Mount St. Helens is the Cascade Range's most active volcano. It has produced more eruptions over the last 4,000 years than any other Cascade Range volcano, according to the United States Geological Survey. One of its biggest eruptions occurred approximately 3,600 years ago; this eruption was four times larger than the 1980 eruption. The mountain's name derives from Lord St. Helens, who was a friend of George Vancouver, the British explorer who discovered the mountain. Mount St. Helens' summit is 8,365 feet above sea level.
Rising over 12,200 feet, Mount Adams is the second-highest mountain in Washington. Its surface area extends into four Washington counties -- Lewis, Skamania, Yakima and Klickitat -- making it one of the widest volcanic mountains in the Cascade Range. Mount Adams' flat summit is the result of multiple cone-building eruptions released from several of the mountain's vents. Although Mount Adams has not erupted in the last 1,000 years, most volcanologists believe it is still active.
The Cascade Range's most recent volcanic explosion occurred when Mount St. Helens erupted on May 18, 1980. Before this eruption, Mount St. Helens had a dome-shaped summit and was the fifth-tallest mountain in Washington. The volcano's blast flattened trees within a 230-square-mile area, and created an ash pillar that rose more than 6,500 feet. The eruption changed Mount St. Helens' peak to a crater with a maximum diameter of nearly two miles. In 1982, the U.S. Congress created the 110,000-acre Mount St. Helens National Volcanic Monument for the purpose of researching the mountain's volcanic activity; the Monument features hiking trails and a visitor's center.
Mount St. Helens and Mount Adams are not the only volcanic mountains in the Cascade Range. Mount Rainier, Washington's tallest mountain at 14,410 feet, is 50 miles north of St. Helens and Adams. Its most recent eruptions occurred during the 19th century. Volcanologists believe that Mount Rainier's extensive glaciers -- 26 in total -- could produce enormous lahars if the volcano erupts. "Lahar" is the scientific term for an avalanche of volcanic debris and boulders mixed with water.
