Watershed disturbance and lahars on the east side of Mount Pinatubo during the mid-June 1991 eruptions
No abstract available.
Book chapter↗
Geology topics
R. J. Janda
Publications and source records attributed to R. J. Janda.
Volcanic mixed avalanches: A distinct eruption-triggered mass-flow process at snow-clad volcanoes
A generally unrecognized type of pyroclastic deposit was produced by rapid avalanches of intimately mixed snow and hot pyroclastic debris during eruptions at Mount St. Helens, Nevado del Ruiz, and Redoubt Volcano between 1982 and 1989. These "mixed avalanches" traveled as far as 14 km at velocities up to ∼27 m/s, involved as much as 10 7 m 3 of rock and ice, and left unmelted deposits of single flow units as thick as 5 m. During flow downslope, heat transfer from hot rocks to snow produced meltwater that partially saturated the mixtures, apparently giving these mixed avalanches mobilities equal to or greater than those of "dry" debris avalanches of similar volume. The avalanches studied for this report began as snow avalanches, triggered during explosive phases of the eruptions by ballistic bombardment of snow slopes by hot pyroclastic debris. Erosion by the avalanches incorporated additional snow, fragments of glacier ice, and other rock debris. Mixed-avalanche deposits were massive, very poorly sorted, and ungraded to inversely graded prior to melting. Although rock debris composed as much as 70 wt% of the frozen samples, the bulk volumetric content of rock debris was only 9% to 36%. Snow and ice composed between 36% and 72% of the frozen samples, with void space making up the remainder of the volume. Deformation and consolidation after melting of the ice content reduced the deposits to loose, porous layers that were only a fraction of their original thickness and that later gave little hint of their origin. After melting and desiccation, the deposits are highly susceptible to erosion and unlikely to be well preserved in the stratigraphic record.
Alaska, Washington↗
Proximal pyroclastic deposits from the 1989-1990 eruption of Redoubt Volcano, Alaska - stratigraphy, distribution, and physical characteristics
More than 20 eruptive events during the 1989-1990 eruption of Redoubt Volcano emplaced a complex sequence of lithic pyroclastic-flow, -surge, -fall, ice-diamict, and lahar deposits mainly on the north side of the volcano. The deposits record the changing eruption dynamics from initial gas-rich vent-clearing explosions to episodic gas-poor lava-dome extrusions and failures. The repeated dome failures produced lithic pyroclastic flows that mixed with snow and glacial ice to generate lahars that were channelled off Drift glacier into the Drift River valley. Some of the dome failures occurred without precursory seismic warning and appeared to result solely from gravitational instability. Material from the disrupted lava domes avalanched down a steep, partly ice-filled canyon incised on the north flank of the volcano and came to rest on the heavily crevassed surface of the piedmont lobe of Drift glacier. Most dome-collapse events resulted in single, monolithologic, massive to reversely graded, medium- to coarse-grained, sandy pyroclastic-flow deposits containing abundant dense dome clasts. These deposits vary in thickness, grain size, and texture depending on distance from the vent and local topography; deposits are finer and better sorted down flow, thinner and finer on hummocks, and thicker and coarser where ponded in channels cut through the glacial ice. The initial vent-clearing explosions emplaced unusual deposits of glacial ice, snow, and rock in a frozen matrix on the north and south flanks of the volcano. Similar deposits were described at Nevado del Ruiz, Columbia and have probably been emplaced at other snow-and-ice-clad volcanoes, but poor preservation makes them difficult to recognize in the geologic record. In a like fashion, most deposits from the 1989-1990 eruption of Redoubt Volcano may be difficult to recognize and interpret in the future because they were emplaced in an environment where glacio-fluvial processes dominate and quickly obscure the primary depositional record. ?? 1994.
Journal of Volcanology and Geothermal Research↗
Eruption-triggered avalanche, flood, and lahar at Mount St. Helens - Effects of winter snowpack
An explosive eruption of Mount St. Helens on 19 March 1982 had substantial impact beyond the vent because hot eruption products interacted with a thick snowpack. A blast of hot pumice, dome rocks, and gas dislodged crater-wall snow that avalanched through the crater and down the north flank. Snow in the crater swiftly melted to form a transient lake, from which a destructive flood and lahar swept down the north flank and the North Fork Toutle River.
Washington↗