Mount Saint Helens eruption 1980. Mount St. Helens is an active stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United States.It is 96 miles south of Seattle and 50 miles northeast of Portland, Oregon.s with most other volcanoes in the Cascade Range, Mount St. Helens is a large eruptive cone consisting of lava rock interlayered with ash, pumice, and other deposits. The mountain includes layers of basalt and andesite through which several domes of dacite lava have erupted.