Volcanic hazards: extent and severity of potential tephra hazard interpreted from layer Yn from Mount St. Helens, Washington (Abstract)
Volcanoes in the conterminous United States erupt infrequently but represent a significant potential hazard. Tephra eruptions can affect broader areas and reach population centers at greater distances from a volcano than any other kind of volcanic event. Lava flows, pyroclastic flows, mudflows, and floods can be more hazardous, but they seldom extend beyond a volcano except along valleys. Severity of risk from tephra depends in part on rate of fall and grain size, but mainly on thickness. Rates of fall from future eruptions in the Cascade Range must be estimated from historic eruptions elsewhere; potential grain sizes and thicknesses can be judged from past tephra eruptions of the Cascade volcanoes themselves. Pumice layer Yn, erupted by Mount St. Helens about BC 2000, exemplifies an extensive and thick tephra resulting from a single eruptive pulse of a Cascade volcano; in thickness and volume it resembles tephra of the type Plinian eruption of Vesuvius in Italy in 79 AD. Layer Yn trends NNE from Mount St. Helens in a long narrow lobe that is much thicker at any given distance than if the layer had formed a wide lobe. On broad ridges where it should be nearly unaffected by thickening or erosion, its present (compacted) thickness is as much as 70 cm at about 50 km from the volcano, 20 cm at 100 km, and 5 cm at about 280 km. Future eruptions like that of layer Yn could produce similar thicknesses in any easterly direction between about NNE and SSE downwind from Mount St. Helens or any other explosive Cascade volcano. Weaker winds toward the west indicate that potential thicknesses are less in westerly directions.
- Research Organization:
- US Geological Survey, Denver
- OSTI ID:
- 5371776
- Journal Information:
- Geol. Soc. Am., Abstr. Programs; (United States), Vol. 9:4
- Country of Publication:
- United States
- Language:
- English
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