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 (half an inch), termed ash, are carried by winds for thousands of miles.
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.
Evidence from past eruptions shows that three Cascade Range volcanoes are capable of erupting massive volumes of volcanic ash—Glacier Peak, Mount 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.
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.
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.