The 1 MA evolution of constructive and destructive processes at the island arc volcano of Montserrat
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Montserrat is a small island arc volcano in the Caribbean island arc. The island comprises three main volcanic centres: Silver Hills, active between 2.5-1 Ma; Centre Hills, active between ~1 to 0.5 Ma; and the Soufriere Hills-South Soufriere Hills volcanic complex, active from ~0.3 Ma. Here an extensive (> 1 Ma) and detailed stratigraphic record is compiled for Montserrat using both the subaerial and submarine (in the form of three International Ocean Drilling Program cores) records. This combined record gives valuable insight into the evolution of volcanic and mass-wasting processes at Montserrat, and may be useful for future hazard mitigation. The stratigraphic record shows that eruptive styles, volcanic intensity and mass-wasting processes have varied through time. Dome-style eruptions have dominated the past 1 Ma of volcanic activity at Montserrat. At the older edifice of Centre Hills, regular large-magnitude explosive eruptions (represented by >1m thick pumiceous sequences onshore) also occurred, but such explosive eruptions are rare at the younger Soufriere Hills-South Soufriere Hills volcanic complex. Periods of heightened volcanic activity occurred between 1.1-0.9 Ma, 0.3 Ma, and 0.2-0.1 Ma. Another period of increased volcanism may have also occurred at ~0.5±0.2 Ma. These coincide with periods of increased, mass-wasting, identified at 1.1-0.9 Ma, 0.6-0.5 Ma, 0.3 Ma, and 0.2-0.1 Ma and suggest that increased volcanic activity may facilitate mass-wasting processes. This may be due to increased deposition of material on the island flanks, or increased seismic activity that can trigger collapses. The ages of the largest landslide deposits (volumes >0.3 km3) observed offshore of Montserrat also coincide with periods of faster sea-level rises. Analysing the global database, large landslides coincide with rapid sea-level rise at other island arc volcanoes, but not at ocean islands. The reason for this difference in behavior is unclear, but maybe associated with differences in island composition and size, or tectonic regimes.Keywords:
Mass wasting
Subaerial
Lahar
Volcanic arc
Submarine volcano
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Abstract Hole U1395B, drilled southeast of Montserrat during Integrated Ocean Drilling Program Expedition 340, provides a long (>1 Ma) and detailed record of eruptive and mass‐wasting events (>130 discrete events). This record can be used to explore the temporal evolution in volcanic activity and landslides at an arc volcano. Analysis of tephra fall and volcaniclastic turbidite deposits in the drill cores reveals three heightened periods of volcanic activity on the island of Montserrat (∼930 to ∼900 ka, ∼810 to ∼760 ka, and ∼190 to ∼120 ka) that coincide with periods of increased volcano instability and mass‐wasting. The youngest of these periods marks the peak in activity at the Soufrière Hills volcano. The largest flank collapse of this volcano (∼130 ka) occurred toward the end of this period, and two younger landslides also occurred during a period of relatively elevated volcanism. These three landslides represent the only large (>0.3 km 3 ) flank collapses of the Soufrière Hills edifice, and their timing also coincides with periods of rapid sea level rise (>5 m/ka). Available age data from other island arc volcanoes suggest a general correlation between the timing of large landslides and periods of rapid sea level rise, but this is not observed for volcanoes in intraplate ocean settings. We thus infer that rapid sea level rise may modulate the timing of collapse at island arc volcanoes, but not in larger ocean‐island settings.
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New 40Ar/39Ar dates from the Jemez Mountain volcanic field (JMVF) reveal formerly unrecognized shifts in the loci of pre-caldera volcanic centers across the northern Jemez Mountains; these shifts are interpreted to coincide with episodes of Rio Grande rift faulting. Early activity in the field includes two eruptive pulses: 10.8–9.2 Ma basaltic to dacitic volcanism on Lobato Mesa in the northeastern JMVF and 12–9 Ma mafic to silicic volcanism in the southwestern JMVF. While 9–7 Ma eruptions persisted in the southern JMVF, a new eruptive center developed on the La Grulla Plateau in the northwestern JMVF (8.7–7.2 Ma), corresponding with a period of rift widening caused by reactivation of Laramide faults in this area. The older 8.7–7.8 Ma mafic lavas emitted from Encino Point and the younger 7.7–7.2 Ma trachyandesite and dacite erupted on the La Grulla Plateau are assigned to a new unit called the La Grulla Formation. The chemical composition of a 640 m stack of lava flows exposed in the northern margin of the Valles caldera changes from dacite to andesite, then back to dacite upsection, becoming slightly more alkalic upward. The shift to more alkalic compositions occurs across a sedimentary break, marking a subtle change in magma source for the older Paliza Canyon Formation and the younger La Grulla Formation lavas. New age constraints from a rhyolite intrusion in the southern JMVF and pumiceous rhyolite deposits in the northern JMVF suggest an episode of localized, 7.6–7.8 Ma rhyolitic volcanism that occurred in the central part of the JMVF between 12–8 Ma Canovas Canyon Rhyolite and 7–6 Ma peak Bearhead Rhyolite volcanism. Younger Bearhead Rhyolite intrusions (7.1–6.5 Ma) are more widespread than previously documented, extending into the northeastern JMVF. Tschicoma Formation dacite erupted at 5 Ma in the Sierra de los Valles and then erupted throughout the northeastern JMVF 5–2 Ma. The more refined geochronology of the JMVF indicates that pre-caldera volcanic centers were characterized by geographically and chemically distinct, relatively short-lived, episodes of activity. Volcanism generally migrated eastward through time in the southern JMVF, but the pattern in the northern JMVF had a more complex east (10–9 Ma) to west (9–7 Ma) to east (5–2 Ma) pattern that reflects the timing of motion on faults. The new ages, coupled with detailed mapping of both volcanic rocks and the Santa Fe Group, document significant pulses of faulting, erosion, and deposition during middle Miocene time and during late Miocene time across the Cañones fault zone in the northern JMVF.
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The western Canaries, relatively little studied until a few years ago from the geological point of view, have however provided decisive data for understanding many of the most important geological problems of the Archipelago, which would probably have been dilucidated earlier, had the study begun with the most recent islands, as occurs in similar chains of oceanic volcanic islands in other parts of the world. To summarize the main geological features and evolutionary characteristics of both islands we emphasize the following stages of development: During the Pliocene, a submarine volcanic edifice or seamount formed in the island of La Palma, made up of pillow lavas, pillow breccias and hyaloclastites, intruded by trachytic domes, plugs of gabbros, and a highly dense dyke swarm. The intense magmatic and dyke intrusion uplifted the searnount up to 1,500 m, tilting it 45-50 to the SW. This intrusive phase was followed by a period of quiescence and erosion of the emerged submarine edifice. The definitive consolidation and progression of the construction of the island continued from at least 1.77 ma in angular and erosive discordance over the submarine basement. The subaerial volcanic reactivation, in which explosive volcanism predominated during the initial stages, producing abundant volcanoclastic and phreatomagmatic materials at the base of the subaerial edifice, persisted in a highly continuous manner until at least 0.41 ma. This initial subaerial stage shaped the northern volcanic shield, formed by the accumulation of several superimposed volcanoes, approximately concentric in relation to one another and the submarine basement. The initial stage of the northern volcanic shield lasted between 1.77 and 1.20 ma, during which period the Garafia volcano was built to a height of 2,500-3,000 m, with steeply sloping flanks, formed predominantly by alkaline basalts with abundant pahoehoe lavas. The rapid growth and progressive instability of the Garafia volcano culminated some 1.20 ma ago in a gravitational landslide of the south flank of the volcanic edifice. The eruptive activity that followed the collapse built the Taburiente volcano, that rests upon a clear angular discordance caused by the landslide. The landslide depression was filled completely some 0.89 ma ago, as shown by the age of the first lavas to overflow the collapse embayment. The filling-in of the depression by the Taburiente volcano lavas finally formed a sequence of horizontal lavas, predominantly alkaline basalts, that ponded against the headwall of the landslide scarp fonning a plateau in the centre of the volcanic shield. Coinciding approximately with the Matuyama-Brunhes boundary (0.78 ma) an important reorganisation of the Taburiente volcano took place, the dispersed emission centres of which progressively concentrated in three increasingly defined rifts (NW, NE and N-S) and subsequently in a central edifice situated at the geometrical centre of the volcanic shield. The abundant emissions of this final stage covered the earlier formations with sequences of lava flows up to 1,000 m thick in places, with the exception of a part of the alignments of cones of the rifts. The basaltic lavas evolved towards more differentiated phonolitic and trachytic terms at the terminal phases of construction of the volcano. One of these rifts, the southern or Cumbre Nueva rift, developed more than the others, possibly because the volcanism already began to migrate southwards, forming a N-S trending dorsal ridge over 2,500 m high. The progressive instability of the Cumbre Nueva rift, due to overgrowth, triggered the gravitational landslide of the western flank, in a process that took place about 560 ka ago, involving the detachment of some 180-200 km3 and the formation of a wide depression (the Valle de Aridane) and the beginning of the formation, by incision and retrogressive erosion, of the Caldera de Taburiente. The activity subsequent to the collapse in the northern shield was preferentially concentrated in the interior of the new collapse basin, quickly building the Bejenado stratovolcano. This activity was coetaneous with that of other residual centres dispersed over the flanks of the shield. The initially basanite lavas of Bejenado volcano evolved to mafic tephrites in differentiated lateral and terminal vents. The activity of the volcanic shield ceased definitively some 0.4 ma ago. After a transition period with a certain degree of activity associated with Bejenado late peripheral vents, volcanism was definitively located until the present in the new Cumbre Vieja volcano, at the south of the island. The oldest Cumbre Vieja lavas have been dated in 123 ka, although the first eruptions of the volcano may be considerably older. During this last stage of volcanism in La Palma a N-S trending rift has been formed, with predominantly basanitic, tephritic and tephri-phonolitic lavas, and intrusions of domes of tephri-phonolites and phonolites, frequently associated with eruptive vents. Numerous submarine eruptive vents, severa1 of which are apparently very recent, have recently been observed and sampled at the prolongation of the Cumbre Vieja rift southwards in the ocean. The foreseeable geological evolution of this rift is similar to that of its Cumbre Nueva predecesor, towards a progressive development and increasing instability, although changes may take place that may modify it towards more stable configurations, fundamentally the submarine progression of the southern tip of the rift, that could redistribute the volume of emitted materials, reduce the aspect ratio of the volcano and, consequently, its instability. The en echelon faults generated during the 1949 eruption have been interpreted as a possible detachment of the western flank of the volcano, although a more favourable hypothesis would be that such faults are surficial and contribute to accommodating the volcano by reducing its instability. A noteworthy aspect is the important role played by the mobility of the general feeding system of the volcanism in shaping the form and structure of the island. If the volcanism had not continually migrated southward since the final stages of construction of the northern shield, the island of La Palma would probably have taken on a similar configuration to that of the islands of El Hierro or Tenerife, in the shape of a triangular pyramid, with triple-armed rifts and landslide lobes between the rifts. The southward migration of volcanism in La Palma left the northern shield extinct, the rifts incomplete and finally configured an island lengthened in a N-S direction. Another point of interest is that the islands of La Palma and El Hierro are the first of the Canaries to form simultaneously, with possibly alternating eruptive activity, at least in the most recent period. This separation in a «dual line» of islands and the greater depth of its oceanic basement account for the long time they have required to emerge since the formation of the prior island of La Gomera. The island of El Hierro is geologically somewhat younger than La Palma and, because it formed over a stationary source of magma, it presents, in comparison, a perfect, concentric development, with superimposed volcanoes and a regular three-armed rift geometry. The activity of the subaerial volcanism began in El Hierro with the development of Tinor volcano on the NE flank of the island (approximately from 1.12 to 0.88 ma), with the emission of massive typical basalts. The volcano developed quickly, with different stages of growth, the eruption of Ventejis volcano being the terminal explosive stage, and probably the precursor of the collapse of the NW flank of the edifice some 882 ka ago. The emissions of the new volcano -El Golfo, approximately 545 to 176.000 ka- totally filled the depression of the lateral collapse of Tinor volcano, the lava flows of which then spilled over the flanks of the earlier volcano. The beginning of the construction of the El Golfo volcano seems to have taken place after a relatively long period of activity, probably coinciding with the maximum development of the Cumbre Nueva rift on La Palma. The initial subaerial activity at El Golfo was characterised by basaltic lavas that evolved to trachybasalts and trachytes, and finally towards more differentiated eruptive episodes indicative of the terminal state of the volcanic activity of the El Golfo volcano. The excessive growth of this volcano triggered the failure of its north flank, generating the spectacular scarp and present El Golfo depression. Subsequent volcanism, from emission vents arranged in a three-armed rift system (rift volcanism, with ages ranging from 145 ka to 2,500 years, with probably prehistoric eruptions), implies the much more moderate continuation of the earlier predominantly basanitic-tephritic volcanic activity. This period may correspond to that of maximum development of the Cumbre Vieja rift, in the island of La Palma.
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Abstract Pacific drill sites offshore Central America provide the unique opportunity to study the evolution of large explosive volcanism and the geotectonic evolution of the continental margin back into the Neogene. The temporal distribution of tephra layers established by tephrochonostratigraphy in Part 1 indicates a nearly continuous highly explosive eruption record for the Costa Rican and the Nicaraguan volcanic arc within the last 8 Myr. The widely distributed marine tephra layers comprise the major fraction of the respective erupted tephra volumes and masses thus providing insights into regional and temporal variations of large‐magnitude explosive eruptions along the southern Central American Volcanic Arc (CAVA). We observe three pulses of enhanced explosive volcanism between 0 and 1 Ma at the Cordillera Central, between 1 and 2 Ma at the Guanacaste and at >3 Ma at the Western Nicaragua segments. Averaged over the long‐term the minimum erupted magma flux (per unit arc length) is ∼0.017 g/ms. Tephra ages, constrained by Ar‐Ar dating and by correlation with dated terrestrial tephras, yield time‐variable accumulation rates of the intercalated pelagic sediments with four prominent phases of peak sedimentation rates that relate to tectonic processes of subduction erosion. The peak rate at >2.3 Ma near Osa particularly relates to initial Cocos Ridge subduction which began at 2.91 ± 0.23 Ma as inferred by the 1.5 Myr delayed appearance of the OIB geochemical signal in tephras from Barva volcano at 1.42 Ma. Subsequent tectonic re‐arrangements probably involved crustal extension on the Guanacaste segment that favored the 2–1 Ma period of unusually massive rhyolite production.
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Abstract Small intra-plate volcanic islands (total height above seafloor <2500 m) have been considered gravitationally stable. Topographic, stratigraphic, structural and new K/Ar data show that the small island of Flores (Azores) is strongly asymmetric and made up of nested volcanic successions. Along the northwestern coastline, ca. 1.2 Ma lava flows are in lateral contact with a younger volcanic unit (ca. 0.7 Ma), reflecting the existence of a steep lateral discontinuity. From the general dip of the lava flows, their age and the arcuate geometry of the contact, we infer a major landslide that removed the western flank of the older volcano. Further inland, E-dipping lava flows at the summit of the island are ca. 1.3 Ma, suggesting another landslide structure that displaced the whole western half of the former volcanic edifice. Available offshore data show a large hummocky field west of Flores, here interpreted as voluminous debris-avalanche deposits. Unlike the eastern and central Azores islands, Flores sits on a relatively stable tectonic setting. Therefore, we propose that small-size volcanic islands can be sufficiently gravitationally unstable to experience recurrent episodes of large-scale mass wasting triggered by mechanisms other than tectonic earthquakes and thus represent an under-evaluated potential source of hazard and, therefore, risk.
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Different stages of volcanic island evolution are evident in the Cenozoic Lesser Antilles arc developed near the eastern edge of the Caribbean Plate. The early stage of seamount evolution is represented by the Kick-em-Jenny submarine volcano near the southern end of the arc. The volcanically active islands known as the Volcanic Caribbées are composed of a series of composite volcanoes and are examples of an emergent volcanic island stage. Those islands that were formerly active, known as the Limestone Caribbées, provide evidence that an erosional to submergent stage occurred in the northern part of the island arc during the Oligocene and Miocene epochs. Growth of the islands and underlying crust is dependent upon the magma production rate along the arc, which has for the last 100 ka ranged from less than 1 km 3 to 40 km 3 . These differences in magma production and volcanic rock composition along the arc are attributed to the obliquity of plate convergence and the plate convergence rates.
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