Eruptive history of Fisher Caldera,Alaska, USA

New field, compositional, and geochronologic data from Fisher Caldera, the largest of 12 Holocene calderas in Alaska, provide insights into the eruptive history and formation of this volcanic system. Prior to the caldera-forming eruption (CFE) 9400 years ago, the volcanic system consisted of a cluster of several small (∼3 km³) stratocones, which were independently active between 66±144 and 9.4±0.2 ka. Fisher Caldera formed through a single eruption, which produced a thick dacitic fall deposit and two pyroclastic-flow deposits, a small dacitic flow and a compositionally mixed basaltic-dacitic flow. Thickness and grain-size data indicate that the fall deposit was dispersed primarily to the northeast, whereas the two flows were oppositely directed to the south and north. After the cataclysmic eruption, a lake filled much of the caldera during what may have been a significant quiescent period. Volcanic activity from intracaldera vents gradually resumed, producing thick successions of scoria fall interbedded with lake sediments. Several Holocene stratocones have developed; one of which has had a major collapse event. The caldera lake catastrophically drained when a phreatomagmatic eruption generated a large wave that overtopped and incised the southwestern caldera wall. Multiple accretionary-lapilli-bearing deposits inside and outside the caldera suggest significant Holocene phreatomagmatic activity. The most recent eruptive activity from the Fisher volcanic system was a small explosive eruption in 1826, and current activity is hydrothermal. Late Pleistocene to Holocene magma eruption rates range from 0.03 to 0.09 km³ ky⁻¹ km⁻¹, respectively. The Fisher volcanic system is chemically diverse, ∼48–72 wt.% SiO₂, with at least seven dacitic eruptions over the last 82±14 ka that may have become more frequent over time. Least squares calculations suggest that prior to the CFE, Fisher Volcano products were not derived from a single, large magma reservoir, and were likely erupted from multiple, compositionally independent magma reservoirs. After the CFE, the majority of products appear to have derived from a single reservoir in which magma mixing has occurred.     

Relations of cinder cones to the magmatic evolution of Mount Mazama,Crater Lake National Park,Oregon

Cinder cones at Crater Lake are composed of high-alumina basaltic to andesitic scoria and lavas. The Williams Crater Complex, a basaltic cinder cone with andesitic to dacitic lava flows, stands on the western edge of the caldera, against an andesite flow from Mount Mazama. Bombs erupted from Williams Crater contain cores of banded andesite and dacite, similar to those erupted during the climatic eruption of Mount Mazama.Major- and trace-element variations exhibit an increase in incompatible elements and a decrease in compatible elements, consistent with crystal fractionation of olivine, plagioclase, clinopyroxene, orthopyroxene, and magnetite. LREE patterns in the rocks are irregular; each successive basalt is enriched in LREE relative to the preceding andesite.Compositional variations in the magmas of the cinder cones suggest that three magmatic processes were involved, partial melting, fractional crystallization, and magma mixing. Partial melting of more than one source produced primary basaltic magma(s). Subsequent mixing and fractional crystallization produced the more differentiated basaltic to andesitic magmas.     

Structure and physical characteristics of pumice from the climactic eruption of Mount Mazama (Crater Lake), Oregon

The vesicularity, permeability, and structure of pumice clasts provide insight into conditions of vesiculation and fragmentation during Plinian fall and pyroclastic flow-producing phases of the ~7,700 cal. year B.P. climactic eruption of Mount Mazama (Crater Lake), Oregon. We show that bulk properties (vesicularity and permeability) can be correlated with internal textures and that the clast structure can be related to inferred changes in eruption conditions. The vesicularity of all pumice clasts is 75-88%, with >90% interconnected pore volume. However, pumice clasts from the Plinian fall deposits exhibit a wider vesicularity range and higher volume percentage of interconnected vesicles than do clasts from pyroclastic-flow deposits. Pumice permeabilities also differ between the two clast types, with pumice from the fall deposit having higher minimum permeabilities (~5᎒^-13 m²) and a narrower permeability range (5-50᎒^-13 m²) than clasts from pyroclastic-flow deposits (0.2-330᎒^-13 m²). The observed permeability can be modeled to estimate average vesicle aperture radii of 1-5 µm for the fall deposit clasts and 0.25-1 µm for clasts from the pyroclastic flows. High vesicle number densities (~10⁹ cm^-3) in all clasts suggest that bubble nucleation occurred rapidly and at high supersaturations. Post-nucleation modifications to bubble populations include both bubble growth and coalescence. A single stage of bubble nucleation and growth can account for 35-60% of the vesicle population in clasts from the fall deposits, and 65-80% in pumice from pyroclastic flows. Large vesicles form a separate population which defines a power law distribution with fractal dimension D=3.3 (range 3.0-3.5). The large D value, coupled with textural evidence, suggests that the large vesicles formed primarily by coalescence. When viewed together, the bulk properties (vesicularity, permeability) and textural characteristics of all clasts indicate rapid bubble nucleation followed by bubble growth, coalescence and permeability development. This sequence of events is best explained by nucleation in response to a downward-propagating decompression wave, followed by rapid bubble growth and coalescence prior to magma disruption by fragmentation. The heterogeneity of vesicle sizes and shapes, and the absence of differential expansion across individual clasts, suggest that post-fragmentation expansion played a limited role in the development of pumice structure. The higher vesicle number densities and lower permeabilities of pyroclastic-flow clasts indicate limited coalescence and suggest that fragmentation occurred shortly after decompression. Either increased eruption velocities or increased depth of fragmentation accompanying caldera collapse could explain compression of the pre-fragmentation vesiculation interval.     

Eruptive history and petrology of Mount Drum volcano,Wrangell Mountains,Alaska

Mount Drum is one of the youngest volcanoes in the subduction-related Wrangell volcanic field (80×200 km) of southcentral Alaska. It lies at the northwest end of a series of large, andesite-dominated shield volcanoes that show a northwesterly progression of age from 26 Ma near the Alaska-Yukon border to about 0.2 Ma at Mount Drum. The volcano was constructed between 750 and 250 ka during at least two cycles of cone building and ring-dome emplacement and was partially destroyed by violent explosive activity probably after 250 ka. Cone lavas range from basaltic andesite to dacite in composition; ring-domes are dacite to rhyolite. The last constructional activity occurred in the vicinity of Snider Peak, on the south flank of the volcano, where extensive dacite flows and a dacite dome erupted at about 250 ka. The climactic explosive eruption, that destroyed the top and a part of the south flank of the volcano, produced more than 7 km³ of proximal hot and cold avalanche deposits and distal mudflows. The Mount Drum rocks have medium-K, calc-alkaline affinities and are generally plagioclase phyric. Silica contents range from 55.8 to 74.0 wt%, with a compositional gap between 66.8 and 72.8 wt%. All the rocks are enriched in alkali elements and depleted in Ta relative to the LREE, typical of volcanic arc rocks, but have higher MgO contents at a given SiO₂, than typical orogenic medium-K andesites. Strontium-isotope ratios vary from 0.70292 to 0.70353. The compositional range of Mount Drum lavas is best explained by a combination of diverse parental magmas, magma mixing, and fractionation. The small, but significant, range in ⁸⁷Sr/⁸⁶Sr ratios in the basaltic andesites and the wide range of incompatible-element ratios exhibited by the basaltic andesites and andesites suggests the presence of compositionally diverse parent magmas. The lavas show abundant petrographic evidence of magma mixing, such as bimodal phenocryst size, resorbed phenocrysts, reaction rims, and disequilibrium mineral assemblages. In addition, some dacites and andesites contain Mg and Ni-rich olivines and/or have high MgO, Cr, Ni, Co, and Sc contents that are not in equilibrium with the host rock and indicate mixing between basalt or cumulate material and more evolved magmas. Incompatible element variations suggest that fractionation is responsible for some of the compositional range between basaltic andesite and dacite, but the rhyolites have K, Ba, Th, and Rb contents that are too low for the magmas to be generated by fractionation of the intermediate rocks. Limited Sr-isotope data support the possibility that the rhyolites may be partial melts of underlying volcanic rocks. Received March 13, 1993/Accepted September 10, 1993     

Rock Mass Strength Assessment and Significance to Edifice Stability, Mount Rainier and Mount Hood, Cascade Range Volcanoes

—Catastrophic edifice and sector failure occur commonly on stratovolcanoes worldwide and in some cases leave telltale horseshoe-shaped calderas. Many of these failures are now recognised as having resulted from large-scale landsliding. These slides often transform into debris avalanches and lahars that can devastate populations downstream of the volcano. Research on these phenomena has been directed mainly at understanding avalanche mechanics and travel distances and related socioeconomic impacts. Few investigations have examined volcanic avalanche source characteristics. The focus of this paper is to 1) describe a methodology for obtaining rock strengths that control initial failure and 2) report results of rock mass strength testing from Mount Rainier and Mount Hood. Rock mass and shear strength for fresh and hydrothermally altered rocks were obtained by 1) utilizing rock strength and structural information obtained from field studies and 2) applying rock mechanics techniques common in mining and civil engineering to the edifice region. Rock mass and intact rock strength differences greatly in excess of one order of magnitude were obtained when comparing strength behavior of fresh and completely altered volcanic rock. The recognition and determination of marked strength differences existing on the volcano edifice and flank, when combined with detailed geologic mapping, can be used to quantify volcano stability assessment and improve hazard mitigation efforts.     

Resistivity monitoring of the tephra barrier at Crater Lake,Mount Ruapehu,New Zealand

The eruptions of Mt Ruapehu in the North Island of New Zealand in 1995 and 1996 caused a tephra barrier to be formed across the outlet of Crater Lake. By 2005 seepage from the refilled lake into the barrier raised the possibility of an eventual collapse of the barrier, releasing a catastrophic lahar down the mountain.As part of an extensive monitoring programme of the tephra barrier, direct current (dc) resistivity surveys were carried out on a number of lines along and across it in order to test whether the extent of the seepage could be measured (and monitored) by geophysical means. Two dimensional inversion of measured apparent resistivity data showed that between the initial measurements, made in January 2005, and February 2006, there was a gradual decrease in resistivity above the old outlet from ~ 50–60 Ωm to ~ 30 Ωm. This gave the first indication that lake water was seeping into the barrier. Between October and December 2006 there was a rapid rise in lake level to only 2 m below the top of the barrier, and a further resistivity survey in January 2007 showed that there had been a further decrease in resistivity throughout the entire barrier with values dropping to < 10 Ωm. The extent of this low resistivity indicated that the barrier was now saturated. At this stage lake water was penetrating the barrier and starting to cause erosion on its downstream side. Catastrophic collapse occurred on 18 March 2007, accompanied by a lahar in the Whangaehu river valley.Subsequent forward 3D numerical modelling of the resistivity structure of the barrier has confirmed that the observed changes in measured resistivity were directly related to the progress of seepage of lake water into the barrier.     

A hydrogeochemical model for stream chemistry,Cascade range,Washington, U.S.A.

A simple mixing model demonstrates that chemical variations in Cascade surface waters reflect flow from three general zones: alpine areas, forested colluvial slopes, and seasonally saturated areas. The chemistry of weathering solutions in alpine portions of the Williamson Creek catchment (North Cascade Range) results from alteration of plagioclase, hornblende, and biotite to kaolinitic material and vermiculite. Surface and shallow groundwater in forested portions of the catchment reflect these reactions, dissolution of small quantities of carbonate, and biologic activity. Both at-a-point and downstream chemical variations are explained quantitatively by the volume of water that originates in each of the hydrogeochemical source areas. Water from the forested colluvial slopes is most significant on an annual basis. However, summer low-flow is a mixture of colluvial waters and dilute solutions from the alpine zone, whereas 10 to 30 per cent of peak flow in snowmelt and rainstorms is produced from seasonally saturated areas. Poor concentration/discharge (C/Q) correlations, typical of Cascade rivers, result from mixing of significant C/Q relations for water leaving each source area. Model predictions could be substantially improved by better data for the effects of temperature, water-contact time, and biologic cycling on the chemistry of soil water from forested zones.     

Acoustic noise and temperature monitoring of the Crater Lake of Mount Ruapehu Volcano

Acoustic signals in Ruapehu Crater Lake, which are now being telemetered via a satellite transmission system, show promise as a possible precursor of increased volcanic activity from Ruapehu. The start of a recent period of rapid heating of Crater Lake was preceded by low-frequency (2 Hz) acoustic signals. These accompanied similar frequency seismic signals, but seemed to be produced independently. Audio-frequency (350–3000 Hz) acoustic noise also showed a very clear peak shortly before the lake temperature started to rise.     

Precambrian volcanics associated with the taum Sauk Caldera,St. Francois Mountains,Missouri, U.S.A.

Petrological studies of 12 volcanic rock units in the northeast segment of the Taum Sauk Caldera, the major structural feature in the western part of the St. Francois Mountains, indicate that they were probably derived from the same magma chamber. These calc-alkalic rocks become progressively silica and alkali rich and calcium poor from the base to the top of the stratigraphic column. In the part of the northeast segment of the caldera studied in detail, the extrusives are over 5 thick and have a volume of over 500 km³. Rock units consisting of ash-flow tuffs, bedded airfall tuffs and lava flows were apparently deposited within a single episode of volcanic activity, since no signs of extensive erosion were observed among them. Although the rocks are completely devitrified, the preservation of pyroclastic and flow features is excellent. These volcanics are exposed representatives of a 1.3–1.4 b.y. old belt of volcanics and associated plutons which extends from southern Ohio to the Texas Panhandle any may represent a belt of continental accretion.     

Implications for the thermal regime of acoustic noise measurements in Crater Lake,Mount Ruapehu,New Zealand

Hydrophone measurements of acoustic noise levels in the Crater Lake of Mount Ruapehu, New Zealand were made on 18 January 1991 from an inflatable rubber boat on the lake. The greatest sound pressures were recorded in the 1–10 Hz band, with sound levels generally decreasing about 20 dB per decade from 10 Hz to 80 kHz. The low frequency noise did not have an obvious relationship to the tremor observed at a seismic station within 1 km of the lake. The comparatively low levels of middle and high frequency sound meant that at the time of measurement, direct steam input did not make a significant contribution to the heating of Crater Lake. This is consistent with the earlier conclusion that during the last decade a major part of the heat input of Crater Lake has come from lake water that was heated below the lake and recycled back into the lake.     

Eruptive history and tectonic setting of Medicine Lake Volcano,a large rear-arc volcano in the southern Cascades

Medicine Lake Volcano (MLV), located in the southern Cascades ∼ 55 km east-northeast of contemporaneous Mount Shasta, has been found by exploratory geothermal drilling to have a surprisingly silicic core mantled by mafic lavas. This unexpected result is very different from the long-held view derived from previous mapping of exposed geology that MLV is a dominantly basaltic shield volcano. Detailed mapping shows that < 6% of the ∼ 2000 km² of mapped MLV lavas on this southern Cascade Range shield-shaped edifice are rhyolitic and dacitic, but drill holes on the edifice penetrated more than 30% silicic lava. Argon dating yields ages in the range ∼ 475 to 300 ka for early rhyolites. Dates on the stratigraphically lowest mafic lavas at MLV fall into this time frame as well, indicating that volcanism at MLV began about half a million years ago. Mafic compositions apparently did not dominate until ∼ 300 ka. Rhyolite eruptions were scarce post-300 ka until late Holocene time. However, a dacite episode at ∼ 200 to ∼ 180 ka included the volcano's only ash-flow tuff, which was erupted from within the summit caldera. At ∼ 100 ka, compositionally distinctive high-Na andesite and minor dacite built most of the present caldera rim. Eruption of these lavas was followed soon after by several large basalt flows, such that the combined area covered by eruptions between 100 ka and postglacial time amounts to nearly two-thirds of the volcano's area. Postglacial eruptive activity was strongly episodic and also covered a disproportionate amount of area. The volcano has erupted 9 times in the past 5200 years, one of the highest rates of late Holocene eruptive activity in the Cascades. Estimated volume of MLV is ∼ 600 km³, giving an overall effusion rate of ∼ 1.2 km³ per thousand years, although the rate for the past 100 kyr may be only half that. During much of the volcano's history, both dry HAOT (high-alumina olivine tholeiite) and hydrous calcalkaline basalts erupted together in close temporal and spatial proximity. Petrologic studies indicate that the HAOT magmas were derived by dry melting of spinel peridotite mantle near the crust mantle boundary. Subduction-derived H₂O-rich fluids played an important role in the generation of calcalkaline magmas. Petrology, geochemistry and proximity indicate that MLV is part of the Cascades magmatic arc and not a Basin and Range volcano, although Basin and Range extension impinges on the volcano and strongly influences its eruptive style. MLV may be analogous to Mount Adams in southern Washington, but not, as sometimes proposed, to the older distributed back-arc Simcoe Mountains volcanic field.     

On the origin of an amphibole-rich vein in a peridotite inclusion from the Lunar Crater Volcanic Field,Nevada, U.S.A.

Late Cenozoic alkali basaltic lavas of the Lunar Crater Volcanic Field (LCVF), located in the center of the Great Basin of the Western U.S.A., contain a diverse suite of nodule samples of the lower crust and upper mantle. This paper documents a composite nodule from the Marcath flow in which an amphibole-bearing wehrlite (59% olivine, 30% clinopyroxene, 6% amphibole) is cut by a 6–9 mm wide vein of andesine-amphibolite (80% kaersutite, 15% andesine, 3% ilmenite). Aside from nodule-basalt reaction at the nodule exterior, there is little chemical variation either within or between individual grains of hydrous and anhydrous phases in the vein and host wehrlite. Furthermore, there is no systematic compositional zoning in the wehrlite relative to vein proximity. The whole-rock major and trace element composition of the vein is similar to a primitive (Mg/(Mg+Fe)=0.692) basaltic liquid and has Al, Fe, Mg, Ca, Mn, Na, K, Zr, Y and Sr contents similar to basalts observed in the LCVF. In contrast to the Sr isotopic equilibrium displayed by vein feldspar and vein amphibole, Sr isotopic disequilibrium is exhibited between the vein (0.70318(4)), wehrlite (0.70322(4)), and host basalt (0.70357(5) n=3). However, the Sr isotopic ratios of older LCVF basalts (0.7030–0.7038; n=14) overlap those of the vein and wehrlite, and the magmatic activity leading to vein and wehrlite formation could be related to this older phase of LCVF volcanism. Petrographic and geochemical evidence is not consistent with a metasomatic origin for the vein and instead supports the view that the vein originated by the intrusion into a wehrlite mass and subsequent crystallization of a relatively primitive alkali basaltic magma in the lower crust or upper mantle. The wehrlite contains olivine of FO₇₁ and probably originated by crystal separation and accumulation from a relatively differentiated basaltic magma in the lower crust or upper mantle.     

The influence of debris flows on channels and valley floors in the Oregon Coast Range,U.S.A.

Debris flows are one of the most important processes which influence the morphology of channels and valley floors in the Oregon Coast Range. Debris flows that initiate in bedrock hollows at heads of first-order basins erode the long-accumulated sediment and organic debris from the floors of headwater, first- and second-order channels. This material is deposited on valley floors in the form of fans, levees, and terraces. In channels, deposits of debris flows control the distribution of boulders. The stochastic nature of sediment supply to alluvial channels by debris flows promotes cycling between channel aggradation which results in a gravel-bed morphology, and channel degradation which results in a mixed bedrock- and boulder-bed morphology. Temporal and spatial variability of channel-bed morphology is expected in other landscapes where debris flows are an important process.     

Eruptive styles and inferences about plumbing systems at Hidden Cone and Little Black Peak scoria cone volcanoes (Nevada,U.S.A.)

We describe two small scoria cone volcanoes, Hidden Cone and Little Black Peak (ages between ~320–390 ka), in the Southwestern Nevada Volcanic Field and discuss their eruption mechanisms and inferences about their plumbing systems. Cone-forming pyroclastic deposits are consistent with eruptive styles ranging from Strombolian to violent Strombolian, and lavas emanated from near the bases of the cones. The volcanoes are monogenetic (rather than polycyclic, as allowed by previous geomorphic interpretations). Vents at each volcano appear to coincide with pre-existing normal faults, consistent with observations at older, deeply eroded volcanoes in the region. The existence of these two volcanoes on a topographically high area (particularly Hidden Cone) provides evidence for short feeder dike lengths (~500 m at the surface). We infer that this short length reflects the small length scale of the mantle source region that was tapped to feed each volcano. Editorial responsibility: J Stix     

The late Pleistocene geomagnetic field as recorded by sediments from Fargher Lake,Washington, U.S.A.

Measurement of the remanent magnetization of a 6.88-m oriented core of soft sediments and tephras from Fargher Lake near Mount St. Helens in southwestern Washington State shows that no significant geomagnetic reversals were recorded in the sediments of the lake. Radiocarbon and palynological dating of the tephra layers from the lake bed indicates deposition during the interval 17, 000–34, 000 years B.P. although geochemical correlation of a prominent tephra layer in the core with tephra set C of Mount St. Helens could mean that the maximum age of the sediments may be at least 36, 000 years B.P. The core was divided into specimens 0.02 m long, each representing approximately 55 years of deposition assuming a constant rate of sedimentation. Pilot alternating field demagnetization studies of every tenth specimen indicated a strong, stable remanence with median destructive field of 15 mT, and the remaining specimens were subsequently demagnetized in fields of this strength. The mean inclination for all specimens exclusive of the unstably magnetized muck and peat from near the surface is 56.1° which is 8° shallower than the present axial dipole field at this site, perhaps because of inclination error in the detrital remanent magnetization of the sediments, although because of the variability in the data, this departure from the axial dipole field may not be significant. The ranges of inclination and declination are comparable to those of normal secular variation at northern latitudes. Although three isolated specimens have remanence with negative inclination, these anomalous directions are due to sampling and depositional effects. Measurement of a second core of 6.86 m length also revealed only normal magnetic polarity, but this result is of little stratigraphic value as this core failed to penetrate the distinctive tephra found near the base of the former core.Studies of a concentrate of the magnetic minerals in the sediments by optical microscopy and X-ray diffraction indicate that the primary magnetic constituent is an essentially pure magnetite of detrital origin. The magnetite occurs in a wide range of grain sizes with much of it of sub-multidomain size (< 15 μm).As a whole, this study provides substantial evidence against the existence of large-scale worldwide geomagnetic reversals during the time interval of Fargher Lake sedimentation, a segment of geological time for which many excursions and reversals have been reported elsewhere.     

Geology and eruptive history of the Rabaul Caldera area, Papua New Guinea

Rabaul Caldera is the most recently active (1937–1943) of four adjoining volcanic centres aligned north-south through the northern extremity of eastern New Britain. Geological mapping after the 1983–1985 Rabaul seismic and deformation crisis has partially revealed a long and complex eruption history dominated by numerous explosive eruptions, the largest accompanied by caldera collapse. The oldest exposed eruptives are the basaltic pre-caldera cone Tovanumbatir Lavas K/Ar dated at 0.5 Ma. The dacitic Rabaul Quarry Lavas exposed in the caldera wall and K/Ar dated at 0.19 Ma, are overlain by a sequence of dacitic and andesitic pyroclastic flow and fall deposits. Uplifted coral reef limestones, interbedded within the pyroclastic sequence on the northeast coast, suggest that explosive eruptions in the Rabaul area had commenced prior to the 0.125 Ma last interglacial high sea level stand. The pyroclastic sequence includes the large Boroi Ignimbrites and Malaguna Pyroclastics both ⁴⁰Ar/³⁹Ar dated at about 0.1 Ma, and the Barge Tunnel Ignimbrite ⁴⁰Ar/³⁹Ar dated at around 0.04 Ma. Few reliable ages exist for the many younger eruptives. These include Holocene ignimbrites of the latest caldera-forming eruptions—the Raluan Pyroclastics variously dated (¹⁴C) at either about 3500 or 7000 yr B.P., and the ca. 1400 yr B.P. Rabaul Pyroclastics. At least eight intracaldera eruptions have occurred since the 1400 yr B.P. collapse, building small pyroclastic and lava cones within the caldera.A major erosional episode is evident as a widespread unconformity in the upper pyroclastic stratigraphy at Rabaul. Lacking relevant radiometric ages, this episode is assumed to have occurred during last glaciation low sea levels and is here arbitarily dated at ca. ?20 ka. At least five, possibly nine, significant ignimbrite eruptions have occurred at Rabaul during the last ?20 ka. The new eruptive history differs considerably from that previously published, which considered ignimbrite eruption and caldera collapse to have first occurred at 3500 yr B.P.Rabaul volcanism has been dominated by two main types: (a) basaltic and basaltic andesite cone building eruptions; and (b) dacitic, and rarely andesitic or rhyolitic, plinian/ignimbrite eruptions of both high- and low-aspect ratio types. The 1400 yr B.P. Rabaul Ignimbrite is a type example of a low-aspect ratio, high-energy, and potentially very damaging eruption. Fine vitric ash deposits, common in the Rabaul pyroclastic sequence, demonstrate the frequent modification of eruptions by external water probably related to early caldera lakes or bays. Interbedding of these fine ashes with plinian pumice lapilli beds suggests that many early eruptions occurred from multiple vents, located in both wet and dry areas.     

Lake-ice collapse trenches in Wisconsin,U.S.A.

A series of trenches about a metre deep, 20 to 30 m wide, and as much as 2 km in length occurs in central Wisconsin, along the east shore of proglacial Lake Wisconsin. They are interpreted to be collapse trenches formed when shore ice melted after being buried beneath an expanding outwash plain.     

Bathymetric and geochemical investigation of Kawah Ijen Crater Lake, East Java, Indonesia

A bathymetric survey of Kawah Ijen crater lake was conducted by acoustic sounding in 1996 to compare the lake morphology with those measured in 1922, 1925 and 1938, and to calculate the present lake volume. Even though the lake experienced several hydrothermal eruptions, the maximum depth became shallower (182 m) than before (200 m), resulting in a reduced lake volume (3.0×10⁷ m³).Fifty-two major and minor constituents including rare earth elements and polythionates (PT) of the lake waters at various depths were determined by ICP-AES, ICP-MS and HPLC, respectively. These ions except for several volatile elements are taken up by lake fringe through congruent dissolution of pyroclastics of Kawah Ijen volcano. Most ions are homogeneously distributed throughout the lake, although PT showed a considerable vertical variation. Rare earth elements (REE) in the Kawah Ijen water as well as those from other hyper-acidic crater lakes show distribution patterns likely due to the three rock dissolution (preferential, congruent and residual) types, and their logarithmic concentrations linearly depend upon the pH values of the lake waters.Using the PT degradation kinetics data, production rates of PT, injection rates of SO₂ and H₂S into the lake were estimated to be 114, 86 and 30 tons/day, respectively. Also travel time of the spring water at the Banyupahit Riverhead from Kawah Ijen was estimated to be 600–1000 days through the consideration of decreasing rates of PT. Molten sulfur stocks containing Sn, Cu, Bi sulfides and Pb-barite exposed on the inner crater slope were presumed to be extinct molten sulfur pools at the former lake bottom. This was strongly supported by the barite precipitation temperature estimated through the consideration of the temperature dependence of Pb-chlorocomplex formation.     

The middle to late Pleistocene geomagnetic field recorded in fine-grained sediments from Summer Lake, Oregon, and Double Hot Springs, Nevada, U.S.A.

A 400,000 year record of the paleomagnetic field has been acquired from 22 meters of middle to late Pleistocene fine-grained sediments from Summer Lake in south-central Oregon and Double Hot Springs in northwestern Nevada. The stratigraphy is based on 55 tephra layers, nine of which have been correlated with tephra layers from other localities on the basis of their distinct major- and trace-element geochemistry and their distinct petrography. The paleomagnetic samples carry a strong and stable magnetization that does not appear to have been affected by the inclination error commonly associated with the magnetization of sediments. The samples have accurately recorded the declination and inclination of the geomagnetic field at or near the time of deposition except for errors arising from rotations of discrete blocks of sediment predominantly about vertical axes. Errors introduced by this type of rotation were corrected by using paleomagnetic directions associated with correlated tephra layers. The Summer Lake paleomagnetic record suggests that secular variations occurred throughout the middle and late Pleistocene often maintaining the same waveform through several oscillations. The amplitudes of these variations were similar to those of Holocene variations, and the periods ranged from 15,000 years to greater than 100,000 years.