Posteruption glacier development within the crater of Mount St. Helens, Washington, USA

The cataclysmic eruption of Mount St. Helens on May 18, 1980, resulted in a large, north-facing amphitheater, with a steep headwall rising 700 m above the crater floor. In this deeply shaded niche a glacier, here named the Amphitheater glacier, has formed. Tongues of ice-containing crevasses extend from the main ice mass around both the east and the west sides of the lava dome that occupies the center of the crater floor. Aerial photographs taken in September 1996 reveal a small glacier in the southwest portion of the amphitheater containing several crevasses and a bergschrund-like feature at its head. The extent of the glacier at this time is probably about 0.1 km². By September 2001, the debris-laden glacier had grown to about 1 km² in area, with a maximum thickness of about 200 m, and contained an estimated 120,000,000 m³ of ice and rock debris. Approximately one-third of the volume of the glacier is thought to be rock debris derived mainly from rock avalanches from the surrounding amphitheater walls. The newly formed Amphitheater glacier is not only the largest glacier on Mount St. Helens but its aerial extent exceeds that of all other remaining glaciers combined.     

Monitoring of geomorphic effects of the 1980 eruptions of Mount St. Helens, Washington, USA

On 18 May 1980, Mount St. Helens erupted explosively with a blast that devastated a 410 km² area, and triggered a debris avalanche exceeding 2.5 billion m³ into the North Fork Toutle River valley. In addition, mudflows radiated out from the stratovolcano cone into all of the major drainages, destroying structures and filling stream channels with sediment. This paper examines the use of geomorphology in the creation of volcanic hazards maps prior to this eruption, the mitigation strategies used, and the subsequent role of geomorphology in subsequent recovery efforts. A sediment budget is presented that summarizes the yield estimated from many geomorphic sources, based on post-eruption aerial monitoring and ground measurements.     

Terpenoid marker compounds derived from biogenic precursors in volcanic ash from Mount St. Helens,Washington

A volcanic-ash sample obtained after the 1980 eruption of Mount St. Helens, Washington, was analyzed for cyclic terpenoid organic compounds and polycyclic aromatic hydrocarbons using capillary gas chromatography-mass spectrometry-computer techniques. Various tricyclic diterpenoid acids and hydrocarbons were identified including dehydroabietic acid, dehydroabietin, dehydroabietane, simonellite, and retene. Preliminary evidence indicates that these compounds were derived from forest soils or atmospheric aerosols or both in the vicinity of coniferous forests. A diagenetic scheme involving three possible pathways for the conversion of abietic acid to retene is presented.     

Electron microprobe analysis of glass shards from tephra assigned to set W,Mount St. Helens,Washington

We have extended the fallout areas for each of two members of tephra-set W, erupted from Mount St. Helens about 1500 ad, by several hundred kilometers beyond the limits mapped in 1975. We traced one member (We) east into Idaho, and the other (Wn) northeast into British Columbia. After using stratigraphic and petrographic observations to assign more than 100 tephra samples to set W, we found 26 of these, selected for chemical analysis, to be closely similar in content of Ca, Fe, and K in glass shards. But improved homogeneity was evident when the 26 sampling localities for tephra W were segregated geographically, east vs. northeast of the volcano. When Ca:Fe:K proportions were plotted on a ternary diagram, there was no overlap of the plotting areas for these two groups of tephra W samples. Without such data, tephra layers We and Wn are currently separable only from stratigraphic and geographic information. Partial glass analysis is also an aid, along with stratigraphic position and petrographic characteristics, in distinguishing tephra W from associated tephra layers. These include tephra layers T and Yn from Mount St. Helens, as well as older tephra layers from Mount Mazama and Glacier Peak.     

Textural and chemical variation in plagioclase phenocrysts from the 1980 eruptions of Mount St. Helens,USA

This study presents major- and trace-element chemistry of plagioclase phenocrysts from the 1980 eruptions of Mount St. Helens volcano. Despite the considerable variation in textures and composition of plagioclase phenocrysts, distinct segments have been cross-correlated between crystals. The variation of Sr and Ba concentration in the melt, as calculated from the concentration in the phenocrysts using partition coefficients, suggests the cores and rims crystallised from compositionally different melts offset by the plagioclase crystallisation vector. In both of these melts Sr and Ba are correlated despite the abundance of plagioclase in the 1980 dacites. We propose that rapid crystallisation of plagioclase upon magma ascent caused a shift in melt composition towards lower Sr and higher Ba, as documented in the rims of the phenocrysts. Although the cores of the phenocrysts crystallised at relatively shallow depths, they preserve the Sr and Ba of the deep-seated melts as they ascended from a deeper region. Further magma ascent resulted in microlite nucleation, which is responsible for a similar shift to even lower Sr concentration as observed in the groundmass of post-18 May 1980 samples. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Ascent-driven crystallisation of dacite magmas at Mount St Helens, 1980–1986

We introduce a novel scheme that enables natural silicic glasses to be projected into the synthetic system Qz-Ab-Or-H₂O in order to relate variations in volcanic glass chemistry to changing pressure (P) and temperature (T) conditions in the sub-volcanic magma system. By this means an important distinction can be made between ascent-driven and cooling-driven crystallisation under water-saturated or undersaturated conditions. In samples containing feldspar and a silica phase (quartz or tridymite), quantitative P-T estimates of the conditions of last equilibrium between crystals and melt can be made. Formation of highly silicic melts (i.e. >77 wt% SiO₂) is a simple consequence of the contraction of the silica phase volume with decreasing pressure, such that high silica glasses can only form by crystallisation at low pressure. Resorption of quartz crystals appears to be a further diagnostic feature of decompression crystallisation. Groundmass and inclusion glasses in dacites from the 1980-1986 eruption of Mount St Helens volcano (WA) span a wide range in SiO₂ (68-80 wt%, anhydrous). The compositions of the least evolved (SiO₂-poor) inclusions in amphibole phenocrysts record entrapment of silicic liquids with Е.4 wt% water, corresponding to a water saturation pressure of ~200 MPa at 900 °C. The compositions of more evolved (higher SiO₂) plagioclase-hosted inclusions and groundmass glasses are consistent with extensive ascent-driven fractional crystallisation of plagioclase, oxide and orthopyroxene phenocrysts and microlites to low pressures. During this polybaric crystallisation, plagioclase phenocrysts trapped melts with a wide range of dissolved water contents (3.5-5.7 wt%). Magmas erupted during the Plinian phase of the 18 May 1980 eruption were derived from a large reservoir at depths of ̈́ km. Subsequent magmas ascended to varying depths within the sub-volcanic system prior to extraction. From glass chemistry and groundmass texture two arrest levels have been identified, at depths of 0.5-1 and 2-4 km. A single dome sample from February 1983 contains groundmass plagioclase, tridymite and quartz, testifying to temperatures of at least 885 °C at 11 MPa. These shallow storage conditions are comparable to those in the cryptodome formed during spring 1980. The corresponding thermal gradient, А.2 °C MPa^-1, is consistent with near-adiabatic magma ascent from ~8 km. We argue that the crystallisation history of Mount St Helens dacite magma was largely a consequence of decompression crystallisation of hot magma beyond the point of water saturation. This challenges the conventional view that phenocryst crystallisation occurred by cooling in a large magma chamber prior to the 1980-1986 eruption. Because the crystallisation process is both polybaric and fractional, it cannot be simulated directly using isobaric equilibrium crystallisation experiments. However, calculation of the phase proportions in water-saturated 910ᆣ °C experiments by Rutherford et al. (1985) over the pressure range 220-125 MPa reproduces the crystallisation sequence and phenocryst modes of Mount St Helens dacites from 18 May 1980. By allowing for the effects of fractional versus equilibrium crystallisation, entrained residual source material, and small temperature differences between nature and experiment, phase compositions can also be matched to the natural samples. We conclude that decompression of water-saturated magma may be the dominant driving force for crystallisation at many other silicic volcanic centres.     

Characterization of dissolved organic materials in surface waters within the blast zone of Mount St Helens,Washington

After the May 18, 1980, eruption of Mount St Helens, the concentration of dissolved organic material in surface waters near the volcano increased significantly as a result of the destruction of the surrounding conifer forest. Low molecular weight organic compounds identified in the blast zone surface waters were derived from pyrolysis of plant and soil organic materials incorporated into pyroclastic flow, mud flow and debris avalanche deposits. A major fraction of the dissolved organic material consisted of high molecular weight, colored, organic acids that are similar in their general properties to aquatic fulvic acids found in more typical surface waters except for greater sulfur contents. The other major fraction of the dissolved organic material consisted of hydrophilic acids, which may include compounds capable of supporting heterotrophic microorganisms, and precursors in the formation of aquatic fulvic acids. The organic chemistry of blast zone surface waters will probably be greatly influenced by the May 18, 1980, eruption for many years.     

Glass geochemical compositions from widespread tephras erupted over the last 200 years from Mount St. Helens

Building reliable chronologies from lake sediments, peat and other paleoenvironmental archives can be challenging, especially for historical times where radiocarbon is unreliable. Nineteenth- and 20th-century eruptions from Mount St. Helens (MSH) provide important chronostratigraphic markers for regional paleoenvironmental studies within this time frame, but are constrained by poorly geochemically characterized tephra and/or limited published data. Here, we present glass geochemistry from the most significant eruptions from this time. This includes proximal, medial and distal deposits of the 18 May 1980 MSH eruption, layer T ( ad 1799/1800), a new tephra that we argue represents the ad 1842 eruption, and the 22 July 1980 eruption that had reported ashfall in Canada. Our results indicate that most tephras ejected during these eruptions, within a time frame of ~200 years, have distinct glass geochemical characteristics that can be used to identify distal deposits for tephrochronological studies. Layer T is on trend with analyses of the 1980 eruption but has a distinct dacitic glass population. The 1980 and ad 1842 eruptions are similar, both having rhyolitic glass compositions, but the ad 1842 event can be differentiated by a more constrained SiO₂ range in the main geochemical population, and the presence of a unique SiO₂ sub-population.     

Mineralogy and phase chemistry of Mount St. Helens tephra sets W and Y as keys to their identification

Voluminous and widespread tephras were produced frequently during the last 36,000 yr of volcanic activity at Mount St. Helens. Numerous tephra sets have been defined by D. R. Mullineaux, J. H. Hyde, and M. Rubin (1975, U.S. Geological Survey Journal of Research, 3, 329–335) on the basis of field relations, FeMg phenocryst assemblage, and ¹⁴C chronology and are valuable marker beds for regional stratigraphic studies. In this study modal abundances and mineral compositions were determined (via petrographic and electron microprobe techniques) for numerous samples of individual layers within tephra sets W and Y to evaluate the degree of compositional variability within and between tephra layers and criteria by which to distinguish among Mount St. Helens and other Pacific Northwest tephras. Although individual layers within a set (e.g., We, Wn) cannot be distinguished from each other on the basis of mineralogic characteristics examined, mineral compositions allow distinction among layers W and Y and other Pacific Northwest tephras (e.g., Mazama, Glacier Peak). FeTi oxide compositions and T-f_O2 estimates derived using coexisting magnetite-ilmenite are especially useful due to the compositional homogeneity of these minerals both within and between samples of a given unit over a wide geographic area. The silicates show more compositional variability than the oxides, but $\text{iO}{}_2\text{Al}{}_2\text{O}{}_3$ contents in hornblende and Fe/Mg ratios in hypersthene aid in distinguishing among Pacific Northwest tephras.     

The role of faulting on surface deformation patterns from pumping-induced groundwater flow (Las Vegas Valley, USA)

Land subsidence and earth fissuring can cause damage in semiarid urbanized valleys where pumping exceeds natural recharge. In places such as Las Vegas Valley (USA), Quaternary faults play an important role in the surface deformation patterns by constraining the migration of land subsidence and creating complex relationships with surface fissures. These fissures typically result from horizontal displacements that occur in zones where extensional stress derived from groundwater flow exceeds the tensile strength of the near-surface sediments. A series of hypothetical numerical models, using the finite-element code ABAQUS and based on the observed conditions of the Eglington Fault zone, were developed. The models reproduced the (1) long-term natural recharge and discharge, (2) heavy pumping and (3) incorporation of artificial recharge that reflects the conditions of Las Vegas Valley. The simulated hydrostratigraphy consists of three aquifers, two aquitards and a relatively dry vadose zone, plus a normal fault zone that reflects the Quaternary Eglington fault. Numerical results suggest that a 100-m-wide fault zone composed of sand-like material produces: (1) conditions most similar to those observed in Las Vegas Valley and (2) the most favorable conditions for the development of fissures to form on the surface adjacent to the fault zone.     

Superimposed deformation in seconds: breccias from the impact structure at Kentland, Indiana (USA)

Breccias from the central uplift at the Kentland, Indiana impact structure have outcrop and microscopic characteristics that give insight into events that may occur in a carbonate-dominated sedimentary sequence in the moments following hypervelocity impact. Three distinct types of brecciated rock bodies — fault breccias, breccia lenses, and breccia dikes — suggest multiple mechanisms of fragmentation. The fault breccias occur along steeply dipping faults that coincide with compositional discontinuities in the stratigraphic succession. The breccia lenses and dikes are less localized in occurrence and show no systematic spatial distribution or orientation. The fault breccias and breccia lenses show no consistent cross-cutting relationships, but both are transected by the breccia dikes. Textural analysis reveals significant differences in particle size distributions for the different breccias. The fault breccias are typically monomict, coarsest and least uniform in grain size, and yield the highest power-law exponent (fractal dimension) in plots of particle size vs. frequency. The polymict dike filling is finest and most uniform in grain size, has the lowest power-law exponent, and is locally laminated and size-sorted. SEM images of the dike-filling breccia show that fragmentation occurred to the scale of microns. Material within the breccia lenses has textural characteristics intermediate between the other two types, but the irregular morphology of these bodies suggests a mechanism of formation different from that of either of the other breccia categories. The breccia lenses and dikes both have sub-mm-scale spheroidal vugs that may have been formed by carbon dioxide bubbles released during sudden devolatilization of the carbonate country rock. Collectively, these observations shed light on the processes that occur during the excavation and modification phases of crater formation in carbonate strata — heterogeneous, polyphase, multiscale deformation accomplished over a time interval of seconds.     

Mid-Holocene Glacier Peak and Mount St. Helens We Tephra Layers Detected in Lake Sediments from Southern British Columbia Using High-Resolution Techniques

A Glacier Peak tephra has been found in the mid-Holocene sediment records of two subalpine lakes, Frozen Lake in the southern Coast Mountains and Mount Barr Cirque Lake in the North Cascade Mountains of British Columbia, Canada. The age–depth relationship for each lake suggests an age of 5000–5080 ¹⁴C yr B.P. (5500–5900 cal yr B.P.) for the eruption which closely approximates the estimated age (5100–5500 ¹⁴C yr B.P.) of the Dusty Creek tephra assemblage found near Glacier Peak. The tephra layer, which has not been reported previously from distal sites and was not readily visible in the sediments, was located using contiguous sampling, magnetic susceptibility measurements, wet sieving, and light microscopy. The composition of the glass in pumice fragments was determined by electron microprobe analysis and used to confirm the probable source of this mid-Holocene tephra layer. Using the same methods, the A.D. 1481–1482 Mount St. Helens We tephra layer was identified in sediments from Dog Lake in southeastern British Columbia, suggesting the plume drifted further north than previously thought. This high-resolution method for identifying tephra layers in lake sediments, which has worldwide application in tephrachronologic/paleoenvironmental studies, has furthered our knowledge of the timing and airfall distribution of Holocene tephras from two important Cascade volcanoes.     

New species of Eopleistocene ground squirrel from Mount Tologoy (West Transbaykalia)

A new species of ground squirrel, Citellus (Citellus) tologoicus, is described from the middle Eopleistocene of Mount Tologoy, West Transbaykalia, and its relationships discussed. --C. E. Ray.     

The July 2007 rock and ice avalanches at Mount Steele, St. Elias Mountains, Yukon, Canada

A large rock and ice avalanche occurred on the north face of Mount Steele, southwest Yukon Territory, Canada, on July 24, 2007. In the days and weeks preceding the landslide, several smaller avalanches initiated from the same slope. The ice and rock debris traveled a maximum horizontal distance 5.76 km with a maximum vertical descent of 2,160 m, leaving a deposit 3.66 km² in area on Steele Glacier. The seismic magnitude estimated from long-period surface waves (M _s) is 5.2. Modeling of the waveforms suggests an estimated duration of approximately 100 s and an average velocity of between 35 and 65 m/s. This landslide is one of 18 large rock avalanches known to have occurred since 1899 on slopes adjacent to glaciers in western Canada. We describe the setting, reconstruct the event chronology and present a preliminary characterization of the Mount Steele ice and rock avalanches based on field reconnaissance, analysis of seismic records and an airborne LiDAR survey. We also present the results of a successful dynamic simulation for the July 24 event.     

GPS measurements of ground deformation induced by water level variations into a granitic aquifer (French Brittany)

We used two semi‐permanent GPS receivers with differential configuration to measure the hydrological subsidence of an aquifer used for water supply, located on a fractured granitic area. The time series of the vertical deformation measurements of the ground surface show a variation of 2 cm between winter and summer. We investigate the relationship between this vertical deformation and hydraulic head variations. We show that this kind of GPS survey allows characterizing part of the hydrological properties of such a heterogeneous aquifer. This is thus a new approach of time continuous monitoring of the deformation related to hydrogeological processes.     

Low-head hyperfiltration through Jurassic-Cretaceous metamorphic Darrington Phyllite discs (from the Northwest Cascades of Washington State, USA)

Membrane behavior in naturally occurring and engineering systems refers to the restriction of solute migration through a membrane relative to the migration of the solvent. Hyperfiltration is the net flux that results when two solutions of different concentration are separated by a membrane and an external force is applied in excess of the osmotic pressure. Clay membranes containing layered fabric have higher efficiencies than membranes with random fabrics. Therefore, low-permeability rocks with a foliated fabric might exhibit membrane properties. Four hyperfiltration experiments were conducted on samples of Darrington Phyllite from the Easton Metamorphic Suite of the Northwest Cascades, Washington (USA). Chloride solutions were passed through thin, intact discs at relatively low heads. At the end of the experiments, dissolved chloride concentrations had increased 110–140 % and calculated reflection coefficients ranged from 0.87 to 0.88; this was attributed to partial solute rejection by the phyllite. Natural scenarios in which phyllite might exhibit membrane properties include (1) shallow perched aquifers bounded by phyllite, (2) overpressured aquifers bounded by phyllite, (3) phyllite-bounded aquifers with significant vertical groundwater flows, and (4) ultrafiltration during metamorphic devolatilization at depth. Membrane processes exhibited by phyllite may also contribute to the formation of some low-temperature ore bodies.     

Extent, timing, and climatic implications of glacier advances Mount Rainier, Washington, U.S.A., at the pleistocene/holocene transition

The North Atlantic Younger Dryas climatic reversal did not cause a glacier advance on Mount Rainier. The glaciers on Mount Rainier seem to have advanced in response to regional or local shifts in climate. However, the Younger Dryas climatic reversal may have affected the Mount Rainier area, causing a cold, but dry, climate unfavorable to glacier advances. Glaciers in the vicinity of Mount Rainier advanced twice during late glacial/early Holocene time. Radiocarbon dates obtained from lake sediments adjacent to the corresponding moraines are concordant, indicating that the ages for the advances are closely limiting. The first advance occurred before 11,300 ¹⁴C yr BP (13,200 cal yr BP). During the North Atlantic Younger Dryas event, between 11,000 and 10,000 ¹⁴C yr BP (12,900 and 11,600 cal yr BP), glaciers retreated on Mount Rainier, probably due to a lack of available moisture, but conditions may have remained cold. The onset of warmer conditions on Mount Rainier occurred around 10,000 ¹⁴C yr BP (11,600 cal yr BP). Organic sedimentation lasted for at least 700 years before glaciers readvanced between 9800 and 8950 ¹⁴C yr BP (10,900 and 9950 cal yr BP).