrowth of this southern ridge caused the volcano to become unstable and triggered a lateral collapse toward the southwest (Fig. 10B). This collapse must have occurred shortly after the time of emplacement of the youngest dated Taburiente lava, which was sam- pled at the top of Cumbre Nueva Ridge (Fig. 3A) and dated as 566 ± 5 ka (Guillou et al., 1997). The geometry and extent of the region removed by the Cumbre Nueva lateral collapse, de- duced from field observations and information obtained from boreholes, will be discussed in detail separately. The eastern headwall of the block was an arcuate scarp parallel to the present-day Cumbre Nueva escarpment, and is now partly concealed by the northern end of Cumbre Vieja volcano (Fig. 10B). The northwestern boundary of the collapse, now entirely eroded away, coincides with the line of Barranco de Las Angustias. The precollapse Cumbre Nueva Ridge may have Figure 9. Evolution of the west coast of Cumbre Vieja volcano through the interaction of vol- reached about 2500 m above sea level, as de- canic eruptions and intense marine erosion during the fall to low sea level. duced from slopes of lava flows at the east and west flanks of the ridge, less than the height of the summit of the Taburiente volcano (perhaps tent of which is uncertain. Precise bathymetric producing Bejenado volcano (Fig. 10C). The 2700–3000 m) of which it is a part. The volume determinations will allow a definition of the sub- postcollapse volcanic activity seems to have con- involved in the collapse assuming this precol- marine part of the collapse and the precise as- tinued almost uninterrupted, as shown by the lapse topography may have been at least sessment of the volume involved. time elapsed between the collapse (ca. 566 ± 5 180–200 km3, the estimated volume of subaerial Following collapse, a new episode of volcanic ka) and the age of the Bejenado volcano (at least rocks removed by the collapse, the seaward ex- activity occurred within the collapse embayment, 549 ± 12 ka). This time interval implies that the

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Figure 10. Geological evolution of (A) the southern rift (Cumbre Nueva Rift) of Taburi- ente volcano, (B) the Cumbre Nueva collapse, and (C) the development of Bejenado volcano. Caldera de Taburiente is shown in C as having been formed by preferential erosion along a drainage system trapped between Bejenado volcano and the northern boundary of the collapse structure, and in D as enlarged by retrogressive erosion.

~100 m of postcollapse sediments have accumu- tion parallel to the northwestern wall of the Cum- since, but we emphasize the importance of the lated in a very short time, probably because the bre Nueva collapse structure, which is a detach- collapse structure, and particularly the north- collapse scarps were very unstable. With the end ment fault (Fig. 10B). Evidence for a fault is western fault boundary, in its localization and de- of Bejenado activity, volcanism ceased com- shown by the fact that the northwestern wall of velopment. pletely or almost completely in the north of the the depression is composed of lavas of the pre- Evidence for a tectonic process initiating the island and retrogressive erosion enlarged the collapse Cumbre Nueva Ridge, whereas the Caldera de Taburiente depression rather than ero- Caldera de Taburiente depression (Fig. 10D) and southeastern wall is composed of lavas of the sion alone can be summarized as follows: produced El Time and other sedimentary units. postcollapse Bejenado volcano. We therefore • The main axis of the depression coincides Emission vents on the flanks of the Bejenado propose that the Caldera de Taburiente may owe with the boundary of the collapse. massif may be contemporaneous with the early its origin to trapping of a drainage system be- • The long axis of the depression is a fault: stages of the subsequent Cumbre Vieja activity. tween the northwestern side of the collapse struc- both walls are composed of precollapse and post- ture and the growing Bejenado volcano, in the collapse lavas. Genesis of the Caldera de Taburiente period after the collapse (Fig. 10C). The trapped • The linearity of the Barranco de Las Angus- drainage system eroded deeply into both the tias is unmatched by the other barrancos in the The Caldera de Taburiente is the most spectac- older and younger volcanic sequences, and even- flanks of Taburiente volcano, which follow a typ- ular topographic feature of La Palma, a 15-km- tually into the underlying Seamount Series. With ically sinuous course. long, 7-km-wide, 2-km-deep depression, with time the Caldera de Taburiente deepened, en- • The Barranco de Las Angustias is about four precipitous bounding cliffs on most sides and a larged, and extended outside the preexisting lat- times deeper (2000 m) than the northern barran- deeply dissected floor. Our observations of age eral collapse structure, incising the El Time sedi- cos of Taburiente volcano, despite the fact that the relationships in this part of La Palma provide im- mentary sequence at its mouth and truncating the latter are older and in the northern flank of the portant constraints on the origin of the Caldera de less-vigorous drainage system east of Pico Beje- volcano, the windward, rainy side of the island. Taburiente. It deeply dissects the Taburiente and nado. This interpretation is similar to the erosive • The estimated volume emptied to develop Bejenado volcanoes, and must therefore postdate model for the formation of the Caldera de Taburi- the depression exceeds 100 km3. The average both, but extends in a northeast-southwest direc- ente proposed by Lyell (1855) and many authors rate of erosion to produce the depression in

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Figure 11. Successive stages in the growth of Cumbre Vieja volcano.

about 500 k.y. is at least 0.2 km3/k.y., consider- cratered vents. This early configuration, if exist- were undermined by erosion of the underlying ably higher than in the older northern barrancos ing, has been lost with time due to the dominant hyaloclastites to form the coastal cliffs. of Taburiente volcano, despite the higher rain- activity along the north-south–trending volcanic After about 20 ka, the island began to grow fall in this part of the island. rift. This dominant ridge may be a much older again (Fig. 11C). Eruptions from the north-south feature than Cumbre Vieja volcano, which has and northeast rift zones produced lavas that de- Cumbre Vieja Volcano: Rift-System grown on top of it since it may have formed by scended the old sea cliffs and produced lava plat- Development the southward growth of the Cumbre Nueva Vol- forms at their base, isolating the cliffs and slow- cano. Indeed, if it is an older feature, the preexis- ing erosion. At this time, the northwest volcanic The evolution of Cumbre Vieja volcano is tence of this ridge and consequent pattern of rift seems to have been less active, maybe an early summarized in Figure 11. After a period of vol- gravitational stresses induced by the topography manifestation of the proposed rift reorganization canic repose following the building of the Beje- (Fiske and Jackson, 1972; McGuire and Pullen, following the platform- and scree-forming series. nado volcano, volcanism continued at the south- 1989) may be the cause of the unequal develop- Since about 7 ka the distribution of volcanic ern edge of the pre-Cumbre Vieja island. A ment of the Cumbre Vieja rifts. vents on Cumbre Vieja has changed almost com- fast-growing volcano developed, leaving the Figure 11A shows the early stage of rapid pletely (Fig. 11D). The northeast– and north- northern part of La Palma inactive. Growth of growth of Cumbre Vieja volcano, from about west–trending volcanic rifts are no longer active. Cumbre Vieja has substantially increased the 125 ka to about 80 ka, which caused a consider- Instead, activity has been concentrated along the submarine and subaerial volume and surface area able enlargement of the island, especially to the north-south–trending rift, which has extended of the island (Fig. 2). An unequally developed south. From about 80 ka to 20 ka, the rate of northward. Most recently, eruptive fissures have triple-rift geometry (Carracedo, 1994) may have growth of the volcano appears to have decreased developed on the western flank of the volcano. been present in the initial stages of development (Fig. 11B). Volcanic activity was insufficient to The reorganization of the volcanic rift system in- of the volcano, as suggested by grouping of keep pace with the increased rate of coastal ero- dicates that the stress field has changed greatly in emission centers and orientation of elongated sion, and as sea level fell the subaerial rocks the past 7 k.y.

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Research, v. 73, p. 141–155. Eruptive Rates Aguas de Argual, in Los Llanos de Aridane, La Guillou, H., Carracedo, J. C., and Day, S. J., 1997, High- Palma, for facilities to study the bore holes in the resolution unspiked K-Ar dating of volcanic rocks from The Cumbre Nueva Ridge of Taburiente vol- area of Los Llanos (La Palma). The Consejería de the western Canary Islands: La Palma and El Hierro. In- ternational Workshop on Immature Oceanic Islands, La cano and Cumbre Vieja volcano show very simi- Política Territorial of the Canarian Government Palma, September 1997, p. 13–16. lar growth rates. The estimated subaerial volume afforded financial and logistic support during Guillou, H., Carracedo, J. C., and Day, S., 1998, Dating of the of the former, prior to collapse, is about 200 km3; field work. We thank Colleen Riley and John upper Pleistocene-Holocene volcanic activity of La Palma using the unspiked K-Ar technique: Journal of Vol- the present subaerial volume of the latter is about Smith for their detailed revision of the manuscript canology and Geothermic Research, v. 86, p. 137–149. 125 km3. The time required for Cumbre Nueva and many helpful comments. Many of the ideas Hernández-Pacheco, A., and Vals, M. C., 1982, The historic eruptions of La Palma Island (Canarias): Arquipelago, Ridge to grow to the collapse threshold is about put forward in this work were greatly clarified Revista de la Universidad de Azores, Series Ciencias Nat- 220 k.y., giving an eruptive rate of about 1 through discussions with colleagues attending the urales, v. 3, p. 83–94. km3/k.y. This value is 100 times lower than the international workshop on oceanic islands (Sep- Hoernle, K., Tilton, G., and Schmincke, H. 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C., Pérez Torrado, F., and Rodríguez Badiola, E., 1996, K-Ar ages and magnetic stratigraphy of MANUSCRIPT RECEIVED BY THE SOCIETY DECEMBER 15, 1997 North American Treaty Organization (NATO) a hotspot-induced, fast grown oceanic island: El Hierro, REVISED MANUSCRIPT RECEIVED JULY 15, 1998 (CRG 940609). We thank the Comunidad de Canary Islands: Journal of Volcanology and Geothermal MANUSCRIPT ACCEPTED AUGUST 12, 1998

Printed in U.S.A.

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