The familiar snow-clad peaks of the Cascade Range are part of a 1,300 km (800 mi) chain of volcanoes, which extends from northern California to southern British Columbia. The volcanoes are the result of the slow slide of dense oceanic crust as it sinks beneath North America (subduction), which releases water and melts overlying rock. This rich volcanic zone contains the well-known landmark volcanoes and approximately 2,900 other known volcanic features ranging from small cinder cones to substantial shield volcanoes.
The time between eruptions is usually measured in decades or centuries, so eruptions are not a part of our everyday experience. However, recent eruptions at Mount St. Helens vividly demonstrate the power and impacts that Cascade volcanoes can unleash when they do erupt.
During the past 4,000 years, periods of eruptive activity at various Cascade volcanoes have lasted for a few to tens of years per century. Locate the volcano closest to you by visiting the map on our home page. Seven Cascade volcanoes have erupted since the beginning of the eighteenth century. Some of those eruptions would have caused considerable property damage, economic disruption, and loss of life if they had occurred today without warning:
As population increases in the Pacific Northwest, areas near the volcanoes are becoming developed and more people and property are at risk. The principal hazards to people in the Pacific Northwest are from lahars and ash fall. Lahars (volcanic mudflows) can destroy buildings and infrastructure. Eruptions that include volcanic ash can be especially dangerous for aircraft, even at long distances from the volcano, because volcanic ash can clog and shut down their engines. To learn more, read about volcanic hazards on this website.
Volcanic threat is the combination of hazards (the dangerous or destructive natural phenomena produced by a volcano) and exposure (the people and property at risk from the volcanic phenomena). Based upon eruption history and distance to population centers, a national volcanic threat assessment (NVEWS) designated nine volcanoes in Washington and Oregon with a "high" or "very high" rating. Very High Threat: Crater Lake, Glacier Peak, Mount Baker, Mount Hood, Mount Rainier, Mount St. Helens, Mount Adams, Newberry, Three Sisters. HIGH: Mount Adams.
Download the full Open File Report 2005-1164.