Lassen Peak is an odd one. Most people have a stereotypical view of what a volcano "should" look like, and to most people, Lassen doesn't fit the stereotype. Yes, it is an isolated peak from most angles, but where is the cone shape, and where is the crater on top?
We were on the last day of our late September field studies course through the Cascades of Northern California, and were having a look at the volcanic features of Lassen Volcanic National Park. In the previous post
we took in a fascinating set of six plug domes, the Chaos Crags, and the debris avalanches that thundered down their flanks forming the Chaos Jumbles.
Like the Chaos Crags, Lassen Peak is a plug dome, a steep rubble-covered peak composed of dacite or rhyolite. It may be the largest plug dome in the world, rising about 2,000 feet above its base (most domes are half of that). It lacks the characteristic cone shape of stratovolcanoes because its lavas were so viscous upon emerging from the depths that they barely flowed at all, forming a steep pile of lava rather than relatively smooth slopes. As the lava cooled, it contracted and broke up, forming the talus slopes that coat most of the mountain. A few rugged cliffs of solid dacite stick out here and there. The peak tops out at 10,457 feet, so it is often coated with snow.
Lassen Peak emerged during a series of eruptions about 28,000 years ago. The entire mountain probably developed in just a few years. Its shape was modified by latest episode of Pleistocene glaciations, but it was otherwise dormant for tens of thousands of years. Until 1914, that is...
In May of 1914 a steam explosion rocked the top of the mountain, producing a small crater on the summit. Over the next year at least 180 explosions blasted away at the summit, ultimately producing a 1,000 foot wide crater. The eruption made news across the country. Brave but foolhardy souls climbed to the summit to have a look, including several men who were standing at the summit on June 14th when B.F. Loomis made the famous images reproduced below (these are pictures I took of the displays in the Loomis Museum at Manzanita Lake).
One of the men, Lance Graham, was walloped by a rock in the shoulder and in the panicky moments that followed was left for dead near the top of the mountain. A rather lurid NY Times story
described him as actually being dead, his arms cut off, a deep gash exposing his heart, and his body nearly torn in half; a later correction
stated : "...he was reported to be dead, but the latest word is that he still is alive, although fatally injured". He apparently survived to live a long life (I recounted this story in one of my classes, and a surprised student piped up saying "Graham? He was my great grandfather!").
On May 19th of 1915, lava emerged from the volcano for the first time.
The thick flow melted thick banks of snow and produced a major mudflow, or lahar
, that traveled many miles down two valleys on the north flank of the mountain, destroying six ranch houses.
|Source: National Park Service. Photograph by R.E. Stinson|
The volcano was quiet for two days, and some may have thought the eruption was subsiding. It wasn't. On May 22nd, a huge explosion lifted ash 30,000 feet into the sky. A plume of hot ash swept down the north flank of the mountain, destroying the forest and all living things over an area of three square miles (these hot ash clouds are called pyroclastic flows or surges). This site is called the Devastated Area, and the demarcation between the old forest and the pyroclastic flow is still clear. The mountain has been quiet since 1917 (except for a puff or two of activity in 1921)
The eruption of Lassen Peak was for sixty years the only volcanic activity to take place in the lower 48 states, and as such was often the main topic of discussion in the volcanoes chapter of geology text books. It was a "nice" eruption in that it displayed many diverse phenomena (steam explosions, ash flows, lava flows, and mudflows), and didn't kill anybody (despite Lance Graham's near miss with mortality). Even though Mt. St. Helens stole all the attention, Lassen Volcanic National Park remains one of the best places I know of to learn about volcanism. And it is a beautiful place to visit.
More in the next post!
An excellent description of the 1914-17 eruption can be seen here: http://www.nps.gov/lavo/naturescience/upload/eruptions_of_lassen_peak.pdf