| On May 18, 1980, after nearly 2 months of local earthquakes and steam eruptions, picturesque Mount St. Helens, a Cascade Range volcano in southwestern Washington, suddenly began a major explosive eruption directed first northward and then upward. The lateral blast, which lasted only the first few minutes of a 9-hour continuous eruption, devastated more than 150 square miles of forest and recreation area, killed countless animals, and left about 60 person dead or missing. The 9-hour eruption, the huge debris avalanche that immediately preceded it, and intermittent eruptions during the following 3 days removed about 4 billion cubic yards (0.7 cubic mile) of new magmatic material and of the old upper and northern parts of the mountain, including about 170 million cubic yards (0.03 cubic mile) of glacial snow and ice. The eruption caused pyroclastic flows and many mudflows, the largest of which produced deposits so extensive and voluminous that they reached and blocked the shipping channel of the Columbia River about 70 river miles from the volcano. The May 18 eruption blew volcanic ash, consisting of pulverized old rock form the mountain's core as well as solidified new lava, more than 15 miles into the air. Winds carried the ash generally eastward across the United States, and, in trace amounts, around the world. The ash, which fell in troublesome amounts as far east as western Montana, severely disrupted travel, caused widespread economic loss, and resulted in other problems that persisted to the end of the first 100 days of the volcano's activity (June 27, 1980). During that time, two other major eruptions (May 25 and June 12) also produced troublesome ashfalls in western Oregon and Washington. -- Foxworthy and Hill, 1982 |
