Eruption Of Mount St Helens Research Articles

After the Blast

After the Blast

High Crystal Number Densities From Mechanical Damage

Laboratory experiments investigating syn-eruptive crystallization are fundamental for interpreting crystal and vesicle textures in pyroclasts. Previous experiments have advanced our understanding by varying decompression and cooling pathways, volatile components, and melt composition. However, they have largely failed to produce the high crystal number densities seen in many cryptodome and dome samples. This is feasibly due to the relatively simple decompression pathways employed in experimental studies. In this study, we approach the problem by exploring non-linear decompression pathways. We present two series of experiments: (1) decompression from low initial starting pressure and (2) a compression-and-release step after the initial decompression. The purpose of each series was to simulate (1) decompression of magma that stalls during ascent and (2) pressure cycling that occurs in non-erupted magma during episodic explosive activity. The experiments were carried out on a synthetic rhyodacite (SiO2 = 69 wt%) held initially at 50 MPa and 885°C then decompressed at rates of 0.026 and 0.05 MPa s−1 to 10 MPa A subset of experiments was then subjected to a compression step to 110 MPa followed by near-instantaneous release back to 10 MPa. A substantial volume fraction of dendritic microlites (ϕxtl = 0.27–0.32, Na = 4.79 × 103 mm–2) formed during the initial hold at 50 MPa; additional crystallization during subsequent decompression to ≥ 10 MPa was minimal, as evidenced by only small increases in crystallinity (ϕxtl = 0.28–0.33) and comparable crystal number densities (4.11–7.81 × 103 mm–2). Samples that underwent recompression followed by a second decompression showed no increase in crystal volume fraction but did show extensive disruption of the initial dendritic, box-work microlite structures that produced high number densities (Na = 43.5–87.2 × 103 mm–2) of small individual crystals. The disruption was driven by a combination of rapid vesiculation, expansion and resulting shear along the capsule walls. From these results, we suggest that high crystal number densities may be a signature of rapid deformation occurring after magma stalling in the subsurface, perhaps related to pressure cycling and accompanying rapid changes in vesicularity during repeated small and shallow-sourced explosions. We compare our experiments to pyroclasts from shallow intrusions that preceded the 18 May 1980 eruption of Mount St Helens. These pyroclasts were erupted both prior to 18 May, during episodic precursory explosive activity, and by the 18 May initial lateral blast. The pattern of precursory activity indicates multiple episodes of pressurization (prior to explosive events) and rapid decompression (during explosive events) that we use to illustrate the significance of our experimental results.

How does tephra deposit thickness change over time? A calibration exercise based on the 1980 Mount St Helens tephra deposit

Tephra layers are frequently used to reconstruct past volcanic activity. Inferences made from tephra layers rely on the assumption that the preserved tephra layer is representative of the initial deposit. However, a great deal can happen to tephra after it is deposited; thus, tephra layer taphonomy is a crucial but poorly understood process. The overall goal of this research was to gain greater insight into the taphonomy of terrestrial tephra layers, specifically the extent to which deposit thickness is altered over time, with implications for tephra volume estimation. We approached this by a)conducting a new survey of the tephra layer from the recent, well-studied eruption of Mount St Helens on May 18th, 1980 (MSH1980); b)modelling the tephra layer thickness using a mathematical technique and c)comparing our results with an equivalent model based on measurements taken immediately after the eruption. In this way, we aimed to quantify any losses and transformations that have occurred. During our study, we collected measurements of tephra layer thickness from 86 locations ranging from <20 to >600 km from the vent. Geochemical analysis was used to identify tephra of uncertain origin. Our results indicated that the extant tephra layer at undisturbed sites was representative of the original deposit: overall, preservation in these locations (in terms of thickness, stratigraphy and geochemistry) had been remarkably good. However, the isopach maps generated from our measurements diverged from isopachs produced in the same way, but derived from the original survey data. Furthermore, our estimates of the quantity of tephra produced during the eruption greatly exceeded previous estimates of the fallout volume. In our study, inaccuracies in the modelled fallout arose from issues of sampling strategy, rather than taphonomy. Our results demonstrate the sensitivity of volcanological reconstructions to measurement location, and the great importance of reliably measured low/zero values in reconstructing tephra deposits.

A new strategy for the estimation of plume height from clast dispersal in various atmospheric and eruptive conditions

Plume height is an important parameter routinely used to characterize and classify explosive eruptions. Though the strategies to estimate key eruption source parameters such as erupted volume and mass flow rate have evolved over the past few decades, the determination of plume height of past eruptions is still mostly based on empirical approaches that do not account for the new developments in plume modelling based on the interaction of plume and wind. Here we present a revised strategy for the retrieval of plume height from field data that accounts for key aspects of plume dynamics and particle sedimentation, which include: i) the effect of wind advection on the buoyant plume, ii) a new parameterization of the gravitational spreading of the umbrella cloud for distances smaller than the radius of the plume, iii) the effect of particle shape on particle sedimentation, iv) the effect of different atmospheric profiles in different climate zones, v) three-dimensional wind, temperature and pressure data, and vi) topography. In particular, as wind can affect the dynamics and height of the plume, new computed sedimentation patterns are more complex and result in non-linear relationships between downwind and crosswind deposition. Our method is tested against observations of the 2011 eruption of Shinmoedake (Japan), the 1980 eruption of Mount St Helens (USA), and the 1991 eruption of Pinatubo (Philippines). These are well-constrained examples of small, intermediate, and high intensity eruptions, respectively. Intensity scenarios are introduced to account for the non-unique relation between plume height and particle sedimentation resulting from wind advection of volcanic plumes. We further demonstrate that needle-like and disk-like particle shapes can have downwind distances 36 to 70% larger than the equivalent spheres. In addition, we find that the effect of latitude on the determination of plume height is more significant for low and intermediate intensity scenarios with a discrepancy between 7 and 20%.

Tephra transformations: variable preservation of tephra layers from two well-studied eruptions

Volcanologists often use terrestrial tephra layers to reconstruct volcanic eruptions. However, the conversion of fresh tephra deposits into tephra layers is poorly understood. To address this knowledge gap, we surveyed tephra layers emplaced by the 1980 eruption of Mount St Helens, USA (MSH1980) and the 1947 eruption of Hekla, Iceland (H1947). We compared our measurements with observations made shortly after the 1947 and 1980 eruptions, to calibrate the subsequent transformation of the tephra deposit. We expected the tephra layers to retain the broad characteristics of the original deposits, but hypothesized (a) changes in thickness and mass loading due to re-working, and (b) positive correlations between thickness and vegetation density. We observed some systematic changes in tephra layer properties with distance from the vent and the main plume axis. However, the preservation of the layers varied both between and within our survey locations. Closed coniferous forest appeared to provide good conditions for the preservation of the MSH1980 tephra, as expected; preservation of the H1947 deposit in sparsely vegetated parts of Iceland was much more variable. However, preservation of the MSH1980 deposit in sparsely vegetated areas of eastern Washington State was also excellent, possibly due to biocrust formation. We concluded that the preservation of tephra layers is sensitive to surface conditions at the time of the eruption. These findings have implications for the reconstruction of past eruptions where eruption plumes span regions of variable surface cover.

West Coast volcanic ashes provide a new continental-scale Lateglacial isochron

The ‘Lateglacial’ period (∼14.7–11.7 cal ka BP) eruptions of Mount St Helens and Glacier Peak in the Cascade Range deposited ash layers (tephras) within a short time span across much of western North America where they form event-stratigraphic marker layers or isochrons. They were deposited at a time which has long been of interest because it represents the transition between two fundamental states of the climate system: the late Pleistocene glacial world when ice sheets were widespread, and the modern interglacial Holocene world. This transition was marked by rapid changes in the distribution of plants, animals and humans on the landscape, and is characterised by short, rapid climate reversals in the warming trend. Yet despite the importance of understanding this period for many areas of palaeoclimatology, palaeobotany and archaeology it remains one of the most difficult for which to develop accurate chronologies because of fluctuations in atmospheric radiocarbon concentration. Hence, the occurrences of distinctive tephra isochrons are valuable for chronological control. Here, we report the first detection of the Mount St Helens set J and Glacier Peak tephras as closely-spaced ‘cryptotephra’ layers (not visible in stratigraphy to the naked eye) in three eastern seaboard lakes and dated to 13.74–13.45 cal ka BP. The presence of these tephras >4000 km from their sources affords an opportunity for continent-wide correlations by providing a high-precision chronological benchmark that is otherwise often lacking in North American studies of palaeoenvironmental change and deglaciation, megafaunal extinction and palaeoindian colonisation.

Magma expansion and fragmentation in a propagating dyke

The influence of magma expansion due to volatile exsolution and gas dilation on dyke propagation is studied using a new numerical code. Many natural magmas contain sufficient amounts of volatiles for fragmentation to occur well below Earth's surface. Magma fragmentation has been studied for volcanic flows through open conduits but it should also occur within dykes that rise towards Earth's surface. The characteristics of volatile-rich magma flow within a hydraulic fracture are studied numerically. The mixture of melt and gas is treated as a compressible viscous fluid below the fragmentation level and as a gas phase carrying melt droplets above it. The numerical code solves for elastic deformation of host rocks, the flow of the magmatic mixture and fracturing at the dyke tip. With volatile-free magma, a dyke fed at a constant rate in a uniform medium adopts a constant shape and width and rises at a constant velocity. With volatiles involved, magma expands and hence the volume flux of magma increases. With no fragmentation, this enhanced flux leads to acceleration and thinning of the dyke. Simple scaling laws allow accurate predictions of dyke width and ascent rate for a wide range of conditions. With fragmentation, dyke behaviour is markedly different. Due to the sharp drop of head loss that occurs in gas-rich fragmented material, large internal overpressures develop below the dyke tip and induce swelling of the nose region, leading to deceleration of the dyke. These results are applied to the two-month long period of volcanic unrest that preceded the May 1980 eruption of Mount St Helens. An initial phase of rapid earthquake migration from the 7–8km deep reservoir to shallow levels was followed by very slow progression of magma within the edifice. Such behaviour can be accounted for by magma fragmentation at the top of a dyke.

Hot science from a volcanic crisis

The eruption of Mount St Helens in 1980 left an indelible mark on the field of volcanology. Janet Fang reports.

Magma ascent rates in explosive eruptions: Constraints from H2O diffusion in melt inclusions

The pre-fragmentation velocity of magma ascending during explosive volcanic eruptions remains difficult to quantify. Here we present a new technique for using syn-eruptive volatile diffusion in imperfectly trapped melt inclusions to obtain a direct estimate of such ascent velocities. H2O diffusion profiles are obtained from back-scattered electron images of synthetic, partially hydrated glasses and tube-shaped melt inclusions. The greyscale intensity of glass in the images shows a good negative linear correlation with melt H2O concentration. Greyscale intensity profiles, extracted using image-processing software, can therefore be calibrated against H2O measured at discrete points by ion microprobe. An advantage of the technique is that concentration profiles can be determined in melt tubes that are too small to analyse directly, with a spatial resolution (≤1 μm) that is considerably better than that obtainable by ion microprobe or FTIR. A finite element model, which incorporates previously published estimates of concentration-dependent H2O diffusivity, is used to fit the resulting continuous concentration profiles. We apply the technique to tube-shaped melt inclusions from the May 18th, 1980 Plinian eruption of Mount St Helens, Washington, USA. The model produces good fits to the data, indicating very rapid ascent times of between 102 and 166 s, which correspond to mean ascent velocities of 37-64 m/s, or mean decompression rates of 0.9-1.6 MPa/s. These are in agreement with previous estimates from petrological studies and numerical modelling.

Ecological Responses to the 1980 Eruption of Mount St Helens

Ecological Responses to the 1980 Eruption of Mount St Helens

Dynamics of seismogenic volcanic extrusion at Mount St Helens in 2004–05

The 2004-05 eruption of Mount St Helens exhibited sustained, near-equilibrium behaviour characterized by relatively steady extrusion of a solid dacite plug and nearly periodic shallow earthquakes. Here we present a diverse data set to support our hypothesis that these earthquakes resulted from stick-slip motion along the margins of the plug as it was forced incrementally upwards by ascending, solidifying, gas-poor magma. We formalize this hypothesis with a dynamical model that reveals a strong analogy between behaviour of the magma-plug system and that of a variably damped oscillator. Modelled stick-slip oscillations have properties that help constrain the balance of forces governing the earthquakes and eruption, and they imply that magma pressure never deviated much from the steady equilibrium pressure. We infer that the volcano was probably poised in a near-eruptive equilibrium state long before the onset of the 2004-05 eruption.

Decadal‐scale change of infiltration characteristics of a tephra‐mantled hillslope at Mount St Helens, Washington

AbstractThe cataclysmic 1980 eruption of Mount St Helens radically reduced the infiltration characteristics of ∼60 000 ha of rugged terrain and dramatically altered landscape hydrology. Two decades of erosional, biogenic, cryogenic, and anthropogenic activity have modified the infiltration characteristics of much of that devastated landscape and modulated the hydrological impact of the eruption. We assessed infiltration and runoff characteristics of a segment of hillslope thickly mantled with tephra, but now revegetated primarily with grasses and other plants, to evaluate hydrological modifications due to erosion and natural turbation. Eruptive disturbance reduced infiltration capacity of the hillslope by as much as 50‐fold. Between 1980 and 2000, apparent infiltration capacities of plots on the hillslope increased as much as ten fold, but remain approximately three to five times less than the probable pre‐eruption capacities. Common regional rainfall intensities and snowmelt rates presently produce little surface runoff; however, high‐magnitude, low‐frequency storms and unusually rapid snowmelt can still induce broad infiltration‐excess overland flow. After 20 years, erosion and natural mechanical turbation have modulated, but not effaced, the hydrological perturbation caused by the cataclysmic eruption. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Darkened Skies and Sparkling Grasses: The Potential Impact of the Mazama Ash Fall on the Northwestern Plains

Mazama ash has long served as an important chronostratigraphic marker for archaeologists working on the northwestern Plains. Despite its thickness and widespread distribution, few archaeologists have examined the potential impact of the ash fall on the plant, animal, and human communities of the area. To some extent, the failure to explore the consequences of this natural disaster reflects the paucity of historical documentation and the lack of current research on the impacts of volcanic eruptions on human communities. The 1980 eruption of Mount St Helens has prompted a renewed interest in the study of volcanoes, including their impacts on climate, plants, animals, and humans. The potential impact of the. Mazama ash fall on the climate, ecology, and human populations of the northwestern Plains is explored in light of this current research.

Spatially structured herbivory and primary succession at Mount St Helens: field surveys and experimental growth studies suggest a role for nutrients

Abstract. 1. The 1980 eruption of Mount St Helens (Washington, U.S.A.) created a 60‐km2 region of primary successional habitat. Since colonising in 1981, the spatial spread of the legume Lupinus lepidus at Mount St Helens, Washington, U.S.A., has afforded intriguing opportunities to study the effect of trophic dynamics on primary succession.2. Insect herbivory on this lupine has exhibited striking spatial structure for over a decade, with inverse density‐dependent damage patterns occurring over both small (10–100 m) and large (1–10 km) spatial scales.3. Hypothesising that lupine nutritional chemistry might underlie the spatial patterns in herbivory, the distribution of elemental macronutrients (nitrogen, phosphorus) across the landscape was characterised.4. Samples of soil and lupine tissue (roots and leaves) were collected from sites along both local and regional gradients in lupine density. On both large and small spatial scales, lupine leaves from low‐density conditions were significantly more nutrient rich.5. In addition, in a laboratory growth study native lepidopteran herbivores that specialise on lupines (Gelechiidae: Filatima sp.) performed better when fed leaves from low‐density, high‐nutrient lupines than on diets of low‐nutrient lupine leaves from high‐density areas a few metres away.6. These data suggest that spatial heterogeneity in lupine nutrient chemistry may underlie the remarkable herbivory gradients witnessed at Mount St Helens.

Pioneer Aeolian Community Development on Pyroclastic Flows after the Eruption of Mount St. Helens, Washington, U.S.A.

The eruption of Mount St Helens in 1980 eliminated all biota from an area of ca. 80 km2 to the north of the crater. The recolonization of the mineral surface of the pyroclastic flows by arthropods was observed from 1981 to 1985. Diverse dispersing arthropods reached the site immediately after the eruption. Representatives of at least 150 families from 18 orders arrived in the devastated zone during the first 3 yr following the eruption. Among these immigrants 19 species showed evidence of established breeding populations on the primary mineral surfaces by 1985. Several carabid beetles in the genus Bembidion, notably B. planatum, B. improvidens, and B. obscurellum were the first arthropods known to breed on the pyroclastic flow; this occurred in spring 1981. Beetles predominated among the pioneers with 15 species of Carabidae, and one each of Agyrtidae, and Tenebrionidae. The hemipteran families Lygaeidae and Saldidae were also early pioneers. Pedestrian taxa (Grylloblattidae and Gryllacrididae) arrived on the pyroclastic flow after 3 yr. Primary succession at Mount St. Helens began with the development of an aeolian community of arthropod predators and scavengers, all of which were supported by an allochthonous resource of arthropod fallout.

Pioneer Aeolian Community Development on Pyroclastic Flows after the Eruption of Mount St. Helens, Washington, U.S.A.

The eruption of Mount St Helens in 1980 eliminated all biota from an area of ca. 80 km2 to the north of the crater. The recolonization of the mineral surface of the pyroclastic flows by arthropods was observed from 1981 to 1985. Diverse dispersing arthropods reached the site immediately after the eruption. Representatives of at least 150 families from 18 orders arrived in the devastated zone during the first 3 yr following the eruption. Among these immigrants 19 species showed evidence of established breeding populations on the primary mineral surfaces by 1985. Several carabid beetles in the genus Bembidion, notably B. planatum, B. improvidens, and B. obscurellum were the first arthropods known to breed on the pyroclastic flow; this occurred in spring 1981. Beetles predominated among the pioneers with 15 species of Carabidae, and one each of Agyrtidae, and Tenebrionidae. The hemipteran families Lygaeidae and Saldidae were also early pioneers. Pedestrian taxa (Grylloblattidae and Gryllacrididae) arrived on the pyroclastic flow after 3 yr. Primary succession at Mount St. Helens began with the development of an aeolian community of arthropod predators and scavengers, all of which were supported by an allochthonous resource of arthropod fallout.

Volcanoes: crucibles of change

Probing the science and mystery of volcanoes, this book chronicles not only their geologic behaviour, but also their profound effect on human life. From Mount Vesuvius to Mount St Helens, the book covers the surprisingly large variety of volcanoes, the subtle to conspicuous signs preceding eruptions and the far-reaching atmospheric consequences. Here scientific facts take on a very human dimension, as the authors draw upon actual encounters with volcanoes, often through firsthand accounts of those how have witnessed eruptions and miraculously survived the aftermath. The book beins with a description of the lethal May 1980 eruption of Mount St Helens, complete with an explanation of how safety officials and scientists tried to predict events and how unsuspecting campers and loggers miles away struggled against terrifying blasts of ash, stone and heat. The story moves quickly to the ways in which volcanoes have enhanced our lives, creating mineral-rich land, clean thermal energy and haunting landscapes that in turn benefit agriculture, recreation, mining and commerce. Religion and psychology embroider the account as the authors explore the impact of volcanoes on the human psyche through tales of volcano gods and attempts to appease them. The book concludes by assisting readers in experiencing these geological phenomena for themselves. A tourist guide to volcanoes outlines over 40 sites throughout the world.

Luminescence dating of loess from the Northwest United states

Thermoluminescence (TL) and infra-red stimulated luminescence (IRSL) ages have been determined for loess from the Columbia Plateau region of southeast Washington and northeast Oregon, U.S.A. The IRSL ED values and ages appear to be tending towards a maximum level lower than that of the TL ED values and ages. In general, the TL ages are in stratigraphic order and agree well with the age control, in the form of 14C and TL dated tephra units derived from various eruptions of Mount St Helens.

Magma fragmentation by rapid decompression

THE processes leading to magma fragmentation and the generation of pyroclastic debris during explosive volcanic eruptions are of fundamental importance in volcanology. Observations of explosive eruptions1–3, as well as theoretical analyses of the underlying processes4, have raised the question of whether the rapid decompression of highly viscous, vesicular magma that results from the collapse of a lava dome or volcanic edifice can, in itself, produce explosive magma fragmentation and pyroclast formation. Here we report the results of a laboratory investigation of pyroclast formation following rapid decompression. Samples of magma from the 1980 eruption of Mount St Helens, when rapidly depressurized from initial pressures of up to 12 MPa (and at temperatures in the range 750–825 °C), fragmented to form pyroclastic products that are in many respects similar to those formed in real eruptions. Moreover, we observe explosive fragmentation at temperatures well below those normally associated with magmatic processes, suggesting that even relatively cool magma bodies can be very hazardous when subjected to rapid unloading events.

About this title

Since the eruption of Mount St Helens in 1980, edifice instability and collapse have been recognized at numerous volcanoes, both currently active and within the geological record, and the phenomena are now recognized as normal occurrences within the life-cycle of all types of volcano. This volume contains a selection of papers, which together form a representative cross-section of contemporary research into volcano instability both on Earth and other terrestrial bodies in the Solar System. The papers are broadly grouped, with the first two summarizing contemporary issues and addressing the development of volcano instability within the Solar System. The following five papers focus upon the different ways in which a volcanic edifice may be destabilized and experience structural failure, while the succeeding four papers examine instability monitoring and hazard implications. The bulk of the volume is devoted to the description and discussion of instability-related processes and products at specific volcanoes or volcanic regions, both submarine, subaerial, and on Mars and Venus, while the final paper examines instabilities within the plumbing system of Stromboli volcanf.