The rhyolitic–andesitic eruptive history of Cotopaxi volcano,Ecuador

At Cotopaxi volcano, Ecuador, rhyolitic and andesitic bimodal magmatism has occurred periodically during the past 0.5 Ma. The sequential eruption of rhyolitic (70–75% SiO₂) and andesitic (56–62% SiO₂) magmas from the same volcanic vent over short time spans and without significant intermingling is characteristic of Cotopaxi’s Holocene behavior. This study documents the eruptive history of Cotopaxi volcano, presenting its stratigraphy and geologic field relations, along with the relevant mineralogical and chemical nature of the eruptive products, in order to determine the temporal and spatial relations of this bimodal alternation. Cotopaxi’s history begins with the Barrancas rhyolite series, dominated by pumiceous ash flows and regional ash falls between 0.4 and 0.5 Ma, which was followed by occasional andesitic activity, the most important being the ample andesitic lava flows (∼4.1 km³) that descended the N and NW sides of the edifice. Following a ∼400 ka long repose without silicic activity, Cotopaxi began a new eruptive phase about 13 ka ago that consisted of seven rhyolitic episodes belonging to the Holocene F and Colorado Canyon series; the onset of each episode occurred at intervals of 300–3,600 years and each produced ash flows and regional tephra falls with DRE volumes of 0.2–3.6 km³. Andesitic tephras and lavas are interbedded in the rhyolite sequence. The Colorado Canyon episode (4,500 years BP) also witnessed dome and sector collapses on Cotopaxi’s NE flank which, with associated ash flows, generated one of the largest cohesive debris flows on record, the Chillos Valley lahar. A thin pumice lapilli fall represents the final rhyolitic outburst which occurred at 2,100 years BP. The pumices of these Holocene rhyolitic eruptions are chemically similar to those of older rhyolites of the Barrancas series, with the exception of the initial eruptive products of the Colorado Canyon series whose chemistry is similar to that of the 211 ka ignimbrite of neighboring Chalupas volcano. Since the Colorado Canyon episode, andesitic magmatism has dominated Cotopaxi’s last 4,400 years, characterized by scoria bomb and lithic-rich pyroclastic flows, infrequent lava flows that reached the base of the cone, andesitic lapilli and ash falls that were carried chiefly to the W, and large debris flows. Andesitic magma emission rates are estimated at 1.65 km³ (DRE)/ka for the period from 4,200 to 2,100 years BP and 1.85 km³ (DRE)/ka for the past 2,100 years, resulting in the present large stratocone.     

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). Received: 10 September 1998 / Accepted: 4 December 1998     

The dynamics and thermodynamics of large ash flows

 Ash flow deposits, containing up to 1000 km³ of material, have been produced by some of the largest volcanic eruptions known. Ash flows propagate several tens of kilometres from their source vents, produce extensive blankets of ash and are able to surmount topographic barriers hundreds of metres high. We present and test a new model of the motion of such flows as they propagate over a near horizontal surface from a collapsing fountain above a volcanic vent. The model predicts that for a given eruption rate, either a slow (10–100 m/s) and deep (1000–3000 m) subcritical flow or a fast (100–200 m/s) and shallow (500–1000 m) supercritical flow may develop. Subcritical ash flows propagate with a nearly constant volume flux, whereas supercritical flows entrain air and become progressively more voluminous. The run-out distance of such ash flows is controlled largely by the mass of air mixed into the collapsing fountain, the degree of fragmentation and the associated rate of loss of material into an underlying concentrated depositional system, and the mass eruption rate. However, in supercritical flows, the continued entrainment of air exerts a further important control on the flow evolution. Model predictions show that the run-out distance decreases with the mass of air entrained into the flow. Also, the mass of ash which may ascend from the flow into a buoyant coignimbrite cloud increases as more air is entrained into the flow. As a result, supercritical ash flows typically have shorter runout distances and more ash is elutriated into the associated coignimbrite eruption columns. We also show that one-dimensional, channellized ash flows typically propagate further than their radially spreading counterparts. As a Plinian eruption proceeds, the erupted mass flux often increases, leading to column collapse and the formation of pumiceous ash flows. Near the critical conditions for eruption column collapse, the flows are shed from high fountains which entrain large quantities of air per unit mass. Our model suggests that this will lead to relatively short ash flows with much of the erupted material being elutriated into the coignimbrite column. However, if the mass flux subseqently increases, then less air per unit mass is entrained into the collapsing fountain, and progressively larger flows, which propagate further from the vent, will develop. Our model is consistent with observations of a number of pyroclastic flow deposits, including the 1912 eruption of Katmai and the 1991 eruption of Pinatubo. The model suggests that many extensive flow sheets were emplaced from eruptions with mass fluxes of 10⁹–10¹⁰ kg/s over periods of 10³–10⁵ s, and that some indicators of flow "mobility" may need to be reinterpreted. Furthermore, in accordance with observations, the model predicts that the coignimbrite eruption columns produced from such ash flows rose between 20 and 40 km. Received: 25 August 1995 / Accepted: 3 April 1996     

Simulating bubble number density of rhyolitic pumices from Plinian eruptions: constraints from fast decompression experiments

Decompression experiments of a crystal-free rhyolitic liquid with ≈ 6.6 wt. % H₂O were carried out at a pressure range from 250 MPa to 30–75 MPa in order to characterize effects of magma ascent rate and temperature on bubble nucleation kinetics, especially on the bubble number density (BND, the number of bubbles produced per unit volume of liquid). A first series of experiments at 800°C and fast decompression rates (10–90 MPa/s) produced huge BNDs (≈ 2 × 10¹⁴ m⁻³ at 10 MPa/s ; ≈ 2 × 10¹⁵ m⁻³ at 90 MPa/s), comparable to those in natural silicic pumices from Plinian eruptions (10¹⁵–10¹⁶ m⁻³). A second series of experiments at 700°C and 1 MPa/s produced BNDs (≈ 9×10¹² m⁻³) close to those observed at 800°C and 1 MPa/s (≈ 6 × 10¹² m⁻³), showing that temperature has an insignificant effect on BNDs at a given decompression rate. Our study strengthens the theory that the BNDs are good markers of the decompression rate of magmas in volcanic conduits, irrespective of temperature. Huge number densities of small bubbles in natural silicic pumices from Plinian eruptions imply that a major nucleation event occurs just below the fragmentation level, at which the decompression rate of ascending magmas is a maximum (≥ 1 MPa/s).     

Recent eruptions of Mount Adams, Washington Cascades, USA

 The postglacial eruption rate for the Mount Adams volcanic field is ∼0.1 km³/k.y., four to seven times smaller than the average rate for the past 520 k.y. Ten vents have been active since the last main deglaciation ∼15 ka. Seven high flank vents (at 2100–2600 m) and the central summit vent of the 3742-m stratocone produced varied andesites, and two peripheral vents (at 2100 and 1200 m) produced mildly alkalic basalt. Eruptive ages of most of these units are bracketed with respect to regional tephra layers from Mount Mazama and Mount St. Helens. The basaltic lavas and scoria cones north and south of Mount Adams and a 13-km-long andesitic lava flow on its east flank are of early postglacial age. The three most extensive andesitic lava-flow complexes were emplaced in the mid-Holocene (7–4 ka). Ages of three smaller Holocene andesite units are less well constrained. A phreatomagmatic ejecta cone and associated andesite lavas that together cap the summit may be of latest Pleistocene age, but a thin layer of mid-Holocene tephra appears to have erupted there as well. An alpine-meadow section on the southeast flank contains 24 locally derived Holocene andesitic ash layers intercalated with several silicic tephras from Mazama and St. Helens. Microprobe analyses of phenocrysts from the ash layers and postglacial lavas suggest a few correlations and refine some age constraints. Approximately 6 ka, a 0.07-km³ debris avalanche from the southwest face of Mount Adams generated a clay-rich debris flow that devastated >30 km² south of the volcano. A gravitationally metastable 2-to 3-km³ reservoir of hydrothermally altered fragmental andesite remains on the ice-capped summit and, towering 3 km above the surrounding lowlands, represents a greater hazard than an eruptive recurrence in the style of the last 15 k.y. Received: 24 June 1996 / Accepted: 6 December 1996     

Viscosity of microlite-bearing rhyolitic obsidians: an experimental study

 To investigate the influence of microlites on lava flow rheology, the viscosity of natural microlite-bearing rhyolitic obsidians of calc-alkaline and peralkaline compositions containing 0.1–0.4 wt.% water was measured at volcanologically relevant temperatures (650–950  °C), stresses (10³–10⁵ Pa) and strain rates (10^–5 to 10^–7 s^–1). The glass transition temperatures (T _g ) were determined from scanning calorimetric measurements on the melts for a range of cooling/heating rates. Based on the equivalence of enthalpic (calorimetric) and shear (viscosity) relaxation, we calculated the viscosity of the melt in crystal-bearing samples from the T _g data. The difference between the calculated viscosity of the melt phase and the measured viscosity for the crystal-bearing samples is interpreted to be the physical effect of microlites on the measured viscosity. The effect of <5 vol.% rod-like microlites on the melt rheology is negligible. Microlite-rich and microlite-poor samples from the same lava flow and with identical bulk chemistry show a difference of 0.6 log₁₀ units viscosity (Pa s), interpreted to be due to differences in melt chemistry caused by the presence of microlites. The only major differences between measured and calculated viscosities were for two samples: a calc-alkaline rhyolite with 1 vol.% branching crystals, and a peralkaline rhyolite containing crystal-rich bands with >45 vol.% crystals. For both of these samples a connectivity factor is apparent, with, for the latter, a close packing framework of crystals which is interpreted to influence the apparent viscosity. Received: 14 March 1996 / Accepted: 30 May 1996     

Modelling the dynamics and thermodynamics of volcanic degassing

 The rates of passive degassing from volcanoes are investigated by modelling the convective overturn of dense degassed and less dense gas-rich magmas in a vertical conduit linking a shallow degassing zone with a deep magma chamber. Laboratory experiments are used to constrain our theoretical model of the overturn rate and to elaborate on the model of this process presented by Kazahaya et al. (1994). We also introduce the effects of a CO₂–saturated deep chamber and adiabatic cooling of ascending magma. We find that overturn occurs by concentric flow of the magmas along the conduit, although the details of the flow depend on the magmas' viscosity ratio. Where convective overturn limits the supply of gas-rich magma, then the gas emission rate is proportional to the flow rate of the overturning magmas (proportional to the density difference driving convection, the conduit radius to the fourth power, and inversely proportional to the degassed magma viscosity) and the mass fraction of water that is degassed. Efficient degassing enhances the density difference but increases the magma viscosity, and this dampens convection. Two degassing volcanoes were modelled. At Stromboli, assuming a 2 km deep, 30% crystalline basaltic chamber, containing 0.5 wt.% dissolved water, the ∼700 kg s^–1 magmatic water flux can be modelled with a 4–10 m radius conduit, degassing 20–100% of the available water and all of the 1 to 4 vol.% CO₂ chamber gas. At Mount St. Helens in June 1980, assuming a 7 km deep, 39% crystalline dacitic chamber, containing 4.6 wt.% dissolved water, the ∼500 kg s^–1 magmatic water flux can be modelled with a 22–60 m radius conduit, degassing ∼2–90% of the available water and all of the 0.1 to 3 vol.% CO₂ chamber gas. The range of these results is consistent with previous models and observations. Convection driven by degassing provides a plausible mechanism for transferring volatiles from deep magma chambers to the atmosphere, and it can explain the gas fluxes measured at many persistently active volcanoes. Received: 26 September 1997 / Accepted: 11 July 1998     

Origin, characteristics, and behaviour of lahars following the 1990 eruption of Kelud volcano, eastern Java (Indonesia)

 In contrast to most twentieth-century eruptions of Kelud volcano (eastern Java), the 10 February 1990 plinian eruption was not accompanied by lake-outburst lahars. However, at least 33 post-eruption lahars occurred between 15 February and 28 March 1990. They swept down 11 drainage systems and travelled as far as 24 km at an estimated mean peak velocity in the range of 4–11 m s^–1. The deposits (volume ≥30 000 000 m³) were approximately 7 m thick 2 km from vent, and 3 m thick 10 km from vent, on the volcaniclastic apron surrounding the volcano. Subtle but significant sedimentological differences in the deposits relate to four flow types: (a) Early, massive deposits are coarse, poorly sorted, slightly cohesive, and commonly inversely graded. They are inferred to record hot lahars that incorporated pumice and scoria from pyroclastic-flow deposits, probably by rapid remobilization of hot proximal pyroclastic flow deposits by rainfall runoff. Sedimentary features, such as clasts subparallel to bedding and thick, reversely to ungraded beds, suggest that these flows were laminar. (b) Abundant, very poorly sorted deposits include non-cohesive, clast-supported, inversely graded beds and ungraded, finer-grained, and cohesive matrix-supported beds. These beds display layering and vertical segregation/density stratification, suggesting unsteady properties of pulsing debris flows. They are interpreted as deposited from segments of flow waves at a middle distance downstream that incorporated pre-eruption sediments. Sedimentological evidence suggests unsteady flow properties during progressive aggradation. (c) Fine-grained, poorly sorted and ungraded deposits are interpreted as recording late hyperconcentrated streamflows that formed in the waning stage of an overflow and transformed downcurrent into streamflows. (d) Ungraded, crudely stratified deposits were emplaced by flows transitional between hyperconcentrated flows and streamflows that traveled farther downvalley (as far as 27 km from the vent). At Kelud, the transformation of flow and behavior occurs within only 10 km of the source, at the apex of the alluvial fans. The rapid change of flow behavior is attributed to the low fines content and to the unsteady flow regime, which may be due to: (a) the rapid deposition of bedload, owing to the break in channel gradient close to the vent and to changes in channel cross-section and roughness; and (b) the very low silt+clay content in the non-cohesive deposits. These deposits mix with water to produce streamflows. Received: 27 June 1997 / Accepted: 5 January 1998     

Late Holocene eruptive activity at Nevado Cayambe Volcano, Ecuador

 Four Late Holocene pyroclastic units composed of block and ash flows, surges, ashfalls of silicic andesite and dacite composition, and associated lahar deposits represent the recent products emitted by domes on the upper part of Nevado Cayambe, a large ice-capped volcano 60 km northeast of Quito. These units are correlated stratigraphically with fallout deposits (ash and lapilli) exposed in a peat bog. Based on ¹⁴C dating of the peat and charcoal, the following ages were obtained: ∼910 years BP for the oldest unit, 680–650 years BP for the second, and 400–360 years BP for the two youngest units. Moreover, the detailed tephrochronology observed in the peat bog and in other sections implies at least 21 volcanic events during the last 4000 years, comprising three principal eruptive phases of activity that are ∼300, 800, and 900 years in duration and separated by repose intervals of 600–1000 years. The last phase, to which the four pyroclastic units belong, has probably not ended, as suggested by an eruption in 1785–1786. Thus, Cayambe, previously thought to have been dormant for a long time, should be considered active and potentially dangerous to the nearby population of the Interandean Valley. Received: 5 July 1997 / Accepted: 21 October 1997     

Subsidence of ash-flow calderas: relation to caldera size and magma-chamber geometry

 Diverse subsidence geometries and collapse processes for ash-flow calderas are inferred to reflect varying sizes, roof geometries, and depths of the source magma chambers, in combination with prior volcanic and regional tectonic influences. Based largely on a review of features at eroded pre-Quaternary calderas, a continuum of geometries and subsidence styles is inferred to exist, in both island-arc and continental settings, between small funnel calderas and larger plate (piston) subsidences bounded by arcuate faults. Within most ring-fault calderas, the subsided block is variably disrupted, due to differential movement during ash-flow eruptions and postcollapse magmatism, but highly chaotic piecemeal subsidence appears to be uncommon for large-diameter calderas. Small-scale downsag structures and accompanying extensional fractures develop along margins of most calderas during early stages of subsidence, but downsag is dominant only at calderas that have not subsided deeply. Calderas that are loci for multicyclic ash-flow eruption and subsidence cycles have the most complex internal structures. Large calderas have flared inner topographic walls due to landsliding of unstable slopes, and the resulting slide debris can constitute large proportions of caldera fill. Because the slide debris is concentrated near caldera walls, models from geophysical data can suggest a funnel geometry, even for large plate-subsidence calderas bounded by ring faults. Simple geometric models indicate that many large calderas have subsided 3–5 km, greater than the depth of most naturally exposed sections of intracaldera deposits. Many ring-fault plate-subsidence calderas and intrusive ring complexes have been recognized in the western U.S., Japan, and elsewhere, but no well-documented examples of exposed eroded calderas have large-scale funnel geometry or chaotically disrupted caldera floors. Reported ignimbrite "shields" in the central Andes, where large-volume ash-flows are inferred to have erupted without caldera collapse, seem alternatively interpretable as more conventional calderas that were filled to overflow by younger lavas and tuffs. Some exposed subcaldera intrusions provide insights concerning subsidence processes, but such intrusions may continue to evolve in volume, roof geometry, depth, and composition after formation of associated calderas. Received: 13 February 1997 / Accepted: 9 August 1997     

Tephra dispersal from Myojinsho, Japan, during its shallow submarine eruption of 1952–1953

 A new and detailed bathymetric map of the Myojinsho shallow submarine volcano provides a framework to interpret the physical volcanology of its 1952–1953 eruption, especially how the silicic pyroclasts, both primary and reworked, enlarged the volcano and were dispersed into the surrounding marine environment. Myojinsho, 420 km south of Tokyo along the Izu–Ogasawara arc, was the site of approximately 1000 phreatomagmatic explosions during the 12.5-month eruption. These explosions shattered growing dacite domes, producing dense clasts that immediately sank into the sea; minor amounts of pumice floated on the sea surface after some of these events. The Myojinsho cone has slopes of almost precisely 21° in the depth range 300–700 m.We interpret this to be the result of angle-of-repose deposition of submarine pyroclastic gravity flows that traveled downslope in all directions. Many of these gravity flows resulted from explosions and associated dome collapse, but others were likely triggered by the remobilization of debris temporarily deposited on the summit and steep upper slopes of the cone. Tephra was repeatedly carried into air in subaerial eruption columns and fell into the sea within 1–2 km of the volcano's summit, entering water as deep as 400 m. Because the fall velocity of single particles decreased by a factor of ∼30 in passing from air into the sea, we expect that the upper part of the water column was repeatedly choked with hyperconcentrations of fallout tephra. Gravitational instabilities within these tephra-choked regions could have formed vertical density currents that descended at velocities greater than those of the individual particles they contained. Upon reaching the sea floor, many of these currents probably continued to move downslope along Myojinsho's submarine slopes. Fine tephra was elutriated from the rubbly summit of the volcano by upwelling plumes of heated seawater that persisted for the entire duration of the eruption. Ocean currents carried this tephra to distal areas, where it presumably forms a pyroclastic component of deep-sea sediment. Received: 5 December 1996 / Accepted: 17 September 1997     

Depositional features and transportation mechanism of valley-filling Iwasegawa and Kaida debris avalanches, Japan

 The depositional features of two valley-filling debris avalanche deposits were studied to reveal their transportation and depositional mechanisms. The valley-filling Iwasegawa debris avalanche deposit (ca. 0.1 km³) is distributed along the valleys at the southeastern foot of Tashirodake Volcano, northern Honshu, Japan. Debris-avalanche blocks range in size from <35 m proximally to <10 m in the distal zone and consist dominantly of fragile materials. Debris-avalanche matrix percentages increase from 35–60% in the proximal zone to 95% in the distal zone. The debris-avalanche matrix is greater in volume (80–90%) at the bottom and margins of the deposit. Normal grading of large clasts and reverse grading of wood logs and branches occur within the debris-avalanche matrix. Preferred orientation of 311 wood logs and branches within the deposit coincide with the interpreted local flow direction. The basal part of the deposit is characterized by (1) erosional features and incorporated clasts of underlying material; (2) a higher proportion (30–50%) of incorporated clasts than the upper part; and (3) reverse grading of clasts. The valley-filling Kaida debris avalanche deposit (50 000 y B.P., >0.3 km³) is distributed along the valleys at the eastern-southeastern foot of Ontake Volcano, central Japan. Debris-avalanche blocks range in size from <25 m proximally to <7 m in the medial zone. Debris-avalanche matrix percentages increase from 50–70% in the proximal zone to 80% in the distal zone. The debris-avalanche matrix is more abundant (80–90%) at the bottom part of the deposit. Deformation structures observed in the debris-avalanche blocks include elongation, folding, conjugate reverse faults, and numerous minor faults in unconsolidated materials. Lithic components within the debris-avalanche matrix tend to have a higher percentage of plucked clasts from the adjacent underlying formations. A Bingham "plug flow" model is consistent with the transportation and depositional mechanisms of the valley-filling debris avalanches. In the plug of the debris avalanche, fragile blocks were transported without major rupturing due to relatively small shear stresses in regions of small strain rate. The debris-avalanche matrix was mainly produced by shearing at the bottom and margins of the avalanche. Valley-filling debris avalanches tend to have smaller debris-avalanche blocks and larger amounts of debris-avalanche matrix than do unconfined debris avalanches. These differences may be due to disaggregation of debris-avalanche blocks by shearing against valley walls and interaction between debris-avalanche blocks and valley walls. Oriented wood logs and branches, reverse grading of clasts at the base, and a higher proportion of incorporated clasts at the base are interpreted to result from shearing along the bottom and valley walls. Received: 25 March 1998 / Accepted: 10 October 1998     

Nature and significance of small volume fall deposits at composite volcanoes: Insights from the October 14, 1974 Fuego eruption,Guatemala

The first of four successive pulses of the 1974 explosive eruption of Fuego volcano, Guatemala, produced a small volume (∼0.02 km³ DRE) basaltic sub-plinian tephra fall and flow deposit. Samples collected within 48 h after deposition over much of the dispersal area (7–80 km from the volcano) have been size analyzed down to 8 φ (4 μm). Tephra along the dispersal axis were all well-sorted (σ _φ = 0.25–1.00), and sorting increased whereas thickness and median grain size decreased systematically downwind. Skewness varied from slightly positive near the vent to slightly negative in distal regions and is consistent with decoupling between coarse ejecta falling off the rising eruption column and fine ash falling off the windblown volcanic cloud advecting at the final level of rise. Less dense, vesicular coarse particles form a log normal sub-population when separated from the smaller (Md_φ < 3φ or < 0.125 mm), denser shard and crystal sub-population. A unimodal, relatively coarse (Md_φ = 0.58φ or 0.7 mm σ _φ = 1.2) initial grain size population is estimated for the whole (fall and flow) deposit. Only a small part of the fine-grained, thin 1974 Fuego tephra deposit has survived erosion to the present day. The initial October 14 pulse, with an estimated column height of 15 km above sea level, was a primary cause of a detectable perturbation in the northern hemisphere stratospheric aerosol layer in late 1974 to early 1975. Such small, sulfur-rich, explosive eruptions may substantially contribute to the overall stratospheric sulfur budget, yet leave only transient deposits, which have little chance of survival even in the recent geologic record. The fraction of finest particles (Md_φ = 4–8φ or 4–63 μm) in the Fuego tephra makes up a separate but minor size mode in the size distribution of samples around the margin of the deposit. A previously undocumented bimodal–unimodal–bimodal change in grain size distribution across the dispersal axis at 20 km downwind from the vent is best accounted for as the result of fallout dispersal of ash from a higher subplinian column and a lower “co-pf” cloud resulting from pyroclastic flows. In addition, there is a degree of asymmetry in the documented grain-size fallout pattern which is attributed to vertically veering wind direction and changing windspeeds, especially across the tropopause. The distribution of fine particles (<8 μm diameter) in the tephra deposit is asymmetrical, mainly along the N edge, with a small enrichment along the S edge. This pattern has hazard significance.     

Receiver function study in northern Sumatra and the Malaysian peninsula

In this receiver function study, we investigate the structure of the crust beneath six seismic broadband stations close to the Sunda Arc formed by subduction of the Indo-Australian under the Sunda plate. We apply three different methods to analyse receiver functions at single stations. A recently developed algorithm determines absolute shear-wave velocities from observed frequency-dependent apparent incidence angles of P waves. Using waveform inversion of receiver functions and a modified Zhu and Kanamori algorithm, properties of discontinuities such as depth, velocity contrast, and sharpness are determined. The combination of the methods leads to robust results. The approach is validated by synthetic tests. Stations located on Malaysia show high-shear-wave velocities (V _S) near the surface in the range of 3.4–3.6 km s^− 1 attributed to crystalline rocks and 3.6–4.0 km s^− 1 in the lower crust. Upper and lower crust are clearly separated, the Moho is found at normal depths of 30–34 km where it forms a sharp discontinuity at station KUM or a gradient at stations IPM and KOM. For stations close to the subduction zone (BSI, GSI and PSI) complexity within the crust is high. Near the surface low V _S of 2.6–2.9 km s^− 1 indicate sediment layers. High V _S of 4.2 km s^− 1 are found at depth greater than 6 and 2 km at BSI and PSI, respectively. There, the Moho is located at 37 and 40 km depth. At station GSI, situated closest to the trench, the subducting slab is imaged as a north-east dipping structure separated from the sediment layer by a 10 km wide gradient in V _S between 10 and 20 km depth. Within the subducting slab V _S ≈ 4.7 km s^− 1. At station BSI, the subducting slab is found at depth between 90 and 110 km dipping 20° ± 8° in approximately N 60° E. A velocity increase in similar depth is indicated at station PSI, however no evidence for a dipping layer is found.     

Large debris avalanches and associated eruptions in the Holocene eruptive history of Shiveluch Volcano, Kamchatka, Russia

 Shiveluch Volcano, located in the Central Kamchatka Depression, has experienced multiple flank failures during its lifetime, most recently in 1964. The overlapping deposits of at least 13 large Holocene debris avalanches cover an area of approximately 200 km² of the southern sector of the volcano. Deposits of two debris avalanches associated with flank extrusive domes are, in addition, located on its western slope. The maximum travel distance of individual Holocene avalanches exceeds 20 km, and their volumes reach ∼3 km³. The deposits of most avalanches typically have a hummocky surface, are poorly sorted and graded, and contain angular heterogeneous rock fragments of various sizes surrounded by coarse to fine matrix. The deposits differ in color, indicating different sources on the edifice. Tephrochronological and radiocarbon dating of the avalanches shows that the first large Holocene avalanches were emplaced approximately 4530–4350 BC. From ∼2490 BC at least 13 avalanches occurred after intervals of 30–900 years. Six large avalanches were emplaced between 120 and 970 AD, with recurrence intervals of 30–340 years. All the debris avalanches were followed by eruptions that produced various types of pyroclastic deposits. Features of some surge deposits suggest that they might have originated as a result of directed blasts triggered by rockslides. Most avalanche deposits are composed of fresh andesitic rocks of extrusive domes, so the avalanches might have resulted from the high magma supply rate and the repetitive formation of the domes. No trace of the 1854 summit failure mentioned in historical records has been found beyond 8 km from the crater; perhaps witnesses exaggerated or misinterpreted the events. Received: 18 August 1997 / Accepted: 19 December 1997     

The influence of trace amounts of water on the viscosity of rhyolites

 As a major volatile in volcanic systems, water has a significant influence on the rheological properties of silicic magmas. This is especially so at minor water contents relevant to the emplacement of silicic lavas. To investigate the influence of water on the viscosity of natural rhyolitic obsidians, a novel strategy has been adopted employing parallel-plate and micropenetration techniques. Viscosities have been determined on three types of material: (a) raw water-bearing obsidians; (b) remelted (1650  °C, 1 atm) degassed glasses of the obsidians; and (c) hydrothermally hydrated (1300  °C, 3 kbar) obsidians. Ten natural rhyolitic obsidians (peraluminous, calc-alkaline and peralkaline) were employed: seven originated from lava flows and contained <0.2 wt.% H₂O, two samples were F-rich from pyroclastic successions, and one was an obsidian cobble with 1.5 wt.% water also associated with pyroclastic units. Melt compositions and water contents were stable during viscometry. The measured decreases in activation energies of viscous flow and viscosity with small amounts of water are much greater than the Shaw calculation scheme predicts. In addition, a marked non-linear decrease in η exists with increasing water content. In contrast to the case for peralkaline rhyolites, 0.1–0.2 wt.% water decreases activation energies significantly (up to 30%) for calc-alkaline compositions. These results have important implications for the ease of near-surface degassing of silicic magmas during emplacement and permit the testing of calculational models for viscosity, largely based on synthetic systems. Received: 7 July 1997 / Accepted: 6 April 1998     

Cristobalite in a rhyolitic lava dome: evolution of ash hazard

Prolonged and heavy exposure to particles of respirable, crystalline silica-rich volcanic ash could potentially cause chronic, fibrotic disease, such as silicosis, in individuals living in areas of frequent ash fall. Here, we show that the rhyolitic ash erupted from Chaitén volcano, Chile, in its dome-forming phase, contains increased levels of the silica polymorph cristobalite, compared to its initial plinian eruption. Ash erupted during the initial, explosive phase (2–5 May 2008) contained approximately 2 wt.% cristobalite, whereas ash generated after dome growth began (from 21 May 2008) contains 13–19 wt.%. The work suggests that active obsidian domes crystallise substantial quantities of cristobalite on time-scales of days to months, probably through vapour-phase crystallisation on the walls of degassing pathways, rather than through spherulitic growth in glassy obsidian. The ash is fine-grained (9.7–17.7 vol.% <4 μm in diameter, the respirable range) and the particles are mostly angular. Sparse, fibre-like particles were confirmed to be feldspar or glass.     

Stabilization of volcanic flanks by dike intrusion: an example from Kilauea

 Dike propagation and dilation increases the compression of adjacent rocks. On volcanoes, especially oceanic shields, dikes are accordingly thought to be structurally destabilizing. As compression is incremented, volcanic flanks are driven outward or downslope and thus increase their susceptibility to destructive earthquakes and giant landslides. We show, however, that the 2-m-thick dike emplaced along the east rift zone of Kilauea in 1983 actually stabilized that volcano's flank. Specifically, production of flank earthquakes dropped more than twofold after 1983 as maximum downslope motion slowed to 6 cm·year^–1 from approximately 40 cm·year^–1 during 1980–1982. As much as 65 cm of deflationary subsidence above Kilauea's summit and upper rift zones accompanied the dike intrusion. According to recent estimates, this deflation corresponds to a reduction in magma-reservoir pressure of approximately 4 MPa, probably about as much as the driving pressure of the 1983 dike. The volume of the dike, approximately 0.10–0.15 km³, is orders of magnitude less than the estimated 200- to 250-km³ volume of Kilauea's reservoir of magma and nearby hot, mushy rock. Thus, deflation of that reservoir reduces the compressional load on the flank over a much larger area than intrusion of the dike adds to it, particularly at the dominant depth of seismicity, 8–9 km. A Coulomb block model for flank motion during intervals between major earthquakes requires the low-angle fault beneath Kilauea's flank to exhibit slip weakening, conducive to earthquake instability. Accordingly, the triggering mechanism of destructive earthquakes, several of which have struck Hawaii during the past 150 years, need not require stresses accumulated by dike intrusions. Received: 27 October 1998 / Accepted: 24 May 1999     

An empirical method for estimating travel times for wet volcanic mass flows

 Travel times for wet volcanic mass flows (debris avalanches and lahars) can be forecast as a function of distance from source when the approximate flow rate (peak discharge near the source) can be estimated beforehand. The near-source flow rate is primarily a function of initial flow volume, which should be possible to estimate to an order of magnitude on the basis of geologic, geomorphic, and hydrologic factors at a particular volcano. Least-squares best fits to plots of flow-front travel time as a function of distance from source provide predictive second-degree polynomial equations with high coefficients of determination for four broad size classes of flow based on near-source flow rate: extremely large flows (>1 000 000 m³/s), very large flows (10 000–1 000 000 m³/s), large flows (1000–10 000 m³/s), and moderate flows (100–1000 m³/s). A strong nonlinear correlation that exists between initial total flow volume and flow rate for "instantaneously" generated debris flows can be used to estimate near-source flow rates in advance. Differences in geomorphic controlling factors among different flows in the data sets have relatively little effect on the strong nonlinear correlations between travel time and distance from source. Differences in flow type may be important, especially for extremely large flows, but this could not be evaluated here. At a given distance away from a volcano, travel times can vary by approximately an order of magnitude depending on flow rate. The method can provide emergency-management officials a means for estimating time windows for evacuation of communities located in hazard zones downstream from potentially hazardous volcanoes. Received: 5 June 1997 / Accepted: 2 February 1998     

Holocene explosive activity of Hudson Volcano, southern Andes

 Fallout deposits in the vicinity of the southern Andean Hudson Volcano record at least 12 explosive Holocene eruptions, including that of August 1991 which produced ≥4 km³ of pyroclastic material. Medial isopachs of compacted fallout deposits for two of the prehistoric Hudson eruptions, dated at approximately 3600 and 6700 BP, enclose areas at least twice that of equivalent isopachs for both the 1991 Hudson and the 1932 Quizapu eruptions, the two largest in the Andes this century. However, lack of information for either the proximal or distal tephra deposits from these two prehistoric eruptions of Hudson precludes accurate volume estimates. Andesitic pyroclastic material produced by the 6700-BP event, including a  1 10-cm-thick layer of compacted tephra that constitutes a secondary thickness maximum over 900 km to the south in Tierra del Fuego, was dispersed in a more southerly direction than that of the 1991 Hudson eruption. The products of the 6700-BP event consist of a large proportion of fine pumiceous ash and accretionary lapilli, indicating a violent phreatomagmatic eruption. This eruption, which is considered to be the largest for Hudson and possibly for any volcano in the southern Andes during the Holocene, may have created Hudson's 10-km-diameter summit caldera, but the age of the caldera has not been dated independently. Received: 31 January 1997 / Accepted: 29 October 1997