Volcano Growth Research Articles

Topographic History of the Maui Nui Complex, Hawai'i, and Its Implications for Biogeography

The Maui Nui complex of the Hawaiian Islands consists of the islands of Maui, Moloka'i, Lana'i, and Kaho'olawe, which were connected as a single landmass in the past. Aspects of volcanic landform construction, island subsidence, and erosion were modeled to reconstruct the physical history of this complex. This model estimates the timing, duration, and topographic attributes of different island configurations by accounting for volcano growth and subsidence, changes in sea level, and geomorphological processes. The model indicates that Maui Nui was a single landmass that reached its maximum areal extent around 1.2 Ma, when it was larger than the current island of Hawai'i. As subsidence ensued, the island divided during high sea stands of interglacial periods starting around 0.6 Ma; however during lower sea stands of glacial periods, islands reunited. The net effect is that the Maui Nui complex was a single large landmass for more than 75% of its history and included a high proportion of lowland area compared with the contemporary landscape. Because the Hawaiian Archipelago is an isolated system where most of the biota is a result of in situ evolution, landscape history is an important determinant of biogeographic patterns. Maui Nui's historical landscape contrasts sharply with the current landscape but is equally relevant to biogeographical analyses.

Internal structure of Puna Ridge: evolution of the submarine East Rift Zone of Kilauea Volcano, Hawai ̀i

Multichannel seismic reflection, sonobuoy, gravity and magnetics data collected over the submarine length of the 75 km long Puna Ridge, Hawai ̀i, resolve the internal structure of the active rift zone. Laterally continuous reflections are imaged deep beneath the axis of the East Rift Zone (ERZ) of Kilauea Volcano. We interpret these reflections as a layer of abyssal sediments lying beneath the volcanic edifice of Kilauea. Early arrival times or ‘pull-up’ of sediment reflections on time sections imply a region of high P-wave velocity ( V p) along the submarine ERZ. Refraction measurements along the axis of the ridge yield V p values of 2.7–4.85 km/s within the upper 1 km of the volcanic pile and 6.5–7 km/s deeper within the edifice. Few coherent reflections are observed on seismic reflection sections within the high-velocity area, suggesting steeply dipping dikes and/or chaotic and fractured volcanic materials. Southeastward dipping reflections beneath the NW flank of Puna Ridge are interpreted as the buried flank of the older Hilo Ridge, indicating that these two ridges overlap at depth. Gravity measurements define a high-density anomaly coincident with the high-velocity region and support the existence of a complex of intrusive dikes associated with the ERZ. Gravity modeling shows that the intrusive core of the ERZ is offset to the southeast of the topographic axis of the rift zone, and that the surface of the core dips more steeply to the northwest than to the southeast, suggesting that the dike complex has been progressively displaced to the southeast by subsequent intrusions. The gravity signature of the dike complex decreases in width down-rift, and is absent in the distal portion of the rift zone. Based on these observations, and analysis of Puna Ridge bathymetry, we define three morphological and structural regimes of the submarine ERZ, that correlate to down-rift changes in rift zone dynamics and partitioning of intrusive materials. We propose that these correspond to evolutionary stages of developing rift zones, which may partially control volcano growth, mobility, and stability, and may be observable at many other oceanic volcanoes.

New insights into Kilauea's volcano dynamics brought by large‐scale relative relocation of microearthquakes

We investigated the microseismicity recorded in an active volcano to infer information concerning the volcano structure and long‐term dynamics, by using relative relocations and focal mechanisms of microearthquakes. There were 32,000 earthquakes of the Mauna Loa and Kilauea volcanoes recorded by more than eight stations of the Hawaiian Volcano Observatory seismic network between 1988 and 1999. We studied 17,000 of these events and relocated more than 70%, with an accuracy ranging from 10 to 500 m. About 75% of these relocated events are located in the vicinity of subhorizontal decollement planes, at a depth of 8–11 km. However, the striking features revealed by these relocation results are steep southeast dipping fault planes working as reverse faults, clearly located below the decollement plane and which intersect it. If this decollement plane coincides with the pre‐Mauna Loa seafloor, as hypothesized by numerous authors, such reverse faults rupture the pre‐Mauna Loa oceanic crust. The weight of the volcano and pressure in the magma storage system are possible causes of these ruptures, fully compatible with the local stress tensor computed by Gillard et al. [1996]. Reverse faults are suspected of producing scarps revealed by kilometer‐long horizontal slip‐perpendicular lineations along the decollement surface and therefore large‐scale roughness, asperities, and normal stress variations. These are capable of generating stick‐slip, large–magnitude earthquakes, the spatial microseismic pattern observed in the south flank of Kilauea volcano, and Hilina‐type instabilities. Rupture intersecting the decollement surface, causing its large‐scale roughness, may be an important parameter controlling the growth of Hawaiian volcanoes.

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.

Do transcurrent faults guide volcano growth? The case of NW Bicol Volcanic Arc, Luzon, Philippines

ABSTRACT This paper shows that although volcanoes can be aligned along transcurrent faults, their growth can in reality be controlled by secondary structures that have a different orientation and kinematics. Assessing the relationships between local structures and volcano growth can be very useful for the evaluation of volcanic hazards and for economic purposes such as geothermal and hydrogeological exploration. In the north‐western Bicol Volcanic Arc (Luzon, Philippines) the Quaternary Labo and Caayunan volcanoes are aligned with the NW‐striking transcurrent Philippine Fault System. They are exceptional examples of composite volcanoes mostly comprising lava domes, where regional tectonic structures and hydraulic radial fracturing have alternated to control volcano growth. Field data and analogue modelling demonstrate that apical depressions affecting the domes reflect the geometry of magma‐feeding fractures. These data, together with alignments of domes and hot springs and other structural information, suggest that magma rose at depth along the NW‐striking transcurrent faults, but that in the uppermost crust magma rose along NE‐striking fractures parallel to the greatest principal stress.

Volatiles in glasses from Mauna Loa Volcano, Hawai'i: implications for magma degassing and contamination, and growth of Hawaiian volcanoes

Glasses from Mauna Loa pillow basalts, recent subaerial vents, and inclusions in olivine were analyzed for S, Cl, F, and major elements by electron microprobe. Select submarine glasses were also analyzed for H2O and CO2 by infrared spectroscopy. The compositional variation of these tholeiitic glasses is dominantly controlled by crystal fractionation and they indicate quenching temperatures of 1,115–1,196 °C. Submarine rift zone glasses have higher volatile abundances (except F) than nearly all other submarine and subaerial glasses with the maximum concentrations increasing with water depth. The overwhelming dominance of degassed glasses on the submarine flanks of Mauna Loa implies that much of volcano's recent submarine growth involved subaerially erupted lava that reached great water depths (up to 3.1 km) via lava tubes. Anomalously high F and Cl in some submarine glasses and glass inclusions indicate contamination possibly by fumarolic deposits in ephemeral rift zone magma chambers. The relatively high CO2 but variable H2O/K2O and S/K2O in some submarine rift zone glasses indicates pre-eruptive mixing between degassed and undegassed magma within Mauna Loa's rift system. Volatile compositions for Mauna Loa magmas are similar to other active Hawaiian volcanoes in S and F, but are less Cl-rich than Lō'ihi glasses. However, Cl/K2O ratios are similar. Mauna Loa and Lō'ihi magmas have comparable, but lower H2O than those from Kilauea. Thus, Kilauea's source may be more H2O-rich. The dissimilar volatile distribution in glasses from active Hawaiian volcanoes is inconsistent with predictions for a simple, concentrically zoned plume model.

Hawaiian lava flows in the third dimension: Identification and interpretation of pahoehoe and ′a′a distribution in the KP‐1 and SOH‐4 cores

Hawaiian lava flows are classified as pahoehoe or 'a'a by their surface morphology. As surface morphology reflects flow emplacement conditions, the surface distribution of morphologic flow types has been used to study the evolution and eruptive history of basaltic volcanoes. We extend this analysis to the third dimension by determining the distribution of flow types in two deep drill cores, the Scientific Observation Hole‐4 (SOH‐4) core, drilled near Kilauea's East Rift Zone (ERZ), and the pilot hole (Kahi Puka‐1 (KP‐1)) for the Hawaiian Scientific Drilling Project (HSDP), drilled through distal flows from Mauna Loa and Mauna Kea. Flows are classified using both internal structures and groundmass textures, with the latter useful when identification based on mesoscopic flow features (e.g., surface morphology and vesicle content and distribution) is ambiguous. We then examine the temporal distribution of pahoehoe and ′a′a flows in proximal (SOH‐4) and distal (KP‐1) settings. Sequence analysis shows that the two flow types are not randomly distributed in either core but instead are strongly clustered. The proximal SOH‐4 core is dominated by thin pahoehoe flows (∼60% by volume), consistent with the common occurrence of surface‐fed pahoehoe flows in near‐vent settings. The distal KP‐1 core has a high proportion of ′a′a (∼58% by volume), although the proportion of pahoehoe and ′a′a varies dramatically throughout the Mauna Kea sequence. Thick inflated pahoehoe flows dominate when the drill site was near sea level, consistent with the numerous inflated pahoehoe fields on the current coastal plains of Kilauea and Mauna Loa. ′A′a flows are abundant when the site was far above sea level. As slope increases from the coastal plains to Mauna Kea's flank, this correlation may reflect the combined effect of long transport distances and increased slopes on flow emplacement. These results demonstrate that flow type and thickness variations in cores provide valuable information about both vent location and local site environment. Observed variations in flow type within the KP‐1 core raise interesting questions about feedback between volcano evolution and flow morphology and suggest that flow type is an important variable in models of volcano growth and related models for lava flow hazard assessment.

Volcanic spreading on Mauna Loa volcano, Hawaii: Evidence from accretion, alteration, and exhumation of volcaniclastic sediments

A transect of submersible dives across the submarine west flank of Mauna Loa volcano yields compelling evidence for volcanic spreading and associated hydrothermal circulation during volcano growth. A frontal bench at the toe of the flank, formerly thought to be a downdropped block of Mauna Loa, contains a mix of volcaniclastic lithologies, including distally derived siltstone, mudstone, and hyaloclastite. The bench is overlain by bedded gravels and subaerially erupted pillow flows derived from local shoreline-crossing lava flows. The volcaniclastic strata in the bench were offscraped, uplifted, and accreted to the edge of the flank, as it plowed seaward into the surrounding moat. The accreted strata underwent significant diagenesis, through deep burial and circulation of hydrothermal fluids expelled from porous sediments beneath the volcano. Timing constraints for bench growth and breakup suggest that catastrophic failure of the subaerial edifice ca. 250–200 ka triggered volcanic spreading by reducing stresses resisting basal sliding and rift-zone inflation. Increased eruptive activity, and westward migration of Mauna Loa9s southwest rift zone, gradually rebuilt the massive flank, arresting slip prior to detachment of the Alika 2 debris avalanche ca. 120 ka.

How old is the Hawaiian biota? Geology and phylogeny suggest recent divergence.

This study quantifies long-term landscape changes in the Hawaiian archipelago relating to dispersal, speciation and extinction. Accounting for volcano growth, subsidence and erosion, we modelled the elevations of islands at time intervals of 0.5 Myr for the last 32 Myr; we also assessed the variation in the spacing of volcanoes during this period. The size, spacing and total number of volcanic islands have varied greatly over time, with the current landscape of large, closely spaced islands preceded by a period with smaller, more distantly spaced islands. Considering associated changes in rates of dispersal and speciation, much of the present species pool is probably the result of recent colonization from outside the archipelago and divergence within contemporary islands, with limited dispersal from older islands. This view is in accordance with abundant phylogenetic studies of Hawaiian organisms that estimate the timing of colonization and divergence within the archipelago. Twelve out of 15 multi-species lineages have diverged within the lifetime of the current high islands (last 5 Myr). Three of these, and an additional seven (mostly single-species) lineages, have colonized the archipelago within this period. The timing of colonization of other lineages remains uncertain.

The Indian Ocean Nodule Field: petrotectonic evolution and ferromanganese deposits

The Indian Ocean Nodule Field (IONF) is significant from several points of view. Roughly bordered by 10°S to 16°30′S and 72°E to 80°E and located within the Central Indian Ocean Basin (CIOB), the field hosts the world's second largest and second high grade manganese nodule deposit, after the equatorial nodule belt in the north Pacific Ocean. Moreover, the crust underlying this field is characterised by unique morphotectonic signatures owing to its formation between 60 and 49 Ma under three variable spreading conditions, fast, intermediate and slow, from the Indian Ocean Ridge System (IORS). The nodule field has been surveyed both extensively (more than 0.4 million km 2 area) and intensively (comprising of a large geophysical data set and geological sample inventory) during the last two decades. Several morphotectonic features, such as seamounts, hills, ridge-normal lineaments and ridge-parallel lineations, have disturbed the apparently smooth topographic gradient (1:7000) of the seafloor here. Variations in the rate of spreading and formation of new oceanic crust along the ridge crest, during more than one episode of India–Eurasia collision, are imprinted in the IONF. Based on the nature of the ridge-parallel lineations, which are related to the rate of crustal accretion, the field is divided in to four sectors: A, B, C, and D, from north to south. Sectors A and C were formed at a fast rate of spreading (90–95 mm/year, half-rate) and sectors B and D were formed at an intermediate (55 mm/year) and slow (26 mm/year) rates, respectively. The predominance of tensional stress in sectors A and C caused asymmetrical flexuring of the seafloor, resulting in widely spaced faults and folds with low amplitude and large wavelength. In contrast, the seafloor flexuring in sector D are closely spaced, long, symmetrical and of high amplitude. The timing and intensity of the collision of India with Eurasia is constrained by the variable intensity of these flexures, suggesting probably a ‘soft’ touch at ∼58 Ma and the hard collision at about 51 Ma. The nodule field hosts several seamounts, both as isolated entities and in linear chains, which are arranged parallel to the flow lines along the direction of absolute motion of the Indian plate. The distribution, morphology and growth patterns of a majority of these seamounts are related to spreading rate, suggesting their formation at the ridge crest. However, many of the seamounts show more than one stage of growth with local intraplate volcanism contributing to the enlargement of the larger ancient seamounts. Varieties of volcanics, such as tholeiitic basalts, spilites, ferrobasalts and pumice, occur within the IONF. The alteration of some of these volcanics has resulted in palagonitisation of the glass and formation of zeolites. Subsequently, during its journey away from the ridge crest to the abyssal areas, the crust underlying the nodule field witnessed intraplate volcanism. This is evident from the addition of younger rocks at the base of the ancient seamounts, inconsistent growth of volcanoes, eruption of ferrobasalt corresponding to the areas of high amplitude magnetic zones and presence of volcanogenic–hydrothermal materials (vhm) of 10 ka age. These findings collectively hint that the IONF is geodynamically unstable and may have been volcanically active in the recent past. During the last 8 Ma, growth of authigenic ferromanganese deposits in the form of manganese nodules and crusts has occurred in the nodule field. The deposits occur at an average water depth of 5000 m. The basinal geomorphology, intraplate tectonic deformations and volcanic eruptions considerably influence the formation, development, morphology, mineralogy and composition of these deposits. The data show that the large seamounts, reverse faults and fracture zones (FZs) supply rock fragments as ‘seeds’ for the nodule formation. The hydrogenous precipitation from the overlying water column is the primary source of metal accumulation in the nodules. The secondary (relatively weak) intraplate eruptions along the base of ancient seamounts or lineaments, subsurface igneous activity and diagenetic remobilisation of metals have also played significant role in the growth and enrichment of the deposit. Based on a large data set, we estimate the contribution of various physico-chemical parameters and model the probable conditions of formation of the ferromanganese deposits in the IONF. The model also hints at the location where the resource exploitation should be rewarding.

Facies analysis of proximal subglacial and proglacial volcaniclastic successions at the Eyjafjallajökull central volcano, southern Iceland

Abstract The long-lived (at least 0.78 Ma) Eyjafjallajökull volcano has been constructed during a period of dramatic climatic shifts, and has produced subaerial lavas and cinder cones, pillow lavas, hyaloclastite, monogenetic volcaniclastic sediments and polygenetic glacio-fluvial sediments. Subaqueous lithofacies are dominant and nine distinct cogenetic lithofacies associations are identified within the successions that are interpreted as the proximal deposits of subglacial eruptions. These associations are typically bound by unconformities and lie directly on thin diamictites or glaciated surfaces; epiclastic sedimentary rocks are absent. The lithofacies include subaqueous sheet lava flows, lobate flows, pillow lavas, breccias, hyaloclastite and hyalotuff generated by eruption of mainly basaltic lavas. Lavas associated with massive hyaloclastite breccias commonly lie on or intrude cogenetic redeposited hyalotuff, indicating rapid changes in style of activity from explosive to effusive. This suggests that the vent was initially subaqueous but became subaerial as meltwater drained away down-slope beneath temperate ice sheets. In other instances, probably under thicker ice, the eruptions were effusive throughout but lava was subjected to intense steam explosivity. There is abundant evidence that volcaniclastic material was transported downslope by mass flow, grain flow and traction currents (i.e. by running water). Lithofacies associations that ponded in ice-dammed water commonly comprise thick redeposited tephra overlain by a ‘passage zone’ of hyaloclastite and then subaerial lavas. These deposits are voluminous but conditions suitable for ponding of large volumes of water were not common in the evolution of this large volcano. During the early stages of volcano growth, eruptive deposits were emplaced on the gentle slopes of the growing cone during both glacial and interglacial periods. Glacial erosion modified the growing edifice and, as a result, younger deposits (<600 ka) tend to be valley-confined. The unconformities produced during glacial advance represent significant time gaps within the succession. Observations support the hypothesis that there were higher volcanic production rates during periods of deglaciation, probably as a result of the rapid decrease in lithostatic pressure as ice melted.

Submarine evidence for large-scale debris avalanches in the Lesser Antilles Arc

Results from a recent marine geophysical survey demonstrate the importance of the process of flank collapse in the growth and evolution of volcanoes along an island arc. The Aguadomar cruise, aboard the French R/V L’Atalante, surveyed the flanks of the Lesser Antilles Arc between the islands of Montserrat and St. Lucia. Analysis of the data shows that flank collapse events occurred on active volcanoes all along the arc and resulted in debris avalanches, some of them being of large magnitude. The debris avalanche deposits display hummocky topography on the swath bathymetry, speckled pattern on backscatter images, hyperbolic facies on 3.5 kHz echosounder data and chaotic units on air gun seismic profiles. They extend from horseshoe-shaped structures previously identified on the subaerial part of the volcanoes. In the southern part of the arc, large-scale debris avalanche deposits were identified on the floor of the Grenada Basin west of active volcanoes on Dominica, Martinique and St. Lucia. The extent of debris avalanche deposits off Dominica is about 3500 km 2. The debris avalanches have resulted from major flank collapse events which may be mainly controlled by the large-scale structure of the island arc and the presence of the deep Grenada Basin. In the northern part of the arc, several debris avalanche deposits were also identified around the island of Montserrat. With smaller extent (20–120 km 2), they are present on the east, south and west submarine flanks of Soufriere Hills volcano which has been erupting since July 1995. Flank collapse is thus a recurrent process in the recent history of this volcano. The marine data are also relevant for a discussion of the transport mechanisms of debris avalanches on the seafloor surrounding a volcanic island arc.

The long-term growth of volcanic edifices: numerical modelling of the role of dyke intrusion and lava-flow emplacement

The contribution of intrusive complexes to volcano growth is attested by field observations and by the monitoring of active volcanoes. We used numerical simulations to quantitatively estimate the relative contributions to volcano growth of elastic dislocations related to dyke intrusions and of the accumulation of lava flows. The ground uplift induced by dyke intrusions was calculated with the equations of Okada (Bull. Seismol. Soc. Am., 75 (1985) 1135). The spreading of lava flows was simulated as the flow of a Bingham fluid. With realistic parameters for dyke statistics and lava-flow rheology we find the contribution of dyke intrusions to the growth of a basaltic shield archetype to be about 13% in terms of volume and 30% in terms of height. The result is strongly dependent on the proportion of dykes reaching the surface to feed a lava flow. Systematic testing of the model indicates that edifices tend to be high and steep if dykes are thick and high, issued from a small and shallow magma chamber, and if they feed lava flows of high yield strength. The simulation was applied to Ko'olau (O'ahu Is., Hawai'i) and Piton de la Fournaise (Réunion Is.) volcanoes. The simulation of Ko'olau with dyke parameters as described by Walker (Geology, 14 (1986) 310; U.S. Geol. Surv. Prof. Pap., 1350 (1987) 961) and with lava-flow characteristics collected at Kilauea volcano (Hawai'i Is.) results in an edifice morphology very close to that of the real volcano. The best fit model of the Piton de la Fournaise central cone, with its steep slope and E–W elongation, is obtained by the intrusion of 10 000 short and thick dykes issued from a very small and shallow magma chamber and feeding only 700 low-volume lava flows. The same method may be applied to the growth of basaltic shields and other volcano types in different environments, including non-terrestrial volcanism.

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.

Near-axis seamount distribution and its relationship with the segmentation of the East Pacific Rise and northern Pacific–Antarctic Ridge, 17°N–56°S

We used satellite-derived gravity anomaly maps and bathymetry data to analyze the distribution of off-axis seamounts on young crust on the flanks of the East Pacific Rise (EPR) and northern Pacific–Antarctic Ridge (PAR), from 17°N to 56°S latitude. We observed large-scale variations in the distribution of the volcanoes which we attribute to variations of the regional magmatic budget of the underlying mantle. Two distinct settings for off-axis volcanism were observed at the scale of second-order segments. The first one favors volcano growth near first- or second-order discontinuities. The second type occurs preferentially near the centers of second-order segments. We infer that the distribution of seamounts is controlled both by the availability of melt under the flanks of the ridge and by the vulnerability of the young lithosphere. Near the northern EPR and in some places near the PAR, the off-axis volcanism results mostly from fracturing of the crust near axial discontinuities, probably due to the thermomechanical stresses that develop in the cooling lithosphere. In the southern EPR, especially in the 14–19°S area, the young lithosphere is probably much more vulnerable to off-axis volcanism. The seamount distribution seems to reflect variations in the off-axis melt production at the scale of second-order segments (∼150 km). We infer that the difference in volcanic setting (close to segment ends or close to segment centers) between the northern and southern EPR results from the difference in spreading rate and, consequently, the variation in rheology of the young lithosphere.

Growth, structure, instability and collapse of Canarian volcanoes and comparisons with Hawaiian volcanoes

Recent onshore and offshore investigations in the Canarian archipelago, especially in the western islands of La Palma and El Hierro, have greatly improved the understanding of the genesis and evolution of these islands and allow interesting comparisons with other hotspot-induced oceanic island volcanoes, especially the Hawaiian archipelago. Genesis induced by hotspot activity and, consequently, shield and post-erosional stages of growth allow the definition of similar main stratigraphic units. The Canarian and Hawaiian volcanoes show common constructional and structural features, such as rift zones, progressive volcano instability and multiple gravitational collapses. However, the Canaries present some important geological differences from the prototypical hotspot volcanoes of the Hawaiian Islands, particularly the absence of significant subsidence; The Canarian volcanoes remain emergent until completely mass-wasted by gravitational collapses and erosion. Volcanic formations over 20 million years are observable in outcrop in the Canaries, including the seamount stages of growth in several of the islands.

Widespread assimilation of a seawater-derived component at Loihi Seamount, Hawaii

Many tholeiitic and transitional pillow-rim and fragmental glasses from Loihi seamount, Hawaii, have high Cl contents and Cl/K 2O ratios (and ratios of Cl to other incompatible components, such as P 2O 5, H 2O, etc.) relative to other Hawaiian subaerial volcanoes (e.g., Mauna Loa, Mauna Kea, and Kilauea). We suggest that this results from widespread contamination of Loihi magmas by a Cl-rich, seawater-derived component. Assimilation of high-Cl phases such as saline brine or Cl-rich minerals (halite or iron–hydroxychlorides) with high Cl/H 2O ratios can explain the range and magnitude of Cl contents in Loihi glasses, as well as the variations in the ratios of Cl to other incompatible elements. Brines and Cl-rich minerals are thought to form from seawater within the hydrothermal systems associated with submarine volcanoes, and Loihi magmas could plausibly have assimilated such materials from the hydrothermal envelope adjacent to the magma chamber. Our model can also explain semiquantitatively the observed contamination of Loihi glasses with atmospheric-derived noble gases, provided the assimilant has concentrations of Ne and Ar comparable to or slightly less than seawater. This is more likely for brines than for Cl-rich minerals, leading us to favor brines as the major assimilant. Cl/Br ratios for a limited number of Loihi samples are also seawater-like, and show no indication of the higher values expected to be associated with the assimilation of Cl-rich hydrothermal minerals. Although Cl enrichment is a common feature of lavas from Loihi, submarine glasses from other Hawaiian volcanoes show little (Kilauea) or no (Mauna Loa, Mauna Kea) evidence of this process, suggesting that assimilation of seawater-derived components is more likely to occur in the early stages of growth of oceanic volcanoes. Summit collapse events such as the one observed at Loihi in October 1996 provide a ready mechanism for depositing brine-bearing rocks from the volcanic edifice into the top of a submarine summit magma chamber.

Results of repeated leveling surveys at Newberry Volcano, Oregon, and near Lassen Peak Volcano, California

Personnel from the U.S. Geological Survey's Cascades Volcano Observatory conducted first-order, class-II leveling surveys near Lassen Peak, California, in 1991 and at Newberry Volcano, Oregon, in 1985, 1986, and 1994. Near Lassen Peak no significant vertical displacements had occurred along either of two traverses, 33 and 44 km long, since second-order surveys in 1932 and 1934. At Newberry, however, the 1994 survey suggests that the volcano's summit area had risen as much as 97±22 mm with respect to a third-order survey in 1931. The 1931 and 1994 surveys measured a 37-km-long, east–west traverse across the entire volcano. The 1985 and 1986 surveys, on the other hand, measured only a 9-km-long traverse across the summit caldera with only one benchmark in common with the 1931 survey. Comparison of the 1985, 1986, and 1994 surveys revealed no significant differential displacements inside the caldera. A possible mechanism for uplift during 1931–1994 is injection of approximately 0.06 km3 of magma at a depth of approximately 10 km beneath the volcano's summit. The average magma supply rate of approximately 1×10–3 km3/year would be generally consistent with the volcano's growth rate averaged over its 600,000-year history (0.7–1.7×10–3 km3/year).

Growth of large volcanoes on Venus: Mechanical models and implications for structural evolution

The structure, tectonics, and evolution of large volcanoes on Venus, as revealed by data from the Magellan mission, appear to be distinct from those on Earth and Mars. To determine the conditions and processes that account for these differences, we model the evolution of stress and deformation in growing volcanic edifices on Venus using the finite element method. Large volcanoes on Venus are characterized by topographically prominent conical edifices surrounded by relatively flat flow aprons. The surfaces of both edifice and apron consist dominantly of radially oriented flows. Tectonic features, usually the surface expression of shallowly intruded dikes, also have predominantly radial orientations. Features similar to Hawaiian‐style linear rift zones or large‐scale flank failure, common on large volcanoes on Earth and Mars, are absent. By comparing predictions of faulting for models with detached and welded basal boundary conditions with the observed tectonic features, we determine that a welded condition is more likely. Horizontal compressive stresses transmitted into edifices across welded basal boundaries are inversely proportional to elastic lithosphere thickness Te. Contravening stress increments from magma chamber expansion, differential thermal contraction, or buoyant loading from beneath the crust or lithosphere are required to reorient principal stresses in the edifice so that magma ascent and radial dike formation are favored. Incremental volcano growth results in stress distributions that decrease with height in the edifice. Dike intrusion is easiest in the uppermost (youngest) layers, consistent with the observed shallow radial dikes on Venus volcanoes. Small values of Te greatly inhibit the formation of large shields, in that conical edifice topography cannot be supported and large contravening stresses are required for further growth. Large volcano formation is much more likely at large Te, where more moderate contravening stresses are sufficient for growth. The near‐surface manifestation of mantle upwelling on Venus may thus depend on Te in that the response to upwellings beneath thin or nonexistent lithosphere is likely to be dominantly ductile, leading to the formation of coronae, but a thick elastic lithosphere is required to support the growth of the largest volcanoes. The transitional value of Te between these modes of evolution likely depends on the horizontal scale of the upwelling.

Cyclic flank-vent and central-vent eruption patterns

Many basaltic and andesitic polygenetic volcanoes have cyclic eruptive activity that alternates between a phase dominated by flank eruptions and a phase dominated by eruptions from a central vent. This paper proposes the use of time-series diagrams of eruption sites on each polygenetic volcano and intrusion distances of dikes to evaluate volcano growth, to qualitatively reconstruct the stress history within the volcano, and to predict the next eruption site. In these diagrams the position of an eruption site is represented by the distance from the center of the volcano and the clockwise azimuth from north. Time-series diagrams of Mauna Loa, Kilauea, Kliuchevskoi, Etna, Sakurajima, Fuji, Izu-Oshima, and Hekla volcanoes indicate that fissure eruption sites of these volcanoes migrated toward the center of the volcano linearly, radially, or spirally with damped oscillation, occasionally forming a hierarchy in convergence-related features. At Krafla, terminations of dikes also migrated toward the center of the volcano with time. Eruption sites of Piton de la Fournaise did not converge but oscillated around the center. After the convergence of eruption sites with time, the central eruption phase is started. The intrusion sequence of dikes is modeled, applying crack interaction theory. Variation in convergence patterns is governed by the regional stress and the magma supply. Under the condition that a balance between regional extension and magma supply is maintained, the central vent convergence time during the flank eruption phase is 1–10 years, whereas the flank vent recurrence time during the central eruption phase is greater than 100 years owing to an inferred decrease in magma supply. Under the condition that magma supply prevails over regional extension, the central vent convergence time increases, whereas the flank vent recurrence time decreases owing to inferred stress relaxation. Earthquakes of M≥6 near a volcano during the flank eruption phase extend the central vent convergence time. Earthquakes during the central eruption phase promote recurrence of flank eruptions. Asymmetric distribution of eruption sites around the flanks of a volcano can be caused by local stress sources such as an adjacent volcano.