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Tephra Fall Is a Widespread Volcanic Hazard

Map modified from image acquired by ATAC with data provided by the Federal Aviation Administration.

Falling volcanic ash can disrupt lives distant from an erupting volcano.

The term tephra defines all pieces of all fragments of rock ejected into the air by an erupting volcano. Most tephra falls back onto the slopes of the volcano, enlarging it. But, billions of smaller and lighter pieces less than 2 mm diameter (less than one tenth of an inch), termed ash, are carried by winds for thousands of miles.

Shaded areas indicate where tephra layers remain from associated ve...
Shaded areas indicate where tephra layers remain from associated very large eruptions. At the time of eruption, lighter ash fall extended to greater distances than shown here.

Falling ash, even in low concentrations, can disrupt human activities hundreds of miles downwind, and drifting clouds of fine ash can endanger jet aircraft thousands of miles away. Aircraft that fly in the dense network of aviation routes across the Cascade Range carry nearly 200,000 people daily over Cascade airspace—an amount equivalent to the population of the City of Spokane, Washington. When it has settled on and near the ground, volcanic ash threatens the health of people and livestock, damages electronics and machinery, and interrupts power generation, water and transportation systems, and telecommunications.

Some ash falls are extensive with far-reaching effects.

Ash from the May 18, 1980 eruption of Mount St. Helens covering the...
Ash from the May 18, 1980 eruption of Mount St. Helens covering the ground and road at a farm in Connell, Washington, approximately 300 km (180 mi) from the volcano.

Evidence from past eruptions shows that three Cascade Range volcanoes are capable of erupting massive volumes of volcanic ash—Glacier PeakMount St. Helens, and Mount Mazama (Crater Lake). While the extent of these ash layers is widespread, minor eruptions of ash from any Cascade Range volcano can cause serious societal disruptions. Keep in mind that the next volcanic ash fall that drifts over your community might be from a distant volcano.

1980 volcanic ash disrupted many lives.

On May 18, 1980, Mount St. Helens sent an enormous column of volcanic ash and gas more than 80,000 feet into the air in less than 15 minutes. As ash clouds drifted over eastern Washington, a rain of ash covered homes, farms, and roads to a depth up to four-inches. Small ash particles penetrated even the mostly tightly sealed structures and disrupted businesses and services on farmlands and in communities. By end of the day, more than 500 million tons of ash had fallen onto parts of Washington, Idaho, and Montana. Many small communities became temporarily isolated, and more than 10,000 people became stranded on roadways because of poor visibility, slippery roads, and ash–damaged vehicles. Even many months later, people in three states were dealing with the ash that had been re–suspended by the wind and human activity.

What would an explosive eruption from Mount St. Helens look like today? View three different computer-generated ash-cloud simulations

Ash may impact areas very far from volcanoes, be prepared.

Westerly winds dominate in the Pacific Northwest sending volcanic ash east and north–eastward about 80–percent of the time, though ash can blow in any direction. Volcanic ash that reaches your community might be from a distant volcano, and not necessarily from the Cascades volcano closest to you. Organizations at many levels – families, businesses, and public services – will benefit from a plan that aims to help them live with reasonable comfort and safety during, and for many months following, significant volcanic ashfall. The Volcanic Ash webpages are intended to help people prepare and recover from volcanic ashfall.

To learn more about ash and tephra, visit the Volcano Hazards Program tephra webpage.