James W. Vallance

The VEI 6, Tierra Blanca Joven pyroclastic sequence (30–90 km3 DRE volume), erupted from Ilopango caldera, El Salvador, in 431 CE, is the product of one of the largest eruptions of the last two millennia. The eruption devastated Central America's Mayan civilization. The eruption began with a short-lived phase of ash and pumice fall deposition and transitioned to a ‘wet’ explosive phase during whic

Mount Rainier is the Pacific Northwest’s iconic volcano. At 4,393 meters and situated in the south-central Cascade Range of Washington State, it towers over cities of the Puget Lowland. As the highest summit in the Cascade Range, Mount Rainier hosts 26 glaciers and numerous permanent snow fields covering 87 square kilometers and having a snow and ice volume of about 3.8 cubic kilometers. It remain

Despite their seeming permanence, volcanoes are prone to catastrophic collapse that can affect vast areas in a matter of minutes. Large collapses begin as gigantic landslides that quickly transform to debris avalanches—chaotically tumbling masses of rock debris that can sweep downslope at extremely high velocities, inundating areas far beyond the volcano. Rapid burial by the debris avalanches them

Volcanic ash deposits (tephra) in western Canada are instrumental in providing independent chronologic control for many archaeological and paleoenvironmental sites. In Alberta, tephra are a key chronologic tool in a region where radiocarbon dates are often unreliable because of the prevalence of carbonate-rich bedrock and other “old carbon” sources, such as coal. However, many studies using tephra

Lahar, an Indonesian word for volcanic mudflow, is a mixture of water, mud, and volcanic rock flowing swiftly along a channel draining a volcano. Lahars can form during or after eruptions, or even during periods of inactivity. They are among the greatest threats volcanoes pose to people and property. Lahars can occur with little to no warning, and may travel great distances at high speeds, destroy

Large explosive eruptions of silicic magma can produce widespread pumice fall, extensive ignimbrite sheets, and collapse calderas. The surfaces of voluminous ignimbrites are rarely preserved or documented because most terrestrial examples are heavily vegetated, or severely modified by post-depositional processes. Much research addresses the internal sedimentary characteristics, flow processes, and

This field trip will provide an introduction to several fascinating features of Mount St. Helens. The trip begins with a rigorous hike of about 15 km from the Johnston Ridge Observatory (9 km north-northeast of the crater vent), across the 1980 Pumice Plain, to Windy Ridge (3.6 km northeast of the crater vent) to examine features that document the dynamics and progressive emplacement of pyroclasti

The oldest postglacial lapilli–ash tephra recognized in sedimentary records surrounding Mount Rainier (Washington State, USA) is R tephra, a very early Holocene deposit that acts as an important stratigraphic and geochronologic marker bed. This multidisciplinary study incorporates tephrostratigraphy, radiocarbon dating, petrography, and electron microprobe analysis to characterize R tephra. Tephra

Lahars occur during volcanic eruptions--or, less predictably, through other processes on steep volcanic terrain--when large masses of water mixed with sediment sweep down and off volcano slopes and commonly incorporate additional sediment and water. Because lahars are water-saturated, both liquid and solid interactions influence their behavior and distinguish them from other related phenomena com

Landslides reflect landscape instability that evolves over meteorological and geological timescales, and they also pose threats to people, property, and the environment. The severity of these threats depends largely on landslide speed and travel distance, which are collectively described as landslide “mobility”. To investigate causes and effects of mobility, we focus on a disastrous landslide that

Lahars are volcanic debris flows that are dubbed primary when triggered by eruptive activity or secondary when triggered by other factors such as heavy rainfall after eruptive activity has waned. Variation in time and space of the proportion of sediment to water within a lahar dictates lahar flow phase and the resultant sedimentary character of deposits. Characteristics of source material and of d

Catastrophic dense granular flows, such as occur in rock avalanches, debris flows and pyroclastic flows, move as fully shearing mixtures that have approximately 60 vol.% solids and tend to segregate to form coarse-grained fronts and leveed channels. Levees restrict spreading of unconfined flows and form as coarse particles that become concentrated in the top of the flow are transported to the fron