Thursday, October 24, 2013

Chaos! The Jumbles and Crags of Lassen Volcanic National Park

Lassen Volcanic National Park is one of the lesser-known and less visited of the national parks in California. Many have heard of Lassen Peak, but the park is located in the northern reaches of the state relatively far from most population centers. It's a fascinating place, and if one is interested in volcanism, there are examples of practically every type of volcanic feature within the borders of the park. How many places in the world have shields, lava plateaus, stratovolcanoes, plug domes, cinder cones, and calderas, as well as active geothermal fields (fumaroles, boiling mudpots, and hot springs; everything except geysers, so Yellowstone can rest easy). The park is also noted for the recency of many of these features, with numerous cones and flows that are less than a few thousand years old (Lassen itself erupted from 1914-1917).

On our recent trip, we approached the park from the north, and the first sight we beheld was beautiful Reflection Lake. In the morning light it was serene and quiet, and it was strange to consider the violence that led to the lake's formation (along with nearby Manzanita Lake). The forest hides the hummocky surface of the landscape that surrounds the lake, but a short distance up the road, the origin of the lake becomes clear.
The Chaos Jumbles is the remains of a large debris avalanche that thundered down the slopes of  nearby Chaos Crags. The lack of soil and vegetation suggests that the avalanche deposits are not old (there are at least three of them), and indeed none of the trees on the slide date to more than three hundred years. The slide traveled on a cushion of compressed air, traveling for more than two miles and climbing some four hundred feet up the slope on the far side of the valley. It blocked several streams, ultimately forming the beautiful lakes.
The rocks that make up the slide are quite interesting as well. They are volcanic, but not the black basaltic rock that people associate with lava flows and volcanoes. It is a pinkish brown volcanic rock called dacite. Dacite is intermediate in composition between rhyolite and andesite, containing the minerals quartz, plagioclase, hornblende and biotite. The volcanic rock also contains enclaves of a darker volcanic rock, an andesitic basalt that shows that two different magma chambers were comingling and mixing. The mixing probably was a factor in the eruptions of the dacite.
An aerial shot of the Lassen vicinity shows the Chaos Crags in sharp outline. They are the barren snow-free peaks to the left of the main cone of Lassen Peak (the Crags lie north of Lassen; the picture is oriented towards the southeast). Given the dense thick forest that surrounds the volcanoes and the barren nature of the Crags, they have to be exceedingly young, and dating indeed places their age at about 1,100 years before the present. Six individual domes were erupted during a period of about 60 years.

The Chaos Crags are excellent examples of plug domes, short steep cones produced by highly viscous lava flows. The lava emerged like toothpaste from the ground, and as it cooled, the surface of the lava contracted, breaking up and forming huge debris piles on the margins of the cone.

The six domes have been creatively named A, B, C, D, E, and F. Domes E and F can be seen in the picture below, while Dome D dominates the bottom picture.

In any other setting, the Chaos Crags would be the central focus of a volcanic park, but the much taller peak of Lassen stands less than two miles away, so they tend to be somewhat ignored. That's too bad because there are few better examples of plug domes to be found anywhere, and the youthfulness of the Jumbles suggests continuing geologic activity. A stop among the chaos of the Crags and Jumbles is a great introduction to the geology of Lassen Volcanic National Park!

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