Mount St. Helens is a cinder cone volcano that formed through the gradual accumulation of cinders and ash at the base of the mountain. Unlike a shield volcano, such as Mauna Loa in Hawaii, cinder cones can rise sharply from the surrounding terrain and maintain a steep, angular profile throughout their existence.
Mount St. Helens emerged from the surrounding countryside through a series of eruptions that began approximately 275,000 years ago. These eruptions came in four major waves of activity. The earliest eruptions consisted of tephra and pyroclastic flows that built up the thick, low-lying base of the mountain. After this base was laid down, the volcano was dormant until around 1,000 B.C., when Mount St. Helens began building up most of its height. Nearly all of Mount St. Helens above the floor of the 1980 crater was laid down during this period in a series of classic cinder eruptions. Until 1980, the mountain maintained the sharp outline of a young, conical volcano and was even compared in shape to Mount Fuji in Japan. The last major event in Mount St. Helens' formation came with its 1980 eruption, when the uppermost 1,300 feet of the mountain exploded in one of the most dramatic eruptions to have been witnessed in modern times.