Prior to 1980, Mount St. Helens had the shape of a conical,
youthful volcano sometimes referred to as the Mount Fuji of
America. During the 1980 eruption the upper 400 m (1,300 ft) of
the summit was removed by a huge debris avalanche, leaving a 2 x
3.5 km (1.2 x 2.2 mi) horseshoe-shaped crater now partially
filled by a lava dome and a glacier. It is primarily an
explosive dacite volcano with a complex magmatic system. Mount
St. Helens was formed during four eruptive stages beginning
about 275,000 years ago and has been the most active volcano in
the Cascade Range during the Holocene. Prior to about 12,800
years ago, tephra, lava domes, and pyroclastic flows were
erupted, forming the older St. Helens edifice, but a few lava
flows extended beyond the base of the volcano. The bulk of the
modern edifice (above the 1980 crater floor) was constructed
during the last 3,000 years, when the volcano erupted a wide
variety of products from summit and flank vents. Historical
eruptions in the 19th century originated from