Volcanic Oceanic Islands Research Articles

Insular species of Afrotropical freshwater crabs (Crustacea: Decapoda: Brachyura: Potamonautidae and Potamidae) with special reference to Madagascar and the Seychelles

The evolutionary relationships between island and mainland faunas of the 24 species of insular freshwater crabs in the Afrotropical region are reviewed in the light of phylogenetic studies. Twenty insular species of freshwater crabs are endemic, and four are also found on the neighboring mainland of Africa. The Atlantic Ocean islands of Sherbro, Bioko, Principe, and São Tomé support five species of Potamonautidae, while the Western Indian Ocean islands of the Seychelles, Zanzibar, Pemba, Mafia, and Madagascar together have 16 species of Potamonautidae, and Socotra has three species of Potamidae. Disjunct distributions of non-endemic insular species of Afrotropical freshwater crabs with conspecifics on the mainland are the result of past lower sea levels that once united islands with the coast. The presence of endemic species of freshwater crabs on oceanic volcanic islands (such as Príncipe and São Tomé) separated from the mainland by deep seas is probably the result of transoceanic dispersal. Endemic genera of freshwater crabs found on oceanic ‘Gondwanan’ islands are derived from ancestral populations on the Eurasian (Socotra) or African (The Seychelles and Madagascar) mainlands that probably reached there by transoceanic dispersal, rather than their being the vicariant descendents of Gondwanan ancestors. Species of freshwater crabs found on islands in the Afrotropical region are either not unique, or are endemic at the species or genus level. The degree of endemism depends on the island’s geological history: whether it is part of the continental shelf, an oceanic island of volcanic origin, or a former part of the ancient continent of Gondwana.

Geomorphological study of long-term erosion on a tropical volcanic ocean island: Tahiti-Nui (French Polynesia)

A quantitative geomorphological study has been made on 27 river basins in Tahiti-Nui volcanic island (French Polynesia) to reconstruct the erosional evolution of a young oceanic island subjected to heavy tropical rainfall. Tahiti-Nui is composed of a main shield volcano cut by two huge landslides on each side of a main E–W rift zone. The northern landslide depression was rapidly buried by the construction of a second shield, the late activity of which overflowed the crest and then filled the southern landslide depression. The island is now volcanically inactive and is deeply dissected by erosion. The present geometries of the river basins are first compared using dimensionless parameters derived from a digital elevation model. The original volcanic surfaces are then reconstructed to estimate the volumes removed by erosion and determine the average rates of long-term erosion. The basins developed on the flanks of the main shield are wider, shallower, and gentler than the basins incising the post-landslide second shield, indicating a higher degree of evolution. Rainfall concentration on the windward (eastern) side of the island also contributed to increase the vertical lowering of the volcanic relief and the enlargement of the valleys. The magnitude of erosion, however, is neither directly linked with the age of the units incised nor with the differential amounts of rainfall. Erosion rates determined over the last 1 Myr range between 10 − 3 km 3 kyr − 1 and 0.25 km 3 kyr − 1 . The highest values occur in the basins incising the main E–W rift zone and/or the lateral rims of the northern and southern landslide depressions. Long-term dissection has thus been enhanced along the geological discontinuities of the eruptive system. Deep erosion was first constrained along the axis of the main E–W rift zone, where numerous dykes compartmentalize the volcanic structure into large unstable blocks. Dykes most probably acted as mechanical discontinuities along which shallow gravitational landslides recurrently occurred. Such mass-wasting episodes produced significant amounts of debris, partly preserved as highly indurated sedimentary breccias of various ages exposed at various locations. Subsequent dissection of Tahiti-Nui was enhanced to the north and to the south, leading to the rapid evolution of the Papenoo and Taharuu drainage systems over the last 500 kyr. Long-term dissection on Tahiti-Nui has been responsible for the removal of at least 350 km 3 of volcanic material from the surface, and for the partial exhumation of a shallow intrusive complex partly composed of coarse-grained plutonic rocks (gabbros and syenites) in the central part of the eruptive system. Structurally controlled erosion is thus a key component of landscape evolution on such high-relief oceanic tropical islands.

Elemental budget and rheology of the oceanic lithosphere

The compositions of the uppermost parts of subducting oceanic plates (sediments and altered crust) are relatively well known. However, the largest part of the downgoing slab, the lithospheric mantle (up to ∼10 times the thickness of the oceanic crust), is not well defined. By virtue of its size and refractory nature, this may be the subducted material with the longest lifespan within the convecting mantle; also, ancient recycled lithosphere may be a source reservoir to modern ocean island volcanism. We present results constraining the chemical compositions of the entirety of subducting tectonic plates, and discuss consequences of variations in lithospheric composition (especially water) for the rheology of subducted plates.

Sedimentology of rocky shorelines: 4. Coarse gravel lithofacies, molluscan biofacies, and the stratigraphic and eustatic records in the type area of the Pleistocene Hulopoe Gravel, Lanai, Hawaii

During the Pleistocene, the south flank of Lanai was the site of repeated erosion and deposition as a consequence of glacio-eustasy superimposed on tectono-isostasy. Throughout this time, terrigenous and bioclastic gravel was generated along mainly rocky shorelines. Gravelly sediment was deposited in topographically low areas of the shoreface and inner shelf that later were uplifted isostatically. The sedimentary lithofacies and biofacies in these deposits are distinctive, but they represent a deposystem and suite of environments that hitherto have not been studied systematically. These unrecognised circumstances have fuelled controversies about the nature and significance of the Hulopoe Gravel, most notably the claim that it represents a mega-tsunami deposit. The Hulopoe Gravel in its type area displays a lithofacies succession that encompasses a complete disconformity-bounded transgressive–regressive cycle on a sand-poor rocky shoreline. This succession incorporates not only a distinctive vertical assemblage of rocky shoreline litho- and biofacies, but also internal disconformities that record episodes of erosion and non-deposition during shoreline migration. Repetition of the lithofacies succession imparts overall stratigraphic order to the Hulopoe Gravel. Four unconformity-bounded transgressive–regressive cycles correspond to eustatic highstands of Oxygen Isotope Stages 5 and 7, superimposed on continuing tectono-isostatic uplift driven by growth of the island of Hawaii. This study of the Hulopoe Gravel demonstrates that the sedimentary record of mixed volcaniclastic-carbonate depositional systems developed on rocky shorelines of tropical and subtropical oceanic island coasts can be preserved. This record may include depositional products of both transgressive and regressive shorelines. Although tsunami and giant submarine landslides are known to occur in the Hawaiian Islands, deposits and features that result from tsunami inundation above the shoreline, such as might be expected from landslide-related mega-tsunami, have not been recognised in the type area of the Hulopoe Gravel. Rocky palaeoshoreline records on oceanic volcanic islands may yield oceanographic and climatic clues that complement data obtained from other palaeoshoreline types.

Volcanology and evolution of the Werribee Plains intraplate, basaltic lava flow-field, Newer Volcanics Province, southeast Australia

Volcanism in the Werribee Plains at the eastern end of the Cenozoic Newer Volcanics Province began ca 4.6 Ma, the formation of eruptive fissures partly controlled by the orientation of pre-existing basement structures. The exposed eruption centres are small-volume scoria cones and lava shields, and both produced significant lava flows. Calculations of the emplacement times of the lava flows suggest the lava shields were active for about 6 months on average. Scoria cone eruptions probably lasted days to weeks. Despite relatively long repose periods between eruptions (about one eruption per 97 000 years), few interflow sediments are preserved. Minimal physical erosion of the lavas probably reflects the shallow gradient of the Werribee Plains and the onset of arid climatic conditions in the late Cenozoic. Inflated pahoehoe lavas are the dominant volcanic facies in the Werribee Plains. Other lava facies identified include spongy (S-type) pahoehoe, ponded lavas, transitional lavas, and a thick, distal aa lava facies. The characteristics of the volcanic facies and their distribution in this volcanic province resemble those in the type example of basaltic plains provinces, the Snake River Plain, USA. Basaltic plains provinces are produced by a unique style of volcanism which has been largely unexplored compared with basaltic volcanism in other tectonic settings (such as flood basalt and ocean island volcanism). This study expands our understanding of the processes involved in the evolution of basaltic plains provinces. The volcanic facies association typical of basaltic plains provinces is better characterised here, thus aiding the future identification of these unique lava flow-fields elsewhere, in both modern and ancient sequences.

Morphological and geological aspects related to large slope failures on oceanic islands: The huge La Orotava landslides on Tenerife, Canary Islands

Huge volcanic landslides are one of the most hazardous geomorphological processes that can occur during the evolution of volcanic ocean islands. The causes of these phenomena, however, are very complex and combine non-volcanic and volcanic factors. In the Canary Islands, more than 20 events have been detected during the last decades. A detailed analysis was carried out for La Orotava amphitheatre on Tenerife in order to understand the relationship between geomorphological and geological aspects and huge volcanic landslides. The results indicated four major features that play a significant role in such mass movements: deep erosive canyons, high coastal cliffs, widespread residual soils and structural axes. High coastal cliffs and deep erosive canyons locally reduce the stability conditions and control both the seaward and the lateral boundary of the landslide. Weak residual soils formed above phonolitic pyroclastic deposits occur repeatedly in the stratigraphic column of La Orotava and are characterised by their large extent. Thus, one of these soils may have evolved into the slip surface of the failure. Part of the head scarp of the amphitheatre is defined by a volcanic rift zone, as indicated by the measurement of dike orientation and a density map of eruptive vents. The four features are not able to trigger a failure, but to destabilise the volcano flank and determine the boundary of the slide. Therefore, information on depth and orientation of canyons; location and height of coastal cliffs; stratigraphic repetition, extension and thickness of residual soils and orientation and density of dikes and eruptive vents along structural axes should be incorporated into a hazard assessment on large landslides on volcanic islands.

Erosion of the submarine flanks of the Canary Islands

Surveying with multibeam echo sounders around old (≫1 Ma) volcanic ocean islands reveals that their submarine flanks contain a strong downslope‐oriented ridge‐and‐valley corrugation, which modifies the original volcanic morphology of lava terraces and cones. By analogy with canyons in other settings, this corrugation was probably caused by channel incision by erosive sedimentary mass flows such as turbidity currents and debris flows. We adapt a method first used in subaerial geomorphology to isolate the erosion depth (exhumation) and apply it to the eroded flanks of the 6–8 Ma Anaga massif of Tenerife. The channels formed around this massif divert around local topographic highs. These highs, which are probably original volcanic cones, are therefore preferentially preserved during erosion, so that their elevations can be used to construct an artificial reference surface. Terrain depth was calculated by subtracting this reference surface from measured bathymetry. Comparison of the terrain depth of the old, eroded submarine flank of Anaga with that of the young, mostly unaltered submarine flank of El Hierro allows us to infer the mean depth of Anaga's submarine erosion, which is ∼100 m. Volcanic terrains can be dated by radiometric methods, so they also provide a way of quantifying long‐term denudation rates. We infer that submarine denudation of Anaga has occurred at comparable rates to that of subaerial lowlands and much slower than denudation of highlands, illustrated locally by the more extensive erosion of the subaerial Anaga edifice.

Chemical composition, distribution, and origin of silicic volcanic ash layers in the Greenland–Iceland–Norwegian Sea: explosive volcanism from 10 to 300 ka as recorded in deep-sea sediments

Explosive ocean island volcanism in the Greenland–Iceland–Norwegian Sea (GIN Sea) is indicated by marine tephra layers at 10–300 ka. Peaks of explosive volcanism occurred in oxygen isotope stages 8, 7, 5 and 1. The depositional age of the tephra was estimated using the oxygen isotope stratigraphy and dating of marine records. Geochemical analyses of the tephra layers show that all originate from Iceland. Here we report the characteristics of tephra from these major Icelandic events in 30 deep-sea cores from the GIN Sea. Our findings provide constraints on the distribution of tephra from the eruption source. For the Vedde Ash (oxygen isotope stage 1) we estimate a minimum fallout area of 2∗10 5 km 2, stretching from central Greenland in the west and southern Sweden in the east, to 71°N in the GIN Sea. The magnitude of the eruption and the regional wind conditions controlled the extent and concentrations of these ash fallout events. Oceanic circulation and differential settling may have affected the distribution and final deposition of ash particles such as bubble wall shards.

Temporal variation and carbonatite contamination in primitive ocean island volcanics from São Vicente, Cape Verde Islands

Primitive samples (MgO>5%) that are representative of the volcanic stratigraphy of São Vicente were analysed for major and trace element compositions and Nd, Sr and Pb isotopes. Mantle-normalized trace element patterns of the samples have negative anomalies of large ion lithophile elements such as Rb and K and positive anomalies for Nb, Ti and Zr, as is typical for some ocean island basalts (OIB). The oldest volcanics (OV) and the youngest volcanics (RV) are picrobasalts and basanites, whereas the intermediate age volcanics (IV) are nephelinites. Carbonatite dykes intrude the OV but are not observed in IV, indicating intrusion in the late part of the OV phase. OV have the most radiogenic Sr, least radiogenic Nd, and highly radiogenic Pb. Thus, OV comprise the most direct representation of a plume-derived HIMU component. The radiogenic isotopes of IV vary between depleted mantle (DM) and HIMU components. The isotopic compositions of RV indicate a predominance of DM. Isotopic differences between carbonatites and basaltic magmas are evidence of separate HIMU sources for these two magma types. Large negative Δ8/4 ( 208Pb/ 204Pb deviation from the Northern Hemisphere Reference Line: NHRL) is a feature of the carbonatites and basaltic rocks younger than OV. The IV also show an overall enrichment in incompatible elements, and have anomalously high CaO, P 2O 5 and Sr, and high Sr/Nd, P/Nd and P/Zr, without concomitant enrichments in Ti and Zr. The negative Δ8/4 and element enrichments of basaltic rocks younger than OV indicate assimilation of carbonatite. The assimilation of carbonatite is thought to have occurred while the basaltic magmas passed through lithosphere that was previously traversed by carbonatite.

The morphology of the submarine flanks of volcanic ocean islands: A comparative study of the Canary and Hawaiian hotspot islands

The submarine flanks of volcanic islands are shaped by volcanic constructional processes, landslides, erosion, sediment deposition and tectonic movements. We use a newly acquired multibeam sonar dataset from the westerly Canary Islands (El Hierro, La Palma and Tenerife) to develop a comparison with the Hawaiian Islands, which suggests differences in the processes constructing and modifying their flanks. Landslides affect the flanks of both island groups. Debris avalanches (fast-moving shallow landslides) have left smooth chutes and blocky deposits in both cases, but blocks within some Hawaiian avalanche deposits are markedly larger. We attribute the larger block sizes in the Hawaiian Islands to the fact that their avalanches were relatively unconfined, whereas many Canary and Hawaiian avalanches with small block sizes appear to have been constrained down narrow chutes, forcing interactions between blocks within the flows and encouraging disintegration. Furthermore, the Hawaiian avalanches with the largest blocks initiated near sea-level, whereas many of the Canary avalanches initiated above sea-level, so hydraulic resistance of water entering cracks may be an additional factor in resisting block disintegration during flow. Slow-moving deep-seated slumps or volcanic spreading have produced submarine benches and tabular escarpments due to thrust faulting adjacent to several Hawaiian rift zones, but are not well-developed in the Canaries. Although volcanic morphology is partly obscured by sedimentation in the Canaries, we are able to interpret lava terraces around the deep flanks of El Hierro which are similar to those found in the Hawaiian Islands. However, cones rather than terraces are the most common volcanic forms in the Canary Islands, implying that flank eruptions have involved magma with significant volatile contents, assuming that volatile contents dictate whether cones or terraces are formed. These differences may ultimately originate from the different building rates of the two island groups. For example, the lack of evidence for high-level magma chambers in the Canaries, associated with their lower outputs, implies that there is less possibility for degassing of magma below the summit before lateral intrusion down rift zones, hence cones rather than lava terraces are more commonly observed. The apparent lack of slumping or volcano spreading could also reflect a lack of driving pressure from extensive high-level magma chambers in the Canaries.

Storage and Transport of Noble Gases in the Subcontinental Lithosphere

Characteristics of the upper mantle are typically deduced from mid-ocean and ocean island volcanics that sample the convecting mantle. However, a substantial fraction of the upper mantle is isolated from this convecting reservoir as part of the subcontinental lithosphere. This is a potentially significant reservoir; not only might there be substantial abundances of noble gases and other trace element constituents, but also re-entrainment of this material into the deeper mantle may ultimately impart unique isotopic signatures to mantle domains sampled at ocean islands (McKenzie and O’Nions 1983). The characteristics of the lithosphere can be deduced from magmas derived by melting of this region and from xenolithic materials entrained by rapidly rising magmas. Unfortunately, magmas reaching the surface subaerially are generally strongly degassed and do not retain measurable noble gas concentrations, although some data can be obtained from phenocrysts. In contrast, noble gases as preserved in ultramafic xenoliths from continental volcanic provinces have provided substantial information regarding lithospheric processes and insights into the evolution of the subcontinental mantle. Diamonds also have received considerable attention, with the potential for preserving noble gases from greater depths and considerably longer time periods. Lithospheric mantle samples often show complex histories of metamorphism, deformation, and fluid interaction, and noble gases can provide constraints on the sources of trace element bearing fluids and the evolution of parent-daughter ratios. Noble gases, in particular He, provide distinctive signatures for volatile sources. While the 3He/4He ratio of radiogenic He is very low (~0.01 RA, where RA = 1.4× 10−6, the atmospheric ratio), He found in mid-ocean ridge basalts (MORB) sampling the asthenospheric upper mantle are much higher (~8 RA). This is due to mixing of radiogenic He with He that was trapped within the mantle since Earth formation with ≥120 R …

Geología y vulcanología de La Palma y El Hierro, Canarias Occidentales

Geología y vulcanología de La Palma y El Hierro, Canarias Occidentales

K–Ar, 40Ar– 39Ar ages and magnetostratigraphy of Brunhes and Matuyama lava sequences from La Palma Island

We have applied geological mapping, magnetostratigraphy and unspiked K–Ar and 40Ar– 39Ar datings to reconstruct the main volcanic stages of the northern part of La Palma, an oceanic volcanic island of the Canaries archipelago. Based on 57 new K–Ar and 40Ar– 39Ar ages, mostly concordant with the astronomic polarity time scale, three main volcanic units are distinguished: the Garafı́a volcano, and the Lower and Upper Taburiente volcanoes. This entirely subaerial activity took place between 1722±25 and 440±8 ka. The oldest stage, corresponding to the Garafı́a volcano, encompasses the post-Olduvai–Cobb Mountain period, with isotopic age determinations ranging from 1722±25 to 1208±8 ka. The oldest lava flows of the Taburiente volcano, which was active between 1.08±0.04 and 833±11 ka, recorded the Jaramillo normal polarity subchron and younger reversed flows were erupted during the upper part of the post-Jaramillo–Matuyama period. All the Upper Taburiente lava flows were erupted during the Brunhes chron and have ages ranging from 770±11 to 440±8 ka. The age of the Garafı́a collapse lies between 1.20±0.05 and 1.08±0.04 Ma, while the Cumbre Nueva collapse occurred at about 550 ka.

Causes and mobility of large volcanic landslides: application to Tenerife, Canary Islands

Giant volcanic landslides are one of the most hazardous geological processes due to their volume and velocity. Since the 1980 eruption and associated debris avalanche of Mount St. Helens hundreds of similar events have been recognised worldwide both on continental volcanoes and volcanic oceanic islands. However, the causes and mobility of these enormous mass movements remain unresolved. Tenerife exhibits three voluminous subaerial valleys and a wide offshore apron of landslide debris produced by recurrent flank failures with ages ranging from Upper Pliocene to Middle Pleistocene. We have selected the La Orotava landslide for analysis of its causes and mobility using a variety of simple numerical models. First, the causes of the landslide have been evaluated using Limit Equilibrium Method and 2D Finite Difference techniques. Conventional parameters including hydrostatic pore pressure and material strength properties, together with three external processes, dike intrusion, caldera collapse and seismicity, have been incorporated into the stability models. The results indicate that each of the external mechanism studied is capable of initiating slope failures. However, we propose that a combination of these processes may be the most probable cause for giant volcanic landslides. Second, we have analysed the runout distance of the landslide using a simple model treating both the subaerial and submarine parts of the sliding path. The effect of the friction coefficient, drag forces and hydroplaning has been incorporated into the model. The results indicate that hydroplaning particularly can significantly increase the mobility of the landslide, which may reach runout distances greater than 70km. The models presented are not considered definite and have mainly a conceptual purpose. However, they provide a physical basis from which to better interpret these complex geologic phenomena and should be taken into account in the prediction of future events and the assessment of landslide related hazards.

Island Edifice Failures and Associated Tsunami Hazards

—Volcanic ocean islands are prone to structural failure of the edifice that result in landslides that can generate destructive tsunamis. These island landslides range enormously in size, varying from small rock falls to giant sector failures involving tens of cubic kilometers of debris. A survey of literature has allowed us to identify twenty-three processes that contribute to edifice collapse. These have been divided into endogenetic and exogenetic sources of edifice failure. Endogenetic sources of instability and failure include unstable foundations, volcanic intrusions, thermal alteration, edifice pore pressures, unbuttressed structures, and buried faults. Exogenetic sources of instability and failure include collapse of subaerial or submarine deposits, endo-upwelling, karst megaporosity, fractures, oversteepening, overloading, sea-level change, marine erosion, weathering including hurricanes, glacial response, volcanic activity, regional uplift or subsidence, tectonic seismicity and anthropogenic agents. While the endogenetic sources dominate during periods of active volcanism and cone building, the exogenetic sources may cause failure at any time. Tsunamis, both small and large, are associated with these edifice failures.

Partial pressure of carbon dioxide in Coral Reef Lagoon waters : Comparative study of atolls and barrier reefs in the Indo-Pacific Oceans

Factors controlling the CO2 system parameters, including the partial pressure of CO2 (PCO2) in coral reef waters, were investigated in three mid-oceanic reefs of the Indo-Pacific region. Surface water PCO2 in the lagoons of Majuro Atoll and Palau barrier reef in the Pacific were 25 µatm and 48 µatm higher than those of the offshore waters, respectively, while South Male Atoll lagoon of the Maldives in the Indian Ocean exhibited relatively small difference in PCO2 compared to the offshore water. Observations from Majuro Atoll and Palau barrier reef are consistent with the view that calcium carbonate production predominates in coral reefs. On the other hand, results from South Male Atoll can be attributed to the thorough flushing of the lagoon, which is connected to the open ocean by numerous deep channels. The offshore-lagoon PCO2 difference depends on system-level net organic-to-inorganic carbon production ratio while reef topography, especially residence time of the lagoon, has a secondary effect on the magnitude of the offshore-lagoon difference. A potential for releasing CO2 might be more evident in an enclosed atoll where the reef water has a longer residence time. Oceanic atoll and barrier reef lagoons, which are in the terminal stage of evolutionary history of oceanic volcanic islands, have the potential to release CO2 to the atmosphere.

Characterising the irregular coastlines of volcanic ocean islands

Volcanic ocean islands grow into irregular shapes because of eruption along radiating volcanic rift zones, caldera collapse, slope failure, coalescence of adjacent volcanoes and marine and subaerial erosion. With the aim of searching for variations with environmental parameters which might reflect variations in these processes, this study calculates and analyses characteristics of island irregularity from the World Vector Shoreline database. Islands are idealised as flat plates bounded by the digitised shorelines from which perimeter distance, elongation and moment of inertia are calculated to represent irregularity. These characteristics are highly variable and do not correlate strongly with any of the parameters studied here, reflecting the complexity of processes shaping island coastlines. On closer scrutiny, some weak variations with seafloor age and spreading rate are due to some highly irregular islands on old seafloor mostly formed at slow-spreading ridges (Canaries, Cape Verde, Comores, Molokai and Madeira). Since most of these islands lie on the African plate which is slowly moving relative to the earth's mantle, this may reflect a fortuitous superposition of adjacent volcanoes by stationary hot-spots, although their embayed outlines suggest that landslides are also important. The analysis also resolves the orientations of islands with respect to the tectonic fabric of their underlying oceanic basement, revealing that islands on seafloor originally created at slow-spreading ridges are preferentially oriented perpendicular, parallel and at 30° to the fabric and that islands on young seafloor show a slight tendency to be aligned fabric-parallel.

Vertical and lateral collapses on Tenerife (Canary Islands) and other volcanic ocean islands: Comment and Reply

Vertical and lateral collapses on Tenerife (Canary Islands) and other volcanic ocean islands: Comment and Reply

Provenance of the detrital component of deep-sea sediments from the SW Pacific Ocean based on mineralogy, geochemistry and Sr isotopic composition

Clay mineralogy and chemical and strontium isotope compositions have been used to determine the detrital components of deep-sea sediments from the SW Pacific Ocean. Continental detritus derived mainly from the New Zealand landmass is characterised by high illite and chlorite contents, and 87Sr/ 86Sr ratios close to 0.7110. This material, eroded from the eastern margin of both islands, is transported into deeper water by turbidity currents and hemipelagic processes, and is carried northwards by the strong Western Boundary Current as far north as the Samoan Basin. Due to the great distance from source and unfavourable prevailing winds, the contribution of aeolian dust from the Australian deserts to the sediment appears to be relatively small, particularly in the northernmost part of the region. In the vicinity of the oceanic volcanic islands of Tonga, Samoa and the Cook Islands, volcaniclastic material including heavy minerals (pyroxene and magnetite), pumice, glass shards (proximally) and smectite clays (distally) is dispersed into the sediments by turbidity currents and airfall. In areas of very low sedimentation distant from landmasses, the sediments contain significant amounts of authigenic material (RSOs) characterised by high concentrations of transition metals and rare earth elements. Strontium isotopes, together with trace element compositions, have proved particularly useful in determining the relative proportions of the detrital types. The contrasting 87Sr/ 86Sr ratios and Sr contents of the various source rocks permit the use of binary mixing relationships whch indicate that sediments less than 35 km from the Cook Islands contain 80% volcanic detritus, and those more than 1200 km away contain up to 20% volcanic detritus.

Vertical and lateral collapses on Tenerife (Canary Islands) and other volcanic ocean islands

Recent studies demonstrate that Tenerife has undergone large lateral collapses. This has led to the suggestion that the Las Canadas caldera, one of the best exposed calderas in the world, is the result of lateral collapse. We have tested this idea using the available structural, stratigraphic, volcanological, and geochronological data. We conclude that the Las Canadas caldera is the result of a complex sequence of vertical collapse events associated with a long history of phonolitic explosive activity in the central part of Tenerife. Our results indicate, however, that vertical collapses may have played a major role in triggering lateral collapses. We propose that the association of vertical and lateral collapse events, such as inferred for Tenerife, can also explain similar sequences of events interpreted to have affected other large volcanic ocean islands.