Ocean Island Volcanoes Research Articles

Geochemistry of Mauritius and the origin of rejuvenescent volcanism on oceanic island volcanoes

We report new trace element concentrations and Sr, Nd, Hf, and Pb isotopic ratios for basalts from the three lava series of Mauritius. Older Series lavas, which represent the shield‐building phase of Mauritius, are isotopically similar to other lavas produced by the Réunion mantle plume. The Intermediate and Younger Series lavas, erupted after a hiatus of millions of years, have more depleted isotopic signatures than the Older Series. Incompatible element abundances and major element compositions suggest that the extent of melting was greatest for the Older Series, smallest for the Intermediate Series, and intermediate for the Younger Series. Volcanic evolution on Mauritius is thus broadly similar to the Hawaiian pattern, with lavas of the rejuvenescent phase being produced by small degrees of melting of a more depleted source than the shield phase. We propose that both shield and rejuvenescent phase magmas are derived from a lithologically heterogeneous or “plum pudding” mantle plume that consists of pods or veins of low‐solidus‐temperature material such as eclogite or pyroxenite embedded in peridotite matrix. The plums have a less depleted isotopic signature than the matrix. In the vertical stem of the plume, melting is zoned: at greatest depth, only the plums melt; at intermediate depth, both plums and matrix melt, and at shallowest depth, only the matrix melts. Shield stage magmas are hybrids of melts produced in all three zones. As the plume flattens out against the lithosphere and is dragged downstream of the stem, some melting continues due to buoyant lateral spreading of the plume. In this region, although both plums and matrix are melting, melts produced carry the isotopic signature of the matrix because incompatible elements in the plums were previously stripped during melting in the vertical stem of the plume. The relatively small degree melts produced in the melting tail, which give rise to rejuvenescent stage volcanism, thus carry the isotopic signature of the matrix. From a consideration of thermal and isotopic equilibrium, we judge the scale of heterogeneity to be about 102 m.

LINKS BETWEEN LONG‐LIVED HOT SPOTS, MANTLE PLUMES, D″, AND PLATE TECTONICS

The existence, spatial distribution, and style of volcanism on terrestrial planets is an expression of their internal dynamics and evolution. On Earth a physical link has been proposed between hot spots, regions with particularly persistent, localized, and high rates of volcanism, and underlying deep mantle plumes. Such mantle plumes are thought to be constructed of large spherical heads and narrow trailing conduits. This plume model has provided a way to interpret observable phenomena including the volcanological, petrological, and geochemical evolution of ocean island volcanoes, the relative motion of plates, continental breakup, global heat flow, and the Earth's magnetic field within the broader framework of the thermal history of our planet. Despite the plume model's utility the underlying dynamics giving rise to hot spots as long‐lived stable features have remained elusive. Accordingly, in this review we combine results from new and published observational, analog, theoretical, and numerical studies to address two key questions: (1) Why might mantle plumes in the Earth have a head‐tail structure? (2) How can mantle plumes and hot spots persist for large geological times? We show first that the characteristic head‐tail structure of mantle plumes, which is a consequence of hot upwellings having a low viscosity, is likely a result of strong cooling of the mantle by large‐scale stirring driven by plate tectonics. Second, we show that the head‐tail structure of such plumes is a necessary but insufficient condition for their longevity. Third, we synthesize seismological, geodynamic, geomagnetic, and geochemical constraints on the structure and composition of the lowermost mantle to argue that the source regions for most deep mantle plumes contain dense, low‐viscosity material within D″ composed of partial melt, outer core material, or a mixture of both (i.e., a “dense layer”). Fourth, using results from laboratory experiments on thermochemical convection and new theoretical scaling analyses, we argue that the longevity of mantle plumes in the Earth is a consequence of the interactions between plate tectonics, core cooling, and dense, low‐viscosity material within D″. Conditions leading to Earth‐like mantle plumes are highly specific and may thus be unique to our own planet. Furthermore, long‐lived hot spots should not a priori be anticipated on other terrestrial planets and moons. Our analysis leads to self‐consistent predictions for the longevity of mantle plumes, topography on the dense layer, and composition of ocean island basalts that are consistent with observations.

Rock mass strength and slope stability of the Hilina slump, Kīlauea volcano, Hawai'i

Stability of the Hilina slump, on the south flank of Kīlauea volcano, Hawai'i, is investigated using slope stability analyses based on rock mass mechanics. The Hilina slump is an active example of the giant landslides that are observed around submarine flanks of ocean island volcanoes. Based on edifice topology, along with derived rock mass strength and deformability parameters, slope stability analyses predict that the geometries of present-day failure surfaces of the Hilina slump are shallow, located at less than ∼3.5 km depth below the upper surface of the edifice. The failure surfaces at the base of the Hilina slump are predicted to be structurally independent from a previously interpreted subjacent detachment at the contact between the Kīlauea edifice and the oceanic crust. Based on these model results, the Hilina slump is envisioned to ride atop the south flank of Kīlauea as it spreads seaward along this slipping basal detachment. Under present-day slope and sea-level configurations, local horizontal ground accelerations greater than ∼0.4–0.6 g are predicted to cause slip along failure surfaces within Hilina slump. Therefore, dynamic stress changes due to slip along the subjacent detachment may potentially trigger slip along failure surfaces of the Hilina slump. Model-derived stability and failure surface geometries can account for the distinct observed distribution of slip along specific Hilina faults following the M fa 7.9 1868 Ka'u and M s 7.2 1974 Kalapana earthquakes. Further, the spatial distribution of south flank aftershocks > M 1.5 recorded between 1950 and 1976 are consistent with the predicted distribution of potential earthquake-induced slip of the Hilina faults.

Rhenium and chalcophile elements in basaltic glasses from Ko’olau and Moloka’i volcanoes: Magmatic outgassing and composition of the Hawaiian plume

The behavior of chalcophile metals in volcanic environments is important for a variety of economic and environmental applications, and for understanding large-scale processes such as crustal recycling into the mantle. In order to better define the behavior of chalcophile metals in ocean island volcanoes, we measured the concentrations of Re, Cd, Bi, Cu, Pb, Zn, Pt, S, and a suite of major elements and lithophile trace elements in moderately evolved (6–7% MgO) tholeiitic glasses from Ko’olau and Moloka’i volcanoes. Correlated variations in the Re, Cd, and S contents of these glasses are consistent with loss of these elements as volatile species during magmatic outgassing. Bismuth also shows a good correlation with S in the Ko’olau glasses, but undegassed glasses from Moloka’i have unexpectedly low Bi contents. Rhenium appears to have been more volatile than either Cd or Bi in these magmas. Undegassed glasses with 880–1400 ppm S have 1.2–1.5 ppb Re and 130–145 ppb Cd. In contrast, outgassed melts with low S (<200 ppm) are depleted in these elements by factors of 2–5. Key ratios such as Re/Yb and Cu/Re are fractionated significantly from mantle values. Copper, Pb, and Pt contents of these glasses show no correlation with S, ruling out segregation of an immiscible magmatic sulfide phase as the cause of these variations. Undegassed Hawaiian tholeiites have Re/Yb ratios significantly higher than those of MORB, and extend to values greater than that of the primitive mantle. Loss of Re during outgassing of ocean island volcanoes, may help resolve the apparent paradox of low Re/Os ratios in ocean island basalts with radiogenic Os isotopic compositions. Plume source regions with Re/Yb ratios greater than that of the primitive mantle may provide at least a partial solution to the “missing Re” problem in which one or more reservoirs with high Re/Yb are required to balance the low Re/Yb of MORB. Lithophile trace element compositions of most Ko’olau and Moloka’i tholeiites are consistent with variable degrees of melting of fertile mantle peridotite. However, light rare earth element (LREE)-enriched glasses have trace element compositions more consistent with a garnet-rich source having a distinctive trace element composition. This provides additional evidence for a unique source component possibly related to recycled oceanic crust contributing to Ko’olau tholeiites.

Rift zone reorganization through flank instability in ocean island volcanoes: an example from Tenerife, Canary Islands

The relationship between rift zones and flank instability in ocean island volcanoes is often inferred but rarely documented. Our field data, aerial image analysis, and 40Ar/39Ar chronology from Anaga basaltic shield volcano on Tenerife, Canary Islands, support a rift zone—flank instability relationship. A single rift zone dominated the early stage of the Anaga edifice (~6–4.5 Ma). Destabilization of the northern sector led to partial seaward collapse at about ~4.5 Ma, resulting in a giant landslide. The remnant highly fractured northern flank is part of the destabilized sector. A curved rift zone developed within and around this unstable sector between 4.5 and 3.5 Ma. Induced by the dilatation of the curved rift, a further rift-arm developed to the south, generating a three-armed rift system. This evolutionary sequence is supported by elastic dislocation models that illustrate how a curved rift zone accelerates flank instability on one side of a rift, and facilitates dike intrusions on the opposite side. Our study demonstrates a feedback relationship between flank instability and intrusive development, a scenario probably common in ocean island volcanoes. We therefore propose that ocean island rift zones represent geologically unsteady structures that migrate and reorganize in response to volcano flank instability.

Giant landslides, mega-tsunamis, and paleo-sea level in the Hawaiian Islands

Landslide tsunami simulations have advanced to the point where the tsunamigenic potential of giant submarine landslides (GSL) can be affirmed, while the subsidence history of different Hawaiian Islands is still subject to debate. We show that mega-tsunamis are a sufficient explanation for the observed pattern of debris height of calcareous marine deposits on some of the Hawaiian Islands. Further, our tsunami simulations, using the Alika GSL as example, can be used to reduce the considerable uncertainty in subsidence history of the different Hawaiian Islands, a current obstacle to interpreting the deposits from large waves. We also show that the onset of interglacials provides a probable explanation for the timing of these giant landslides over at least the last five million years. The climate change mechanism both explains the confusion with eustatic sea-level rise and provides a reasonable triggering mechanism for giant landslides from oceanic island volcanoes.

Experiments on rift zone evolution in unstable volcanic edifices

Large ocean island volcanoes frequently develop productive rift zones located close to unstable flanks and sites of older major sector collapses. Flank deformation is often caused by slip along a décollement within or underneath the volcanic edifice. We studied how such a stressed volcanic flank may bias the rift zone development. The influence of basal lubrication and lateral flank creep on rift development and rift migration is still poorly constrained by field evidence; here our analog experiments provide new insights. We injected colored water into gelatin cones and found systematic orientations of hydro-fractures (dikes) propagating through the cones. At the base of the cone, diverse friction conditions were simulated. By variation of the basal creep conditions we modeled radial dike swarms, collinear rift zones and three-armed rift systems. It is illustrated that a single outward-creeping flank is sufficient to modify the entire rift architecture of a volcano. The experiments highlight the general unsteadiness of dike swarms and that the distribution and alteration of weak substratum may become a major player in shaping a volcano’s architecture.

Compositional variation during monogenetic volcano growth and its implications for magma supply to continental volcanic fields

Individual volcanoes of continental monogenetic volcanic fields are generally presumed to erupt single magma batches during brief eruptions. Nevertheless, in two unrelated volcanic fields (the Waipiata volcanic field, New Zealand, and the Miocene–Pliocene volcanic field in western Hungary), we have identified pronounced and systematic compositional differences among products of individual volcanoes. We infer that this indicates a two-stage process of magma supply for these volcanoes. Each volcano records: (1) intrusion of a basanitic parent magma to lower- to mid-crustal levels and its subsequent fractionation to form a tephritic residual melt; (2) subsequent transection of this reservoir by a second batch of basanitic melt, with tephrite rising to the surface at the head of the propagating basanite dyke. Eruption at the surface then yields initial tephrite, typically erupted as pyroclasts, followed by eruption and shallow intrusion of basanite from deeper in the dyke. By analogy with similar tephrite–basanite eruptions along rift zones of intraplate ocean-island volcanoes, we infer that fractionation to tephrite would have required decades to centuries. We conclude that the two studied continental monogenetic volcanic fields demonstrate a consistent history of early magmatic injections that fail to reach the surface, followed by capture and partial eruption of their evolved residues in the course of separate and significantly later injections of basanite that extend to the surface and erupt. This systematic behaviour probably reflects the difficulty of bringing small volumes of dense, primitive magma to the surface from mantle source regions. Ascent through continental crust is aided by the presence in the dyke head of buoyant tephrite captured during transection of the earlier-emplaced melt bodies.

Nature and origin of toreva remnants and volcaniclastics from La Palma, Canary Islands

New data pertaining to the origin of volcaniclastic fan delta deposits (the El Time group) and previously unreported toreva remnants (intact landslide blocks) are presented in terms of their relationships with the Quaternary flank destruction of Bejenado volcano on La Palma, Canary Islands. Bejenado volcano is the heavily dismantled relic of a stratocone which developed at the confluence of the Cumbre Nueva embayment and the Caldera de Taburiente collapse amphitheatre. Both of these giant landslide structures formed at around 0.55 Ma after ∼0.2 Ma of gravitational spreading on the western and eastern flanks of the Taburiente–Cumbre Nueva shield volcanic complex. Evidence is presented on the basis of stratigraphic, photogeologic and Geographic Information System studies that both the toreva remnants and the El Time group can be related to the sequential collapses of the north flank and summit region of Bejenado inside the Caldera de Taburiente and the northernmost part of the adjoining Cumbre Nueva embayment. The collapses probably occurred between 0.44 and 0.4 Ma and they involved a volume of around 5 km 3. The toreva remnants display a deformed volcanic architecture, traversed by discontinuities which developed during impact and post-collapse dislocation during slip. They are composed of layer-differentiated scoria with ancillary ankaramite and basanite–tephrite lavas and occasional dikes. They rest upon a thin veneer of sheared volcaniclastic deposits in relative proximity to the disfigured north flank of Bejenado. The collapse event(s) is distinguished by its extremely low mobility due to the deceleration of the debris avalanche mass within the confines of the Caldera de Taburiente. The high friction coefficient ( H/ L=0.22) reflects this topographic factor. Reworking of the avalanche matrix gave rise to the accumulation and offshore progradation of the fan delta deposits at the SW base of Bejenado. Contemporaneous fluvial sediments, sourced by the retrogressive erosion of the pre-Bejenado collapse scarps, were fed into the subaerial confines of the Cumbre Nueva embayment. Piecemeal scarp failure and the extrusion of substratum from beneath the scarps also contributed to the sediment budget. The fluvial sedimentation rates subsequently increased with the onset of Cumbre Vieja volcanism prior to 0.125 Ma. These processes have contributed to the development of the flat-floored submarine channels which extend onto the volcaniclastic apron west of La Palma. These new data give a fresh insight into the role of epiclastic processes, landsliding and volcanism in the morphology and sediment mass balance of an oceanic island volcano system.

CARRACEDO, J. C. &amp; DAY, S. 2002. Canary Islands. Classic Geology in Europe Series Volume 4. viii + 294 pp. Harpenden: Terra Publishing. Price £14.95 (paperback). ISBN 1 903544 07 6

The Canary Islands in the Northeast Atlantic are a wonderful place to study volcanic, structural and sedimentological aspects of ocean island volcanoes. Their geology is varied and superbly exposed, with over fourteen million years of submarine and subaerial, effusive and explosive volcanic activity, intrusion, caldera collapse, landslides, and erosion recorded. Visitors should not be put off by the recent over-sensationalist TV documentaries predicting future landslides and the inundation of Florida and New York by tsunami! The presence of voluminous felsic explosive products, sediments and uplifted submarine successions, make the Canaries more varied than the Hawaiian islands with which they are often compared. With low-cost flights, accommodation and car-hire, they have become particularly accessible to northern Europeans. To my knowledge Canary Islands is the only field guidebook that attempts to cover the geology of all seven main islands in the Canary archipelago. This is an ambitious remit, and the authors …

Giving birth to hotspot volcanoes: Distribution and composition of young seamounts from the seafloor near Tahiti and Pitcairn islands

Apart from being popular holiday destinations, oceanic-island volcanoes such as Hawaii, Tahiti, or the Canaries provide magmas that yield valuable information about the interior of our planet. Until recently, studies have concentrated on the easily accessible, subaerial parts of the volcanoes, largely ignoring their earlier-formed, submarine parts. These submarine parts, however, provide critical information about how the mantle begins to melt and about the lowest-melting-point mantle components-information not available from the subaerial volcanoes but highly relevant for the chemical evolution of the whole mantle. We present here compositional information from small (<500 m) volcanoes on the seafloor near Tahiti and Pitcairn Islands and show that these small volcanoes erupt only highly differentiated magmas. These early melts are derived exclusively from the most trace element-enriched, isotopically extreme mantle component, evidence that this component has the lowest melting temperature and is the first product of melting of a new batch of mantle. The geochemical mantle components (enriched mantle EM-I, EM-II) proposed in the 1980s to explain the compositional variations among oceanic volcanoes worldwide appear in reality to represent distinct rock masses in the mantle.

Ups and downs on spreading flanks of ocean-island volcanoes: Evidence from Mauna Loa and Kīlauea

Submarine-flank deposits of Hawaiian volcanoes are widely recognized to have formed largely by gravitationally driven volcano spreading and associated landsliding. Observations from submersibles show that prominent benches at middepths on flanks of Mauna Loa and Kīlauea consist of volcaniclastic debris derived by landsliding from nearby shallow submarine and subaerial flanks of the same edifice. Massive slide breccias from the mature subaerial tholeiitic shield of Mauna Loa underlie the frontal scarp of its South Kona bench. In contrast, coarse volcaniclastic sediments derived largely from submarine-erupted preshield alkalic and transitional basalts of ancestral Kīlauea underlie its Hilina bench. Both midslope benches record the same general processes of slope failure, followed by modest compression during continued volcano spreading, even though they record development during different stages of edifice growth. The dive results suggest that volcaniclastic rocks at the north end of the Kona bench, interpreted by others as distal sediments from older volcanoes that were offscraped, uplifted, and accreted to the island by far-traveled thrusts, alternatively are a largely coherent stratigraphic assemblage deposited in a basin behind the South Kona bench.

InSAR observations of the 1995 Fogo, Cape Verde, eruption: Implications for the effects of collapse events upon island volcanoes

Fogo volcano, Cape Verde Islands, erupted in April 1995 after 43 years of dormancy. About 46 106 m3 of lava erupted during 7.5 weeks from vents on the SW flank of Pico do Fogo into Cha das Caldeiras. Interferograms obtained from 1993–1998 ERS SAR data show ground deformation due to the feeder dike but lack evidence for any volcano‐wide deformation related to volume changes of a shallow magma reservoir. This suggests that Fogo is fed from a relatively deep, mantle‐lithospheric source (&gt;16.5 km depth), consistent with petrological data. This supports the concept that Cha das Caldeiras is the fill of a collapse scar created by a giant landslide, and not a collapse caldera overlying a central, shallow magma chamber. The disturbance of the magmatic system by giant landslides may explain why some oceanic island volcanoes with high magma supply rates currently lack well‐developed shallow reservoirs.

Olivine-hosted melt inclusions in Hawaiian picrites: equilibration, melting, and plume source characteristics

Olivine-hosted melt inclusions in tholeiitic picrites from five Hawaiian volcanoes (Koolau, Mauna Loa, Kilauea, Loihi, and Hualalai) have major and trace element compositions that illustrate the magmatic characteristics of ocean island volcanoes and the nature of mantle plumes. The geochemistry of these melt inclusions reflects the well known geochemical features that distinguish Hawaiian shield volcanoes, but with considerably greater diversity than whole rock compositions, providing a higher resolution of the magmatic processes contributing to Hawaiian plume magmatism. Naturally quenched inclusions from Kilauea, Mauna Loa, and Hualalai have been modified by crystallization of olivine on the walls of the inclusion and diffusive interaction with the host crystal. In contrast, melt inclusions in two Loihi picrites have not been affected by re-equilibration with their host olivines, reflecting a relatively brief interval between crystallization of the olivines and eruption of these lavas. Corrected major element compositions of experimentally melted inclusions from two Koolau picritic tholeiites are similar to those of erupted lavas from this volcano and document the presence of Koolau melts with at least 14% MgO. Trace element characteristics of melt inclusions from Mauna Loa, Kilauea, and Loihi can be produced by melting of a moderately depleted, garnet lherzolite source. The extent of melting generally increases from Loihi<Kilauea<Mauna Loa, although rare inclusions from Mauna Loa also indicate contributions of relatively small degree (2–4%) melts to these lavas. Extent of melting and isotopically defined source components appear to be linked in the melting regime, with the Mauna Loa component being sampled preferentially at larger degrees of melting. Melt inclusions with trace element characteristics indicating a recycled basaltic eclogite source were not found at Mauna Loa or any of the other volcanoes. Compositions of the Koolau inclusions do require a unique source component, however, possibly reflecting contributions from ancient lithosphere, either within the plume or entrained in the upper mantle.

Trace metal suites in Antarctic pre-industrial ice are consistent with emissions from quiescent degassing of volcanoes worldwide

Abstract Trace metals are more abundant in atmospheric load and deposition material than can be due to rock and soil dusts and ocean salt. In pre-industrial ice from coastal west Antarctica, dust and salt account for only a few percent of the lead, cadmium, and indium that is present in most samples, less than half in any sample. For these trace metals, the deposition rate to the pre-industrial ice is approximately matched by the output rate to the atmosphere by quiescent (non-explosive) degassing of volcanoes worldwide, according to a new estimate. The basis of the match is the masses and proportions of the metals, and the proportions of Pb isotopes, in ice and in volcano emissions. The isotopic compositions of Pb in ice are similar to those of a suite of ocean island volcanoes, mostly in the southern hemisphere. The natural baseline values for pre-industrial atmospheric deposition fluxes of trace metal suites at Taylor Dome, and the worldwide quiescent volcano emissions fluxes to which they are linked, constitute a reasonably well-constrained baseline component for deposition fluxes of metals in modern times.

Submarine growth and internal structure of ocean island volcanoes based on submarine observations of Mauna Loa volcano, Hawaii

A recent model for the submarine growth of Hawaiian volcanoes indicates that these volcanoes are composed mainly of fragmental lava debris formed as lavas enter the ocean. This model has major implications for locating earthquake hypocenters and for the landslide hazard potential of these and other ocean island volcanoes. Observations from submersible dives and analyses of volcanic glasses collected from the western submarine flank of Mauna Loa indicate that subaerially erupted pillow lavas are abundant at depths of 950 to 1900 m below sea level. Fragmental lava is an important component of ocean island volcanoes, as witnessed during the most recent eruption of Kilauea volcano, but probably is the dominant lithology only in the upper 1 km of the submarine section. A submarine dike complex was discovered 17 km west of the assumed axis of Mauna Loa9s southwest rift, which indicates that its intrusive complex is much broader than previously suspected (∼20 km vs. ∼8 km). The great width of this dike complex may be a consequence of crustal unloading following the South Kona landslide or a normal feature of Hawaiian rift zones that was previously unrecognized.

Impacts of volcanism on pre-European inhabitants of Taveuni, Fiji

Taveuni is a Fijian ocean-island volcano that sporadically erupted throughout the Holocene. The 437-km2 island is an active monogenetic volcanic field with a constantly shifting locus of activity along a single apparent rift axis. Although the eruptions were not large ( ≤VEI 2), unexpected shifts in Taveuni volcanism had the potential to affect habitation sites. Since known human settlement of the Fiji Group (ca. 950–750 BC), there have been at least 58 eruptions on Taveuni. Up to 25 of these eruptions potentially affected pre-European inhabitants of the island and at least four former occupation sites are known to have been affected by volcanic products. Despite apparent earliest settlement of Taveuni post-dating other nearby islands by up to 600 years, volcanism probably did not hinder or stall settlement of Taveuni compared with neighbouring islands. However, a period of voluminous eruptions between 300 and 500 AD covered much of south Taveuni with lava and/or thick tephra, apparently causing abandonment of at least this portion of Taveuni until approximately 1100 AD. Most eruptions were not of catastrophic proportions and, due to their localised effects, re-settlement was rapid in marginal unaffected areas. Localised stories and a relict place name survive to describe former eruption locations and effects since approximately 120–320 AD. Knowledge of the impacts on Taveuni's past inhabitants forms the basis of volcanic disaster-mitigation strategies to minimise future effects on the current 14,500 residents.

Extrinsic controls on the evolution of Hawaiian ocean island volcanoes

Extrinsic parameters that affect the evolution of magmatic systems within and beneath ocean island volcanoes include physical variables such as confining pressure, which controls magma degassing, and temperature of the underlying lithosphere and crust, which controls magma crystallization during ascent. Other extrinsic parameters are environmental variables coupled to the hydrosphere and atmosphere such as hydrothermal circulation systems and even rainfall. All these extrinsic factors interact with intrinsic parameters, such as magma supply rates or composition, to modulate the evolution of magma chambers and the petrologic processes that take place within them.

Fernandina Volcano's evolved, well‐mixed basalts: Mineralogical and petrological constraints on the nature of the Galapagos plume

Fernandina Volcano, the most active of Galapagos volcanoes and the one most directly overlying the Galapagos hotspot, is one of a small number of active, plume‐related oceanic island volcanoes with a well‐documented recent eruptive record. Whole rock and glass analyses of lava and tephra from all 12 known eruptions from 1958 to the present show them to be evolved (Mg # (= Mg/Mg + Fe2+) of 0.47–0.56), plagioclase‐phyric, clinopyroxene‐ and olivine‐bearing tholeiites. Thermodynamic modeling indicates that this phase assemblage erupted at 1100–1130°C and is stable only at low pressures (&lt;0.1–0.2 GPa), thereby representing crystallization within a shallow magma chamber underlying the well‐defined, 850‐m‐deep, 5 by 6.5 km caldera. These tholeiites are remarkably homogenous, as is shown by narrow ranges in Zr/Y (5.7–6.3), (La/Sm)N (1.4–1.7), and (La/Yb)N (3.8–4.3). Prehistoric Fernandina lavas, selected for petrographic and geographic variation over the entire subaerial volcano, are of remarkably similar composition (e.g., Zr/Y is 5.2–6.1, (La/Sm)N is 1.5–1.6, and (La/Yb)N is 3.4–4.1), with a single example of a more primitive lava having an Mg # of 0.63, with 8.5% FeO at 8% MgO. Cr‐spinel and glass melt inclusions in plagioclase, together with zoned olivine, give evidence for a range of parental melts in the recent shallow magmatic system, with Mg # as high as 0.62. Extensive, shallow magma mixing is required to explain these variations in mineral and inclusion chemistry, indicating more thermal variation within the shallow magmatic system than is implied by the relatively monotonous whole rock compositions. Apparently, Fernandina's shallow magma chamber has physically blocked passage of more primitive magmas to the surface and homogenized the erupted lavas by extensive mixing. Small variations in lava isotopic and compatible element composition, combined with the need to resupply melt to maintain thermal balance, indicate that replenishment of the magma chamber is semicontinuous, with resupply occurring of the order of a decade or less. Plagioclase‐dominated, evolved Fernandina lavas are in sharp contrast to more primitive, olivine‐dominated Hawaiian lavas; the total lack of primitive melts in the shallow Fernandina magmatic system implies that substantial fractionation and heat loss occurred during melt transportation from the mantle source to the edifice magma chamber, despite the fact that these melts were transported through relatively warm, young, and thin lithosphere adjacent to the Cocos‐Nazca ridge. The diffuse nature of Galapagos volcanism, with 20th century eruptions from eight distinct volcanoes over a 20,000 km2 area, further supports this interpretation and suggests that the Galapagos plume is relatively diffuse, weak, and significantly less thermally intense than the vigorous Hawaiian plume. It may in fact be waning.

The 40Ar/39Ar age dating of the Madeira Archipelago and hotspot track (eastern North Atlantic)

The 40Ar/39Ar ages for 35 volcanic rocks and 14C ages for two charcoal samples from the Madeira Archipelago and Ampère Seamount (eastern North Atlantic) are presented. The volcanic evolution of Madeira can be divided into a voluminous shield stage (&gt;4.6–0.7 Ma) and a subsequent low‐volume posterosional stage (&lt;0.7–0 Ma). Volcanism during the shield stage originated from a two‐armed rift system, composed of the E–W oriented Madeira rift arm and the N–S oriented Desertas rift arm. Average growth rates for the submarine (5500 km3/Ma) and subaerial (100–150 km3/Ma) shield stages on Madeira are among the lowest found for ocean island volcanoes. It is proposed that Madeira represents the present location of a &gt;70 Myr old hotspot which formed Porto Santo Island (11.1–14.3 Ma), Seine, Ampère (31 Ma), Corral Patch and Ormond (65–67 Ma [Féraud et al., 1982, 1986]) Seamounts, and the Serra de Monchique (70–72 Ma [McIntyre and Berger, 1982]) complex in southern Portugal. Age and spatial relationships result in a calculated absolute African plate motion above the hotspot of 1.2 cm/yr around a rotation pole located at 43°36′N/ 24°33′W.