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The spectacular eruption of Mount St. Helens on 18 May 1980 electrified scientists and the public. Photodocumentation of the colossal landslide, directed blast, and ensuing eruption column—which reached as high as 25 kilometers in altitude and lasted for nearly 9 hours—made news worldwide. Reconnaissance of the devastation spurred efforts to understand the power and awe of those moments (Figure 1). The eruption remains a seminal historical event—studying it and its aftermath revolutionized the way scientists approach the field of volcanology. Not only was the eruption spectacular, but also it occurred in daytime, at an accessible volcano, in a country with the resources to transform disaster into scientific opportunity, amid a transformation in digital technology. Lives lost and the impact of the eruption on people and infrastructure downstream and downwind made it imperative for scientists to investigate events and work with communities to lessen losses from future eruptions.

Laboratory experiments with dry particles of different sizes and different densities, and different particle-liquid mixtures flowing down rectangular chutes show the relative importance of segregation mechanisms. In uniform steady flow experiments with binary mixtures of small and large particles, the small particles fall downward and the large particles migrate upward. In slow, dry, frictional flows, the downstream segregation is so efficient that zones of 100% small and large particles separated by a concentration jump form. In rapid, dry, collisional flows, segregation is less efficient because of diffusive mixing. Diffusive mixing smoothes vertical concentration profiles so that concentration jumps blur or disappear. The presence of a viscous fluid inhibits size segregation. Little to no segregation occurs when liquid density matches particle density.

Mount Rainier volcano has produced many large debris flows and debris avalanches during the last 10,000 years. These flows have periodically traveled more than 100 kilometers from the volcano to inundate parts of the now-populated Puget Sound Lowland. Meteorological floods also have caused damage, but future effects will be partly mitigated by reservoirs. Mount Rainier presents the most severe flow risks of any volcano in the United States. Volcanic debris flows (lahars) are of two types: (1) cohesive, relatively high clay flows originating as debris avalanches, and (2) noncohesive flows with less clay that begin most commonly as meltwater surges. Three case histories represent important subpopulations of flows with known magnitudes and frequencies. The risks of each subpopulation may be considered for general planning and design. A regional map illustrates the extent of inundation by the case-history flows, the largest of which originated as debris avalanches and moved from Mount Rainier to Puget Sound. The paleohydrologic record of these past flows indicates the potential for inundation by future flows from the volcano. A map of the volcano and its immediate vicinity shows examples of smaller debris avalanches and debris flows in the 20th century.

1. U.S. Geological Survey, Cascades Volcano Observatory, 5400 MacArthur Blvd., Vancouver, WA 986612. Washington State Dept. of Natural Resources, Division of Geology and Earth Resources, P.O. Box 47007, Olympia, WA 985043. McGill University, Department of Civil Engineering and Applied Mechanics, 817 Sherbrooke St. West, Montreal, QC, H3A 2K6, Canada

Following 18 years of relative quiescence, Mount St. Helens volcano (MSH) became restless and began erupting again during September–December 2004. On 23 September, the U.S. Geological Survey's (USGS) David A. Johnston Cascades Volcano Observatory (CVO) and the Pacific Northwest Seismograph Network (PNSN) at the University of Washington detected the onset of a shallow earthquake swarm beneath the 1980–1986 lava dome. The Mount St. Helens Emergency Response Plan defines three alert levels that differ from normal background activity: Level 1, Notice of Volcanic Unrest (unusual activity detected); Level 2, Volcano Advisory (eruption likely but not imminent); and Level 3, Volcano Alert (eruption imminent or in progress).

Mount St. Helens is an active volcano that hosts glacier ice within its crater. Although the common picture of volcano/glacier interactions is one of rapid meltwater generation when hot material is brought into contact with snow and ice [e.g.,Major and Newhall, 1989], there have been practically no observable hydrological consequences of the ongoing episode of silicic lava dome emplacement at Mount St. Helens. The glaciological consequences have nonetheless been dramatic: The crater glacier has been cut in half since the dome growth began in September 2004, and the resulting ice bodies have in succession been squeezed between the growing lava dome and the crater wall.

We conducted scaled analogue sandbox models of caldera formation in order to understand the effects of chamber depth and orientation on the spatial and temporal development of calderas. Dry sand contained in a 1-m-diameter cylinder served as a crustal rock analogue, and a water-filled 0.6-m-diameter rubber bladder served as an analogue magma chamber. Scaling parameters included a length ratio ( L *) of 2.5 × 10 –5 and a stress ratio (σ*) of 1.8–2.4 × 10 –5 . In contrast to some previous analogue models, the viscosity of the fluid in the chamber and its withdrawal rate were properly scaled. Generally, deformation began with broad sagging, followed by an arcuate or linear outward-dipping fault that formed on one side of the caldera. This fault propagated laterally around the caldera in both directions, sometimes joining other faults, and typically forming an overall polygonal structure. As subsidence continued, the caldera grew incrementally outward and progressively formed a series of concentric outward-dipping faults. Lastly, a peripheral zone of extension and pronounced sagging, and commonly an inward- dipping outer fault related to extension, developed at the surface. As the depth of the chamber increased, (1) the area of faulting decreased, (2) the symmetry of the caldera was affected, and (3) the coherence of the subsiding block decreased. Tilting the chamber caused highly asymmetric subsidence to occur. In this case, faults formed first where the bladder was shallowest. Subsidence then shifted rapidly to where the bladder was deepest, producing an elongate trapdoor caldera that was deepest where the bladder was deepest. Our experiments highlight the roles of sagging and faulting during caldera subsidence. Surface fault patterns both in our experiments and at natural calderas are frequently not circular. The aspect ratio of the block above the magma chamber controls the shape of the caldera, which is frequently polygonal. The faults at natural calderas determine locations and migration of eruptive vents, the degree of subsidence, the style of postcal dera resurgent magmatism, and the extent of hydrothermal circulation. Our experiments reveal details of how calderas grow outward incrementally and demonstrate that asymmetric subsidence along linear and arcuate faults is common to many calderas.

At 4392 m high, glacier-clad Mount Rainier dominates the skyline of the southern Puget Sound region and is the centerpiece of Mount Rainier National Park. About 2.5 million people of the greater Seattle-Tacoma metropolitan area can see Mount Rainier on clear days, and 150,000 live in areas swept by lahars and floods that emanated from the volcano during the last 6,000 years (Figure 1). These lahars include the voluminous Osceola Mudflow that floors the lowlands south of Seattle and east of Tacoma, and which was generated by massive volcano flank-collapse. Mount Rainier's last eruption was a light dusting of ash in 1894; minor pumice last erupted between 1820 and 1854; and the most recent large eruptions we know of were about 1100 and 2300 years ago, according to reports from the U.S. Geological Survey.

Concentrated grain-fluid mixtures in rock avalanches, debris flows, and pyroclastic flows do not behave as simple materials with fixed rheologies. Instead, rheology evolves as mixture agitation, grain concentration, and fluid-pressure change during flow initiation, transit, and deposition. Throughout a flow, however, normal forces on planes parallel to the free upper surface approximately balance the weight of the superincumbent mixture, and the Coulomb friction rule describes bulk intergranular shear stresses on such planes. Pore-fluid pressure can temporarily or locally enhance mixture mobility by reducing Coulomb friction and transferring shear stress to the fluid phase. Initial conditions, boundary conditions, and grain comminution and sorting can influence pore-fluid pressures and cause variations in flow dynamics and deposits.

During the late Pleistocene the ancestral edifice of Citlaltepetl volcano (also known as Pico de Orizaba) collapsed to form a clay-rich deposit that extends 85 km from its source, has a volume of 1.8 km 3 , and covers an area of 143 km 2 east of the volcano. The deposit has clay content ranging from 10 to 16% and contains secondary alteration minerals such as smectite and kaolinite. The deposit's features suggest that it had an origin as a sector collapse of hydrothermally altered rock that transformed from a debris avalanche to a cohesive lahar very close to its source. The presence of glacier ice and a hydrothermal system during late Pleistocene times apparently provided a source of pore water which enhanced the hydrothermal alteration of the summit of Citlaltepetl and was the origin of most of the water for the lahar. This deposit and several others suggest that glaciated volcanoes are sites where hydrothermal alteration and resulting cohesive lahars are most likely. Although cohesive lahars and debris avalanches both have origins as sector collapses, cohesive lahars are more mobile than similar-sized debris avalanches. Thus potential hazard of edifice collapse at glaciated volcanoes, especially those with large volumes of hydrothermally altered rock, includes the possibility of large-volume cohesive lahars.