West Crater

West Crater, a small andesitic lava dome with associated lava flows, is part of a Quaternary volcanic field in southern Washington consisting primarily of small basaltic and basaltic andesite cinder cones and shield volcanoes, oriented along a NW-SE zone to the SE of St. Helens. This 20-km-wide zone extends from Marble Mountain, north of Swift Reservoir, to south of Trout Creek Hill. The Pleistocene basaltic Trout Creek Hill shield volcano produced a lava flow about 340,000 years ago that traveled 20 km SE, temporarily damming the Columbia River. At least three vents in this field are of Holocene age: West Crater, a small cone at Hackamore Creek, and a phreatic crater at the summit of Bare Mountain. The latest eruptions at these locations have been dated at about 8,000 years before present, and have included emplacement of an andesitic lava dome and associated lava flow at West Crater and formation of a phreatic explosion crater at Bare Mountain.