The eruptive history of Mount St. Helens began about 40,000 years ago with dacitic volcanism, which continued intermittently until about 2,500 years ago. This activity included numerous explosive eruptions over periods of hundreds to thousands of years, which were separated by apparent dormant intervals ranging in length from a few hundred to about 15,000 years.
The range of rock types erupted by the volcano changed about 2,500 years ago. Since then, Mount St. Helens repeatedly has produced lava flows of andesite, and produced basalt on at least two occasions. Other eruptions during the last 2,500 years produced: dacite and andesite pyroclastic flows and lahars as well as dacite, andesite, and basalt airfall tephra. Major intervals of dormancy over the last 2,500 years range in length from about 200 to 700 years.
Learn more: USGS Cascades Volcano Observatory