Mt. St. Helens

Mount St. Helens is a young volcano that developed over the last 40,000 years within a highly dissected terrain of Tertiary volcanic and metavolcanic rocks. Major valleys surrounding the volcano were extensively glaciated during Pleistocene glacial advances, but mass wasting, fluvial, and other erosion processes have carved complex landforms that are not dominated by the signature of any single process. Mount St. Helens, like most other Cascade volcanoes, is a great cone of rubble consisting of lava rock interlayered with pyroclastic and other deposits. Volcanic cones of this internal structure are called composite cones or stratovolcanoes. Mount St. Helens includes layers of basalt and andesite through which several domes of dacite lava have erupted.

SOURCE: USGS < http://vulcan.wr.usgs.gov/Volcanoes/MSH/description_msh.html>