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Mt. St. Helens National Volcanic Monument
Mount St. Helens: The Volcano


The Mount St. Helens Volcano has shown continuous intermittent volcanic activity since the 1980 eruption, including minor eruptions of steam and ash, and occasional pyroclastic flows. More than a dozen extrusions of lava have built a mound-shaped lava dome in the new crater. Goat Rocks on the northern flank is one of the older lava domes. Experts monitoring the volcano believe there will be adequate warning of the next eruption.

The most recent seismic activity occurred October 2004 and consisted of steam and ash eruptions and lava extrusions resulting in the significant expansion of the lava dome.

Understandably, Mount St. Helens has been studied intensively, and more is known about its eruptive history than that of any other Cascade volcano. It is a relatively "young" volcano, perhaps 40,000 to 50,000 years old. The cone that partly collapsed in 1980 is only 2,200 years old. In the last 515 years, it is known to have produced 4 major explosive eruptions and dozens of lesser eruptions. An eruption in 1480 A.D., was about 5 times larger than the May 18, 1980 eruption,

The May 18, 1980 eruption came after nearly 2 months of thousands of local earthquakes and hundreds of steam eruptions, and the outward growth of the volcano's entire north flank by more than 80 meters. A magnitude 5.1 earthquake struck beneath the volcano, sending the volcano's bulging north flank sliding in the largest landslide in recorded history. This in turn triggered a lethal lateral blast of hot gas, steam, and rock debris lasting only a few minutes, but which swept across the landscape as fast as 1,100 kilometers per hour. The next 9-hours of continuous eruption, were followed by 3 days of intermittent eruptions, then 5 smaller explosive eruptions over a period of 5 months.

Altogether, about 4 billion cubic yards of new and old lava material and about 170 million cubic yards of glacial snow and ice were deposited over the landscape, as a result of the May 18 eruption. Some debris and mud-flows, were so voluminous that they reached and blocked the shipping channel of the Columbia River about 70 river miles from the volcano. The eruption itself blew volcanic ash more than 15 miles into the air, within minutes. Winds carried about 490 tons of ash generally eastward across the United States and, in trace amounts, around the world. The ash fell in troublesome amounts as far east as western Montana. About 60 persons were dead or missing.

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