My home state, Washington, is home to five major composite volcanoes or stratovolcanoes: Mount Baker, Glacier Peak, Mount Rainier, Mount St. Helens, and Mount Adams.
These volcanoes are part of the Cascade Range, a volcanic arc that stretches from southwestern British Columbia to Northern California.
While some of Washington’s Volcanoes are quite famou – EVERYONE knows about Mt St Helens, for instance, and everyone knows that Mt Rainier looms over Seattle’s skyline – some others are a bit more obscure, but no less scenic, or less interesting… they just haven’t made as much “noise” recently as Rainier or especially St Helens.
All Washington volcanoes except for Mount Adams have erupted since the birth of the United States in 1776, and all of them, as well as several to our South in Oregon, have active seismicity and/or geothermal activity. The USGS provides a compilation of eruptive histories and hazards from major Cascade peaks in Washington and Oregon.
In the thick of things
Washington is situated at a convergent continental margin, the collisional boundary between two tectonic plates. The Cascadia subduction zone, which is the convergent boundary between the North America plate and the Juan de Fuca plate, lies offshore from northernmost California to southernmost British Columbia. The two plates are converging at a rate of about 3-4 centimeters per year (about 2 inches per year); in addition, the northward-moving Pacific plate is pushing the Juan de Fuca plate north, causing complex seismic strain to accumulate. Earthquakes are caused by the abrupt release of this slowly accumulated strain.
The southern Washington Cascades are seismically active. Most earthquakes occur along the 100-kilometer-long, north-northwest trending St. Helens seismic zone, where most focal mechanisms show dextral slip parallel to the trend of the zone and consistent with the direction of plate convergence. Other crustal earthquakes concentrate just west of Mount Rainier and in the Portland (Oregon) area. Few earthquakes occur north of Mount Rainier or south of Mount Hood.
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