Mount St. Helens and Mount Hood provide excellent depositional records of the broad spectrum of volcanic hazards that involve the flow or fall of volcaniclastic particles. This field-trip guide provides an in-depth introduction to the deposits, including criteria that are observable in the field to aid in differentiating between pyroclastic density current, pyroclastic-fall, debris-avalanche, lahar, water-flood, and glacial deposits. The guide also introduces the Holocene eruptive histories of Mount St. Helens and Mount Hood and discusses the processes responsible for deposit emplacement.
This self-guided field trip provides a road log, GPS coordinates, descriptions of what you will see and how to interpret the deposits, Geologic field-trip guide of volcaniclastic sediments from snow- and ice-capped volcanoes—Mount St. Helens, Washington, and Mount Hood, Oregon.
Middle Sister is the product of a profound 50,000–15,000-year-ago eruptive episode that also built South Sister. At 1.6 mi3 (7 km3), Middle Sister's eruptive volume is modest, but its diverse chemistry, sudden onset and abrupt end are intriguing.
Eruptions in the Three Sisters volcanic cluster prior to 50,000 years ago were exclusively basaltic. But lava flows erupted from 50,000 to 37,000 years ago at Middle Sister were chemically diverse, with basaltic andesite, a high-silica rhyolite, and andesite produced from the mixing of a rhyolite and mafic magma (rhyolite and rhyodacite also erupted at South Sister during this time). Between 37,000 to 27,000 years ago, volcanism diminished near Middle Sister and flared up at South Sister, with abundant andesite and dacite lava flows covering South Sister, and several rhyolite flows erupting on its flanks. From 27,000 to 15,000 years ago, Middle Sister erupted mafic, intermediate, and silicic lava flows and then ceased to erupt.
The temporary quadrupling of the eruption rate and introduction of andesite/dacite compositions are a profound departure from the productive, but consistently mafic, earlier eruptive history of the Three Sisters volcanic cluster. The eruption of rhyolite starting about 50,000 years ago and the mixing of mafic material with rhyolite implies development of a more complex (fractionating) magmatic system that waxed 50,000–30,000 years ago, culminated 30,000-20,000 years ago, then waned by 15,000 years ago. The Sisters are notable because the detailed mapping and high-resolution geochronology show that two adjacent stratovolcanoes (Middle and South Sisters) were concurrently active over the same short, but measurable, interval.
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.
Lahars form in many ways. They commonly occur when eruptions melt snow and ice on snow-clad volcanoes; when rains fall on steep slopes covered with fresh volcanic ash; when crater lakes, volcano glaciers or lakes dammed by volcanic debris suddenly release water; and when volcanic landslides evolve into flowing debris. Lahars are especially likely to occur at erupting or recently active volcanoes.
Lahars can occur with little to no warning, and may travel great distances at high speeds, destroying or burying everything in their paths. Because lahars are so hazardous, USGS scientists pay them close attention. They study lahar deposits and limits of inundation, model flow behavior, develop lahar-hazard maps, and work with community leaders and governmental authorities to help them understand and minimize the risks of devastating lahars.
Read more and download this new USGS Fact Sheet, Lahar—River of volcanic mud and debris.