Large Volcanoes Research Articles

Volcano spreading and fault interaction influenced by rift zone intrusions: Insights from analogue experiments analyzed with digital image correlation technique

Large volcanoes tend to be structurally unstable and subject to various forms of deformation and mass wasting. Although morphological changes associated with flank deformation and flank instability are similar on many islands, the details of the structural expression, the fault geometry and the size of the mobile flanks vary significantly. Flank instability on ocean island volcanoes is thought to be related to two principal mechanisms: gravitational spreading and intrusions into a rift zone. In this paper, we first summarize typical structures observed in nature, and then experimentally investigate structures characteristic of volcanic spreading and rift zone intrusions, in order to better understand the interplay of these two mechanisms. We reproduced unstable, intruding and deforming volcanic flanks in a sand-box and systematically studied faults developing during (i) gravitational spreading, (ii) rift zone intrusion, and (iii) combined spreading and intrusion. We recorded the experiments with a digital camera and measured deformation by applying a digital image correlation technique. This enables us to study high resolution strain fields, locate faults and analyze their related activities. The end-member models show that typical spreading structures are listric normal faults, whereas typical rift zone intrusion structures include a graben at the surface and a subhorizontal detachment at depth. Concurrent or alternating simulations of spreading and rift zone intrusions reveal the interplay of the developed faults; some are hindered while others are more encouraged to slip. For instance, while the pure spreading end-member promotes internal faulting within the unstable flank, the occurrence of rift zone intrusions hinders this type of faulting and hence may even stabilize part of the volcanic flank. We apply this understanding to Kilauea (Hawaii) and Piton de la Fournaise (La Réunion) in an attempt to better identify the related mechanisms of intrusions, flank instability and sector collapse. Important implications arise for geological and geophysical field data, such as seismicity or geodetic measurements.

Bathymetric constraints on the tectonic and volcanic evolution of Deception Island Volcano, South Shetland Islands

AbstractDeception Island is the largest volcano in the actively extending Bransfield Basin, a marginal basin situated behind the extinct South Shetland Islands arc. Deception Island has been well studied but its submerged flanks have not. A multibeam bathymetry survey was conducted around the island in 2005. Data from the flooded caldera show no evidence for recent localized resurgence. The gently-sloped bottom of the caldera basin is consistent with either a broad zone of resurgence on its east side associated with trap door deformation or with higher rates of sediment supply from the east side of the island. Around the island, numerous tectonic and volcanic features on the volcano's east and west flanks are nearly all aligned with the regional strike (~060°) of the Bransfield rift and there is very little evidence for the other fault populations that have been identified on the island. We infer that models that link the ongoing tectonic development of Deception Island to complex regional tectonics are less likely than models in which the dominant regional extension in Bransfield Strait is modulated by the local effects of caldera collapse and possibly a small right-lateral transfer zone offsetting the primary extension axes in the Central and Western Bransfield Basins.

Types of collapse calderas

Three main types of collapse calderas can be defined, 1) summit caldera: those formed at the top of large volcanoes, 2) classic caldera: semi-circular to irregular-shaped large structures, several km in diameter and related to relatively large-volume pyroclastic products, and 3) graben caldera: explosive volcano-tectonic collapse structures from which large-volume, ignimbrite-forming eruptions occurred through several fissural vents along the graben master faults and the intra-graben block faults. These in turn can collapse at least with three styles: 1) Piston: when the collapse occurs as a single crustal block; 2) Trap-door: when collapse occurs unevenly along one side while the opposite side remains with no collapse; 3) Piece-meal: when collapse occurs as broken pieces of the crust on top of the magma chamber.

Tropical mountain glaciers on Mars: Altitude-dependence of ice accumulation, accumulation conditions, formation times, glacier dynamics, and implications for planetary spin-axis/orbital history

Fan-shaped deposits up to ∼ 166 , 000 km 2 in area are found on the northwest flanks of the huge Tharsis Montes volcanoes in the tropics of Mars. Recent spacecraft data have confirmed earlier hypotheses that these lobate deposits are glacial in origin. Increased knowledge of polar-latitude terrestrial glacial analogs in the Antarctic Dry Valleys has been used to show that the lobate deposits are the remnants of cold-based glaciers that formed in the extremely cold, hyper-arid climate of Mars. Mars atmospheric general circulation models (GCM) show that these glaciers could form during periods of high obliquity when upwelling and adiabatic cooling of moist air favor deposition of snow on the northwest flanks of the Tharsis Montes. We present a simulation of the Tharsis Montes ice sheets produced by a static accumulation pattern based on the GCM results and compare this with the nature and extent of the geologic deposits. We use the fundamental differences between the atmospheric snow accumulation environments (mass balance) on Earth and Mars, geological observations and ice-sheet models to show that two equilibrium lines should characterize ice-sheet mass balance on Mars, and that glacial accumulation should be favored on the flanks of large volcanoes, not on their summits as seen on Earth. Predicted accumulation rates from such a parameterization, together with sample spin-axis obliquity histories, are used to show that obliquity in excess of 45° and multiple 120,000 year obliquity cycles are necessary to produce the observed deposits. Our results indicate that the formation of these deposits required multiple successive stages of advance and retreat before their full extent could be reached, and thus imply that spin-axis obliquity remained at these high values for millions of years during the Late Amazonian period of Mars history. Spin-axis obliquity is one of the main factors in the distribution and intensity of solar insolation, and thus in determining the climate history of Mars. Unfortunately, reconstruction of past climate history is inhibited by the fact that the chaotic nature of the solution makes the calculation of orbital histories unreliable prior to about 20 Ma ago. We show, however, that the geological record, combined with glacial modeling, can be used to provide insight into the nature of the spin-axis/orbital history of Mars in the Late Amazonian, and to begin to establish data points for the geologically based reconstruction of the climate and orbital history of Mars.

Holocene tephras in lake cores from northern British Columbia, Canada

Sediment cores recovered from alpine and subalpine lakes up to 250 km apart in northern British Columbia contain five previously unrecognized tephras. Two black phonolitic tephras, each 5–10 mm thick, occur within 2–4 cm of each other in basal sediments from seven lakes in the Finlay River – Dease Lake area. The upper and lower Finlay tephras are slightly older than 10 220 – 10 560 cal year B.P. and likely originate from two closely spaced eruptions of one or two large volcanoes in the northern Cordilleran volcanic province. The Finlay tephras occur at the transition between deglacial sediments and organic-rich postglacial mud in the lake cores and, therefore, closely delimit the termination of the Fraser Glaciation in northern British Columbia. Sediments in Bob Quinn Lake, which lies on the east edge of the northern Coast Mountains, contain two black tephras that differ in age and composition from the Finlay tephras. The lower Bob Quinn tephra is 3–4 mm thick, basaltic in composition, and is derived from an eruption in the Iskut River volcanic field about 9400 cal years ago. The upper Bob Quinn tephra is 12 mm thick, trachytic in composition, and probably 7000–8000 cal years old. A fifth tephra occurs as a cryptotephra near the top of two cores from the Finlay River area and is correlated to the east lobe of the White River tephra (ca. 1150 cal year B.P.). Although present throughout southern Yukon, the White River tephra has not previously been documented this far south in British Columbia. The tephras are valuable new isochrons for future paleoenvironmental studies in northern British Columbia.

Proposed re‐dating of the European ice core chronology by seven years prior to the 7th century AD

Progress in the study of past climate change requires integration of historical documentation, tree‐ring data and ice‐core chemistry. Larsen et al. (2008) re‐evaluate volcanic acid signals in the Dye‐3, GRIP and NGRIP ice cores and identify large eruptions at 529 ± 2 and 533–534 ± 2; the latter claimed to cause the ‘dry fog’ of 536–537 and environmental consequences to 550; this conflicts with previous suggestions of a two stage environmental event; one in 536, the second c.540. Understanding the full range of natural hazards requires critical evaluation of the happenings in the mid‐6th century. This paper will show that 4 spaced tree‐ring responses to extreme environmental conditions in the 6th century can be used to re‐locate 4 key ice‐acid dates. Moving the ice‐core chronology forward 7 years places two large volcanoes in 536 and 541 explaining the extended duration of the 536–550 effects and anchoring the ice chronology.

238U– 230Th– 226Ra disequilibria in andesitic lavas of the last magmatic eruption of Guadeloupe Soufriere, french Antilles: Processes and timescales of magma differentiation

Andesitic products of the last magmatic eruption at La Soufrière in Guadeloupe (1530 AD) were analyzed for 238U– 230Th– 226Ra in order to constrain the timescales of magmatic processes preceding the eruption. Magma composition evolved during eruption from acid andesitic scoria falls and flows to andesitic lavas during the final phase of dome extrusion, with some evidence of mixing in banded pumices at the base of the first eruptive unit. Most of the least differentiated products show 238U and 226Ra excesses relative to 230Th, like many other arc lavas. The ( 226Ra/ 230Th) activity ratios of whole rocks then decrease with the degree of differentiation from 1.507 to 0.983, whereas the ( 230Th/ 238U) ratios increase from 0.942 to 1.003. The groundmasses of the least differentiated lava show a ( 226Ra/ 230Th) ratio higher than 1, but all the other samples are in secular equilibrium. These variations can be modeled by time-dependent differentiation from a common parental melt. Assuming a closed-or open-magma chamber behaviour (closed or open system), the duration of magma differentiation ranges from 30 to 70 kyr with crystallization rates ranging from 0.012 to 0.019 kyr − 1 . 238U– 230Th disequilibria have also been measured in minerals from one of the least differentiated products and provide crucial information on their age of crystallization, their origin and the crystal dynamics in the magma chamber. A crystallization age of 34.9 ± 8.3 kyr (2 σ) is obtained using U–Th internal isochrons on pyroxenes, which is older than the differentiation time of their host lava given by time-dependant differentiation models, but younger than the differentiation time of the most evolved magma compositions. Older cumulate crystals were thus recycled in the erupted magmas, consistent with petrologic evidence for crystal-melt disequilibria. The calculated ( 226Ra/ 230Th) ratios of mineral assemblages (> 1 for most samples) suggest the presence of plagioclases younger than 8000 yr, in agreement with the fact that plagioclases do not plot on the U–Th isochron defined by pyroxenes. This study thus exemplifies the hybrid nature of arc andesitic magmas such as the 1530 AD Soufriere magma, which is generated by mixing of crystals and melts produced by fractional crystallization of a common parental magma. Our results also confirm that the magma chamber below large volcanoes such as La Soufrière of Guadeloupe is long-lived (up to tens of thousands of years). This timescale is much longer than the cooling times predicted by a closed-system thermal model, suggesting that the magma chamber was sustained by inputs of fresh and hot magma and behaves as an open system.

Temporal development of the Atherton Basalt Province, north Queensland

The Atherton Basalt Province is centred on the Atherton Tableland, ∼50 km southwest of Cairns in north Queensland. Forty-eight K – Ar age determinations and four U/Th analyses from these basalts provide information on the distribution of the volcanics over time. Volcanism commenced at 7.1 Ma and continued into the Early Holocene, with volumetric peaks in activity occurring 3.5 – 3 Ma and 2 – 1 Ma. The province shows a change with time from eruptions of voluminous lava flows that built relatively large shield volcanoes, to the production of less voluminous lavas and pyroclastics associated with cinder cones during the last million years. Phreatomagmatic, maar-forming eruptions are also preserved among the most recent eruptions. Previous radiocarbon dating of swamp sediments suggested that volcanic activity may have occurred within the past 10 000 years. No systematic change over time is apparent in the location of the volcanoes of the province. Rather, a source region ∼80 km in diameter for the province as a whole has evolved over time. Extensive partial melting in the upper mantle may have led to the ascent and adiabatic melting of lesser volumes of mantle material from deeper levels. Changes in lithospheric stresses in north Queensland, caused by the docking of the Ontong Java Plateau and subsequent development of northward subduction at the San Cristobal Trench, may have allowed the ascent of magma from this mantle region.

Asymmetric generation of oceanic crust at the ultra‐slow spreading Southwest Indian Ridge, 64°E

We describe topographic, gravity, magnetic, and sonar data from a Southwest Indian Ridge spreading segment near 64°E, 28°S. We interpret these to reveal crustal structure, spreading history, and volcanic and tectonic processes over the last 12 Myr. We confirm that the crust is some 2 km thicker north of the ridge axis, though it varies along and across axis on scales of ∼10 km and 4 Myr. The plate separation rate remained approximately constant at 13 ± 1 km Myr−1, but half‐spreading rates were up to 40% asymmetric, varying between faster‐to‐the‐north and faster‐to‐the‐south on a 4 Myr timescale. Topography shows a dominant E–W lineation normal to the N–S spreading direction. This is superficially similar to faulted abyssal hill terrain of the Mid‐Atlantic Ridge (MAR), but inferred fault scarps are 3–4 times more widely spaced and have greater offsets. Conjugate pairs of massifs on either plate are interpreted as volcanic constructions similar to the large volcano currently filling the median valley at the segment center. They have temporal spacings of ∼4 Myr and are thought to reflect episodic melt focusing along an otherwise melt‐poor ridge. Additionally, there are places, mainly on the southern plate, where lineated topography is replaced by a much blockier topography and embryonic ocean core complexes similar to those recently reported on the MAR near 13°N. There is generally more extrusive volcanism on the northern plate and more tectonism on the southern one. Extrusive volcanism has propagated westward from the segment center since 2 Ma. The FUJI Dome core complex and adjacent seafloor to its east and west appear to be part of a single coherent block, capped by extrusive rock near the segment center, exposing gabbro via a detachment fault over the Dome and probably exposing deeper crust or upper mantle farther west near the segment end. Magnetic anomalies are continuous along this block. We suggest that at its eastern boundary the detachment is simply welded onto magmatically emplaced crust to the east in a similar way to young crust being welded to the old plate at ridge‐transform intersections.

風土としての日本の水―自然地理の視点から―

This article summarizes the nature and the behavior of waters in Japan from the viewpoint of physical geography. Because Japan is located in a humid temperate zone lacking a severe dry season, a water deficit is not detected in normal years in terms of water balance analysis. Mean annual specific discharge is between 3 m3/s/100 km2 and 5 m3/s/100 km2 in many river basins, although it is more than 5 m3/s/100 km2 in rainy or snowy regions. Snow is an important source of rivers in Japan.Lakes are relatively abundant. Basin profiles are characterized by steep slopes, reflecting the general landform of Japan. Lakes of volcanic origins and coastal lakes are widely distributed. Strong thermal stratification is also a characteristic of lakes. There are about 210, 000 irrigation ponds, which is one of the special features in Japan.Heavy snowfalls in mountain ranges in northern Japan create perennial snow patches. As summer is very hot, the occurrence of snow patches needs an accumulation of snow of more than 20 m.Groundwater is also abundant, especially at flood plains, alluvial fans, and bases of large volcanoes. On plains where rice paddies are distributed, the groundwater level rises during the irrigation period due to seepage. Groundwater is also influenced by human activities as rivers and lakes.

MOC observations of the 2001 Mars planet-encircling dust storm

From 15 September 1997 through 21 January 2006, only a single planet-encircling martian dust storm was observed by MGS-MOC. The onset of the storm occurred on 26 June 2001 ( L s = 184.7 ° ), earliest recorded to date. It was initiated in the southern mid-to-low latitudes by a series of local dust storm pulses that developed along the seasonal cap edge in Malea and in Hellas basin ( L s = 176.2 ° – 184.4 ° ) . The initial expansion of the storm, though asymmetric, was very rapid in all directions (3–32 m s −1). The main direction of propagation, however, was to the east, with the storm becoming planet encircling in the southern hemisphere on L s = 192.3 ° . Several distinct centers of active dust lifting were associated with the storm, with the longest persisting for 86 sols (Syria–Claritas). These regional storms helped generate and sustain a dust cloud (“haze”), which reached an altitude of about 60 km and a peak opacity of τ dust ∼ 5.0 . By L s = 197.0 ° , the cloud had encircled the entire planet between 59.0° S and 60.0° N, obscuring all but the largest volcanoes. The decay phase began around L s ∼ 200.4 ° with atmospheric dust concentrations returning to nominal seasonal low-levels at L s ∼ 304.0 ° . Exponential decay time constants ranged from 30–117 sols. The storm caused substantial regional albedo changes (darkening and brightening) as a result of the redistribution (removal and deposition) of a thin veneer of surface dust at least 0.1–11.1 μm thick. It also caused changes in meteorological phenomena (i.e., dust storms, dust devils, clouds, recession of the polar caps, and possibly surface temperatures) that persisted for just a few weeks to more than a single Mars year. The redistribution of dust by large annual regional storms might help explain the long period (∼30 years) between the largest planet-encircling dust storms events.

Structure and emplacement of mud volcano systems in the South Caspian Basin

The term “mud volcano system” is coined to describe the set of structures associated with a constructional edifice (mud volcano) and feeder complex that connects the volcano to its source stratigraphic unit. Three-dimensional (3-D) seismic data from the South Caspian Basin are used to investigate the structural elements and evolution of these systems. Mud volcano systems initiate via early, kilometer-scale, biconic edifices termed “pioneer” cones. These are fed by fluidization pipes tens of meters in width. Subsequent kilometer-scale mud volcanoes grew via persistent extrusion, fed by numerous additional fluidization pipes injected in the country rock. This subvolcanic intrusion complex creates a densely intruded, cylindrical zone, similar in cross section to gryphon swarms observed at an outcrop onshore. Wall rock erosion and compaction of the intruded zone leads to the collapse of a downward-tapering cone enveloping the cylindrical zone, capped by ring faults that define a kilometer-scale caldera that downthrows the overlying mud volcano. Mud volcanoes get buried during basin subsidence and can look like intrusive laccoliths at first glance on seismic data. Reactivation of mud flow through a conduit system generates a stack of superimposed mud volcanoes through time. Large volcanoes continue to dewater during burial and may locally remobilize. This model of mud volcano evolution has similarities with igneous and salt tectonic systems. To reduce drilling and geologic uncertainty, mud volcano system extent and impacts on a reservoir can be assessed on 3-D seismic data. Simon Stewart received his Ph.D. from the Imperial College London in 1992. In 1992–2000, he was seismic interpreter at Amerada Hess, in support of exploration and appraisal drilling in the North Sea. In 2000, he moved to BP and has since worked on structural geology and well planning in the Americas, Middle East, and the former Soviet Union. His special interest is in the integration of structural geology and seismic interpretation. Richard Davies received his Ph.D. from the University of Edinburgh in 1995. In 1995–2003, he was with Mobil and ExxonMobil, working on field development and exploration, the North Sea, the west of Shetlands, and the west Niger Delta deep-water slope. In 2003–2005, he was a senior lecturer in earth sciences at Cardiff University, United Kingdom. Currently, he is professor and director of CeREES (Center for Research into Earth Energy Systems), Durham University, United Kingdom. His special interest is in seismic-scale expression of fluid migration and diagenesis.

The geological evolution of Atai Mons, Venus: a volcano–corona ‘hybrid’

Atai Mons, Venus, is classified as a large volcano yet displays characteristics that are typical of coronae. Coronae are widespread volcano-tectonic features that are apparently unique to Venus. Detailed analysis of Atai Mons using photogeological and geophysical techniques illustrates that it has experienced a complex history and may still be active. We show that processes found at both coronae and large volcanoes have occurred throughout the history of Atai Mons with no definite temporal preference. It does not have an evolution that can be described simply as the evolution of one feature into another. These results suggest that large volcanoes and coronae are more intimately linked than previously thought, and that detailed mapping and gravity surveys are reliable ways of establishing the evolution of hybrid features.

Gravitational spreading and formation of new rift zones on overlapping volcanoes

AbstractLarge volcanic edifices are often shaped by the coalescence of adjacent volcanoes as well as intrusive rift zones and gravitational spreading. To better understand the structure of such volcanoes we designed analogue experiments simulating gravitational spreading of an edifice made by overlapping cones of different age, and examined the formation of rift zones. The results allow distinction of two main rift geometries. (i) Spreading edifices of similar age that partly overlap, tend to develop a rift zone approximately perpendicular to the boundary of both volcanoes. Such a rift zone causes two volcanoes to grow together and form an elongated topographic ridge. (ii) Partly overlapping volcanoes of different age are spreading at different rates and thus form a rift zone parallel to the boundary of both volcanoes. Such a rift zone causes two volcanoes to structurally separate. The results are widely applicable for large volcanoes subject to rifting and flank spreading, which we demonstrate for Réunion Island and for southern Hawaii.

Adequacy of the In Situ Observing System in the Satellite Era for Climate SST

Abstract A method is presented to evaluate the adequacy of the recent in situ network for climate sea surface temperature (SST) analyses using both in situ and satellite observations. Satellite observations provide superior spatiotemporal coverage, but with biases; in situ data are needed to correct the satellite biases. Recent NOAA/U.S. Navy operational Advanced Very High Resolution Radiometer (AVHRR) satellite SST biases were analyzed to extract typical bias patterns and scales. Occasional biases of 2°C were found during large volcano eruptions and near the end of the satellite instruments’ lifetime. Because future biases could not be predicted, the in situ network was designed to reduce the large biases that have occurred to a required accuracy. Simulations with different buoy density were used to examine their ability to correct the satellite biases and to define the residual bias as a potential satellite bias error (PSBE). The PSBE and buoy density (BD) relationship was found to be nearly exponential, resulting in an optimal BD range of 2–3 per 10° × 10° box for efficient PSBE reduction. A BD of two buoys per 10° × 10° box reduces a 2°C maximum bias to below 0.5°C and reduces a 1°C maximum bias to about 0.3°C. The present in situ SST observing system was evaluated to define an equivalent buoy density (EBD), allowing ships to be used along with buoys according to their random errors. Seasonally averaged monthly EBD maps were computed to determine where additional buoys are needed for future deployments. Additionally, a PSBE was computed from the present EBD to assess the in situ system’s adequacy to remove potential future satellite biases.

New multibeam mapping and geochemistry of the 30°–35° S sector, and overview, of southern Kermadec arc volcanism

New multibeam mapping and whole-rock geochemistry establish the first order definition of the modern submarine Kermadec arc between 30° and 35° S. Twenty-two volcanoes with basal diameters > 5 km are newly discovered or fully-mapped for the first time; Giggenbach, Macauley, Havre, Haungaroa, Kuiwai, Ngatoroirangi, Sonne, Kibblewhite and Yokosuka. For each large volcano, edifice morphology and structure, surficial deposits, lava fields, distribution of sector collapses, and lava compositions are determined. Macauley and Havre are large silicic intra-oceanic caldera complexes. For both, concentric ridges on the outer flanks are interpreted as recording mega-bedforms associated with pyroclastic density flows and edifice foundering. Other stratovolcanoes reveal complex histories, with repeated cycles of tectonically controlled construction and sector collapse, extensive basaltic flow fields, and the development of summit craters and/or small nested calderas. Combined with existing data for the southernmost arc segment, we provide an overview of the spatial distribution and magmatic heterogeneity along ∼780 km of the Kermadec arc at 30°–36°30′ S. Coincident changes in arc elevation and lava composition define three volcano–tectonic segments. A central deeper segment at 32°20′–34°10′ S has basement elevations of > 3200 m water-depth, and relatively simple stratovolcanoes dominated by low-K series, basalt–basaltic andesite. In contrast, the adjoining arc segments have higher basement elevations (typically < 2500 m water-depth), multi-vent volcanic centres including caldera complexes, and erupt sub-equal proportions of dacite and basalt–basaltic andesite. The association of silicic magmas with higher basement elevations (and hence thicker crust), coupled with significant inter- and intra-volcano heterogeneity of the silicic lavas, but not the mafic lavas, is interpreted as evidence for dehydration melting of the sub-arc crust. Conversely, the crust beneath the deeper arc segments is thinner, initially cooler, and has not yet reached the thermal requirements for anatexis. Silicic calderas with diameters > 3 km coincide with the shallower arc segments. The dominant mode of large caldera formation is interpreted as mass-discharge pyroclastic eruption with syn-eruptive collapse. Hence, the shallower arc segments are characterized by both the generation of volatile-enriched magmas from crustal melting and a reduced hydrostatic load, allowing magma vesiculation and fragmentation to initiate and sustain pyroclastic eruptions. Proposed initiation parameters for submarine pyroclastic eruptions are water-depths < 1000 m, magmas with 5–6 wt.% water and > 70 wt.% SiO 2, and a high discharge rate.

Eruption of the dacite to andesite zoned Mateare Tephra, and associated tsunamis in Lake Managua, Nicaragua

The dacite to andesite zoned Mateare Tephra is the fallout of a predominantly plinian eruption from Chiltepe peninsula at the western shore of Lake Managua that occurred 3000–6000 years ago. It comprises four units: Unit A of high-silica dacite is stratified, ash-rich lapilli fallout generated by unsteady subplinian eruption pulses affected by minor water access to the conduit and conduit blocking by degassed magma. Unit B of less silicic dacite is well sorted, massive pumice lapilli fallout from the main, steady plinian phase of the eruption. Unit C is andesitic fallout that is continuous from unit B except for the rapid change in chemical composition, which had little influence on the ongoing eruption except for a minor transient reduction of the discharge rate and access of water to the conduit. After this, discharge rate re-established to a strong plinian eruption that emplaced the main part of unit C. This was again followed by water access to the conduit which increased through upper unit C. The lithic-rich lapilli to wet ash fallout of unit D is the product of the fully phreatomagmatic terminal phase of the eruption. A massive well-sorted sand layer, the Mateare Sand, replaces laterally variable parts of unit A and lowermost part of unit B in outcrops up to 32 m above present lake level. The corresponding interval missing in the primary fallout can be identified by comparing the composition of pumice entrained in the sand, and pumice from the local base of unit B on top of the sand, with the compositional gradient in undisturbed fallout. The amount of fallout entrained in the sand decreases with distance to the lake. The Mateare Sand occurs at elevations well above beach levels and its widespread continuous distribution defies a fluviatile origin. Instead, it was produced by lake tsunamis triggered by eruption pulses during the initial unsteady phase of activity. Such tsunamis could threaten areas not affected by fallout, and represent a hazard of particular importance in Nicaragua where two large lakes host several explosive volcanoes.

Terrestrial record of post‐eocene climate history in marie byrd land, west antarctica

ABSTRACT. Hydrovolcanic deposits, interbedded tills and recycled microfossils, together with erosion anomalies in the Marie Byrd Land (MBL) landscape, each provide a portion of the record of glaciation and deglaciation events from late Oligocene to the present. We have attempted to synthesize these data sources to provide a more complete record, and to reconcile them with climatic events recorded elsewhere in Antarctica and the deep sea. The MBL data suggest that the late Oligocene was marked by the development of an ice cap at Mount Petras, where the MBL dome was beginning to rise from a near‐sea‐level position. Furthermore, unusually advanced cirque development in the dome crest area is difficult to explain unless there was a period of effective cirque erosion in that area between c. 25 and 15 Ma BP. These inferences are consistent with evidence from the Ross Sea for an expansion of the West Antarctic Ice Sheet (WAIS) in mid‐Miocene (15–17 Ma BP) time.The deep sea oxygen isotope proxy record has been interpreted to show the inception of West Antarctic glaciation around 6 Ma BP. This can perhaps be reconciled with the terrestrial record if one considers (1) the observation that large volume changes in the WAIS cannot produce a ω18O signal that is significantly outside limits of error, and (2) that the landscape in West Antarctica has evolved from very low regional relief in the Oligocene, to more than 2 km of local relief in the present day, as a result of dome uplift since c. 27 Ma BP, and the growth of large volcanoes since c. 19 Ma BP.

Internal structure and evolution of a volcanic rift system in the eastern North Atlantic: the Desertas rift zone, Madeira archipelago

The three elongated Desertas Islands form the top of a 60-km-long NW–SE-striking submarine ridge southeast of Madeira (NE Atlantic). The alignment of eruptive centres and parallel dyke swarms indicates that the islands represent a deeply eroded volcanic rift zone. Detailed field studies combined with 40Ar/ 39Ar age determinations and geochemical analyses reveal the internal structure and evolution of this rift, which may serve as a type example of North Atlantic rift zones. The northern rift end is composed mainly of lava flows up to several meter thickness and few scoria. The central part is characterised by a 100- to 200-m-thick basal sequence of tuff as a result of explosive eruptions due to magma–sea water interaction during the early subaerial stage. These layers are overlain by subhorizontal lava flows reflecting the change from dominantly explosive to more effusive volcanism during island growth. The southern part is dominated by a basal tuff sequence, scoria and cinder cones, minor lava flows and sills up to 30 m in thickness. In general, bomb- and scoria-rich deposits, dykes and sills increase in frequency towards the south. In contrast to rift systems of Hawaiian volcanoes such as Kilauea, we did not find any volcanological or petrological indication of a central caldera underlain by a shallow magma reservoir feeding the rift. 40Ar/ 39Ar ages show that subaerial Desertas volcanism lasted from >5.1 to 1.9 Ma overlapping with the shield-building phases of Madeira island. Desertas lavas show no temporal or spatial variation in chemistry, except for samples from the top of the islands, erupted between ∼3.4 and 2 Ma, which are strongly REE and Y enriched. We suggest that the Desertas ridge represents a discrete volcano that became interconnected with Madeira by growth to the north–northeast. Major volcanic centres may have been located at the southern end of the ridge and/or near its central part. A conceivable explanation for the formation of the elongated Desertas rift is a local gravitative stress field causing preferred extension between the Madeira and Desertas edifices as they progressively overlapped. Our model thus emphasises the importance of edifice amalgamation and gravitative stress in the evolution of large volcanoes and their rift zones. The initiation of Desertas volcanism could be the result of a weak pulsating mantle plume and irregular motion of the African plate, combined with flexural stresses within the lithosphere due to the loads of Madeira and Porto Santo islands. Cessation of volcanism coincides with the exclusive occurrence of Y- and REE-enriched lavas that may be related to sediment assimilation. Our results indicate that models for Hawaiian rifts cannot necessarily be transferred to hotspot volcanoes associated with weak plumes and/or irregular plate motion.

Resurfacing styles and rates on Venus: assessment of 18 venusian quadrangles

We have quantitatively assessed the resurfacing sources and styles in eighteen mapped venusian quadrangles, about 30% of the venusian surface. Each quadrangle was split into 0.5° by 0.5° boxes, which were then identified as corona materials, large volcano materials (>100 km diameter), intermediate volcano materials (10–100 km), small edifice materials (<10 km), flow materials from rifts or fractures, plains without an identifiable source, impact crater materials and highly deformed materials, or data gaps. We find that coronae resurface approximately 21%, small edifices 22% and large volcanoes about 6% of the surfaces analyzed. Plains with no identifiable source account for an average of 35% of the surface assessed. Small edifices resurface on a scale of 10–100 s of km 2; large edifices resurface areas of 10 4–10 5 km 2. Coronae have greatly varying amounts of associated volcanism, with some coronae producing negligible flow deposits and others producing deposits of 10 4–10 6 km 2. The areas identified as plains with no visible source occur on small scales (10 2 km 2) to large scales (> 10 5 km 2). Our results indicate that the majority of plains resurfacing by volcanism can be tied to an identifiable source, that fields of small edifices contribute more to resurfacing than we had anticipated, and that resurfacing styles do not appear to have evolved over the time period represented by the surface geology in the mapped quadrangles. All of the units that we quantified occur throughout the histories of the regions mapped. We favor plains resurfacing to have occurred over at least 100 myr, which implies terrestrially reasonable resurfacing rates.