Glacier Peak is one of the most active of Washington's volcanoes. The volcano formed during the Pleistocene epoch, about one million years ago, and since the most recent ice age, it has produced some of largest and most explosive eruptions in the state. When continental ice sheets retreated from the region, Glacier Peak began to erupt regularly, erupting explosively five times in the past 3,000 years. It has erupted repeatedly during at least six periods; two of these eruptions have been among the largest in Washington.