Learning From Experience - Tephra

How much tephra is too much tephra?

The 1980 eruption of Mount St. Helens shows that even thin accumulations of tephra can profoundly disrupt social and economic activity over broad areas. For example, the Washington communities of Yakima, Ritzville, and Spokane experienced significant disruptions in transportation, business activity, and community services when 6 to 80 millimeters (1/4 to 3 inches) of tephra fell. The greater the amount of tephra that fell, the longer a community took to recover. Residents found that tephra falls of less than 6 millimeters (1/4 inch) were a major inconvenience, and that falls of more than 17 millimeters (2/3 inch) were a disaster. Nonetheless, all three communities returned to nearly normal activities within two weeks. (From Hoblitt and others, 1995)

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