scholarly journals Rockslide-debris avalanche of May 18, 1980, Mount St. Helens Volcano, Washington

1996 ◽  
Author(s):  
Harry Glicken
Keyword(s):  
Debris Avalanche ◽  
Mount St Helens

Wind dispersed seeds and plant recovery on the Mount St. Helens debris avalanche

1989 ◽  
Vol 67 (5) ◽  
pp. 1434-1441 ◽  
Author(s):  
Virginia H. Dale
Keyword(s):  
Plant Density ◽  
Plant Cover ◽  
Debris Avalanche ◽  
Long Distance ◽  
Spider Webs ◽  
Climate Conditions ◽  
Disturbed Areas ◽  
Successional Species ◽  
Average Plant ◽  
Mount St Helens

Seed dispersal and plant establishment were monitored for 4 years on the debris avalanche created by the 1980 eruption of Mount St. Helens. The number of plants on the deposit increased over time to a high of almost 2 plants/m2 by 1983. The number of species per 250-m2 plot has increased to a mean of 10.3 in 1983 with 76 species being present over the entire deposit. Four years after the eruption only 30% of the species present before the eruption had reestablished themselves, and average plant cover was less than 1%. The debris avalanche has been invaded primarily via wind-dispersed seed of early successional species that survived or have become established in adjacent disturbed areas. Most of the early successional species on the avalanche have plumed seeds that are adapted not only for long distance dispersal, but also for being trapped in wet areas or by spider webs. Fluctuations in the density of seeds dispersed to the deposit were related to variation in precipitation. Neither seed abundance nor plant density correlated with absolute distance to a seed source or soil texture conditions. Colonization patterns are more influenced by the available biota and prevailing climate conditions than by substrate alterations resulting from the eruption.

Understanding seismic path biases and magmatic activity at Mount St Helens volcano before its 2004 eruption

2020 ◽  
Vol 222 (1) ◽  
pp. 169-188 ◽  
Author(s):  
S Gabrielli ◽  
L De Siena ◽  
F Napolitano ◽  
E Del Pezzo
Keyword(s):  
Time Window ◽  
Debris Avalanche ◽  
Coda Waves ◽  
Source Inversion ◽  
Borehole Data ◽  
Lapse Time ◽  
Scattering Attenuation ◽  
Seismic Coda ◽  
Velocity Anomalies ◽  
Mount St Helens

SUMMARY In volcanoes, topography, shallow heterogeneity and even shallow morphology can substantially modify seismic coda signals. Coda waves are an essential tool to monitor eruption dynamics and model volcanic structures jointly and independently from velocity anomalies: it is thus fundamental to test their spatial sensitivity to seismic path effects. Here, we apply the Multiple Lapse Time Window Analysis (MLTWA) to measure the relative importance of scattering attenuation vs absorption at Mount St Helens volcano before its 2004 eruption. The results show the characteristic dominance of scattering attenuation in volcanoes at lower frequencies (3–6 Hz), while absorption is the primary attenuation mechanism at 12 and 18 Hz. Scattering attenuation is similar but seismic absorption is one order of magnitude lower than at open-conduit volcanoes, like Etna and Kilauea, a typical behaviour of a (relatively) cool magmatic plumbing system. Still, the seismic albedo (measuring the ratio between seismic energy emitted and received from the area) is anomalously high (0.95) at 3 Hz. A radiative-transfer forward model of far- and near-field envelopes confirms this is due to strong near-receiver scattering enhancing anomalous phases in the intermediate and late coda across the 1980 debris avalanche and central crater. Only above this frequency and in the far-field diffusion onsets at late lapse times. The scattering and absorption parameters derived from MLTWA are used as inputs to construct 2-D frequency-dependent bulk sensitivity kernels for the S-wave coda in the multiple-scattering (using the Energy Transport Equations—ETE) and diffusive (AD, independent of MLTWA results) regimes. At 12 Hz, high coda-attenuation anomalies characterize the eastern side of the volcano using both kernels, in spatial correlation with low-velocity anomalies from literature. At 3 Hz, the anomalous albedo, the forward modelling, and the results of the tomographic imaging confirm that shallow heterogeneity beneath the extended 1980 debris-avalanche and crater enhance anomalous intermediate and late coda phases, mapping shallow geological contrasts. We remark the effect this may have on coda-dependent source inversion and tomography, currently used across the world to image and monitor volcanoes. At Mount St Helens, higher frequencies and deep borehole data are necessary to reconstruct deep volcanic structures with coda waves.

Plant Succession on the Mount St. Helens Debris-Avalanche Deposit

2006 ◽  
pp. 59-73 ◽  
Author(s):  
Virginia H. Dale ◽  
Daniel R. Campbell ◽  
Wendy M. Adams ◽  
Charles M. Crisafulli ◽  
Virginia I. Dains ◽  
...  
Keyword(s):  
Debris Avalanche ◽  
Plant Succession ◽  
Debris Avalanche Deposit ◽  
Mount St Helens

Les lahars; depots, origines et dynamique

2000 ◽  
Vol 171 (5) ◽  
pp. 545-557 ◽  
Author(s):  
Frank Lavigne ◽  
Jean-Claude Thouret
Keyword(s):  
Debris Flow ◽  
Sediment Concentration ◽  
Debris Avalanche ◽  
Fluids Mechanics ◽  
Experimental Works ◽  
Deposition Processes ◽  
Mechanics Concepts ◽  
Dispersive Pressure ◽  
Mount St Helens

Abstract A lahar is a flowing mixture of rock debris and water (other than normal streamflow) from a volcano, which encompasses a continuum from debris flows (sediment concentration > or =60% per volume) to hyperconcentrated streamflows (sediment concentration from 20 to 60% per volume). Debris flow deposits are poorly sorted and massive with abundant clasts. Lahars can be either syn-eruptive, post-eruptive or have a non-eruptive origin. Four types of lahars can be generated during an eruption, based on distinct sources of water (i.e. ice, snow, crater lake, river, and rain) that allow the sediments to be removed and incorporated in the lahar (e.g., Mount St.-Helens in 1980, Nevado del Ruiz in 1985). Post-eruptive lahars, which are rain-triggered, occur during several years after an eruption (e.g., still occurring at Pinatubo). Non-eruptive lahars are flows generated on volcanoes without eruptive activity, particularly in the case of a debris avalanche or a lake outburst (e.g., Kelud or Ruapehu). Lahars flow as pulses, whose velocity and discharge are much higher than those of streamflows, including catchments similar in size. Sediment transport capacity of lahars is exceptional, owing to buoyancy, dispersive pressure, and to the amount of cohesive clay and silt. However, the finding of recent experimental works indicates that even clay-rich lahar mixtures have little true cohesion. Therefore, the typical classification of lahars into "cohesive" and "non cohesive" seems to be inappropriate at present. Besides, past work on lahar mechanics used models based on the Bagnold's or the Bingham's theories. Recent advances in experimentation show that a lahar has specific rheological properties: it moves as a surge or series of surges, driven by gravity, by porosity fluctuation, and by pore fluid pressures, in accordance with the Coulomb grain flow model. Grain size distribution and sorting control pore pressure distribution. Lahar mechanics depend on much more than steady-state rheology, because lahars are highly unsteady and typically heterogeneous flows. Lahar can show a succession of debris flow phases, hyperconcentrated flow phases, and sometimes transient streamflow phases. Therefore, some fluids-mechanics concepts and terminology, such as "viscous", "laminar" or "non-Newtonian" are inappropriate to describe the mechanical properties of lahars. Processes of deposition are complex and poorly known. Interpretation of massive and unsorted lahar deposits commonly ascribe the deposition regime to a freezing en masse process. However, recent laboratory experiments highlight that debris-flow deposits may result from incremental deposition processes.

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