There are two active volcanoes on the West Coast which erupted during the 20th century. Both are in the Cascade Range. The better-known is Mount St. Helens northeast of Portland, Oregon in Washington state. Its enormous eruption on May 18, 1980 was well-documented by the media and made it famous.
The lesser-known is Mt. Lassen.
Located 50 miles east of Redding in the rugged wilds of northeast California, Lassen is the worlds largest lava dome volcano. Lava dome or plug dome volcanoes contain a thick and pasty dacite lava which is high in silica. That means its more acidic and cooler and does not flow very far, bulging up instead. Most do not reach above 2,000 feet but Lassen climbs to 10,457 feet.
The mountain is the southernmost of the Cascade volcanoes, and like them caused by the grinding of the Pacific plate under the North American plate. Unlike most of them, it has no glaciers or year-round snow pack. It grew out of ancient Mt. Tehama, active 600,000 years ago but gradually worn down by erosion until Lassen was born some 25,000 years ago.
Little is known about Lassens early eruptions. In 1914, the few people in the area didnt even know when the last one had occurred. Most thought it dead.
On Memorial Day, May 30, 1914 without warning it suddenly began to erupt. The scientifically curious rushed to examine it. The first crater was approximately 40 feet wide and 150 feet long with deep fissures reaching out in all directions. Steam and sulphur fumes emanated from it; and lava and ash up to two feet deep extended out for about 200 feet around it.
The eruptions continued through the summer into the fall. There may also have been others during the winter months when clouds cloaked the peak. Some of the eruptions lasted only moments with puffs of rock and ash. Others lasted up to half an hour and flung out chunks of lava up to one-hundred pounds. These eruptions led to only one casualty an explorer named Lance Graham was hit by a flying rock on June 14 which cut his shoulder and broke his collar bone. The crater continued to grow; and as the snow pack melted in the spring of 1915, water seeped into the lava chambers below it. On May 19, the mountain began to erupt pasty lava over its rim as well as large vertical ash clouds. This melted the remaining snow, and on May 21, a great flood of water and mud flowed down the east side of the peak. This "lahar," or mud flow, was measured at 18 feet deep in some places. Hot rocks of up to 30 tons were flung up out of the crater and swept along with the mud and debris which rushed down Hat Creek and Lost Creek. Trees, bridges, fences and farm buildings were destroyed; but fortunately, no lives were lost. The area was evacuated and a good thing because the following day came the "Great Eruption of 1915."
A huge blast blew the top off Mt. Lassen. A mushroom cloud of steam and ash roared up five to seven miles above the peak. Ash fell as far east as Elko and Winnemucca, Nevada. What went largely unseen, however, was a lateral blast of super-heated ash and steam called a "nuee ardente" which followed parts of the previous days mud flow and destroyed an estimated five-and-a-half-million board feet of timber. It left behind a moonscape still visible today but not often seen because most tourists to the area do not drive the twisting roads around the mountain to its northeast quadrant.
The volcanos 107 eruptions in 1914-15 also did not gain much national or international attention because the outbreak of World War One captured most of the newspaper headlines of the period. Lassens activity tapered off, ending in 1921 but it is still considered active because it has erupted within recorded history and because the area contains much hydrothermal activity.
What is ironic is how quickly it was forgotten by the people who should have remembered the geologists of Washington, Oregon, California and the U.S. Geological Survey.
When Mount St. Helens started showing signs of activity, they should have reviewed in detail what happened at Lassen. But they didnt. I personally interviewed one of the geologists working on St. Helens and he said he and his colleagues were almost totally unfamiliar with Lassens similarity. He was much more aware of a similar eruption on Martiniques Mt. Pelee in May 1902 (which gave us many of the terms like "lahar" and "nuee ardente). To give him credit, he went off to check immediately after the interview; and the zones around the mountain were then revised to reflect Lassens May 22 "double blast."
And four days short of the 65th anniversary of that great eruption, St. Helens erupted in exactly the same way.