Wednesday, February 3, 2010

The Other California: Chaos! And Jumbles Aren't Always Word Puzzles.

Continuing our exploration of the "Other California", those places in the state that don't always show up on the postcards...

We are in Lassen Volcanic National Park, a place that is on all the postcards, but the park receives roughly a tenth the visitation of a place like Yosemite, somewhere around 350,000 people a year. In the last post, we visited the volcano that isn't there, Mt. Tehama, a large stratovolcano just south of Lassen Peak that succumbed to erosion during the last 400,000 years. In the aerial photo of that post, another barren looking group of mountain peaks could be seen to the left of the Lassen Peak (likewise in the picture below). Aside from a cinder cone in the eastern part of the park that erupted in the 1600's, these cones, the Chaos Crags, are the youngest volcanoes in park, having erupted about 1,100 years ago.

The Chaos Crags are a series of six consecutively erupted plug domes similar in nature to Lassen Peak. They are named, very creatively I might add, domes A, B, C, D, E, and F. The lava, midway in composition between rhyolite and andesite, is called dacite, which usually is highly viscous (resistant to flow). As a result, the term 'lava flow' hardly applies; these volcanoes squeeze out of the ground in a manner similar to toothpaste. Seen from ground level, the domes look barren, steep, and maybe even a bit...unstable. In that context, look at the forest in the foreground of the picture below (taken from the east side of the Crags): it is an old-growth forest of fir and lodgepole pines. No soils have accumulated on the slopes of the cones in a thousand years, so almost no trees grow there.

A look from the west side of the Crags presents a different picture; notice the open forest and stunted trees. What's going on here?

The answer comes to us, courtesy of GoogleEarth (a wonderful thing, that GoogleEarth) below. From the barren side of Dome C (lower right hand side), a series of three large debris avalanches flowed northwest until they slammed into the side of Table Mountain and turned to the west. The avalanche deposit is more than two miles long, and covers 2 1/2 square miles. It blocked Manzanita Creek, forming two attractive lakes, Manzanita and Reflection. Carbon 14 dating of trees in the debris indicate an age of around 300 years, practically yesterday in geological terms. The jumbled and chaotic landscape of the avalanche is called the Chaos Jumbles (that is a great geologic name!).

I remember camping several decades ago at Manzanita Lake, and thinking what a nice campsite it was. The sites were spread out in this lumpy landscape so RV's weren't parked side-by-side. I didn't know at the time that it was because I was in the midst of a geologically recent avalanche. The National Park Service came to this realization in the 1970's (or was it right after the St. Helens eruption? Someone help me here!), and worrying about possible future events, closed a large portion of the campground, as well as some of the area facilities including a camp store and the historic Loomis Museum. In recent years the Loomis Museum has reopened (and has some great geologically themed exhibits).

A close examination of the boulders in the avalanche deposit reveals some interesting features. The dacite includes some large crystals of plagioclase, hornblende, biotite and quartz. It also includes some large blebs of an entirely different rock (called enclaves). These blebs (blobs?) are fragments of a more basaltic magma that bubbled up into the dacite magma when it was still molten. These incidents of magma mixing play a huge role in controlling and influencing the magma systems underneath the Lassen region, and may even provoke eruptions (see Clynne, 1999, for example). Research is continuing on the area.

An excellent picture of a dime (oh, and some dacite with an enclave)

References and Resources:

An excellent geologic map of Lassen and Chaos Crags is available online here.

An excellent geologic road guide to the Lassen National Park area, as well as the Sierra Nevada and Great Valley region around Chico is available from the National Association of Geoscience Teachers, Far Western Section (proceeds from the sale support a scholarship program for geology majors): Convergent Margin Tectonics in and Around Chico, Northern California, 2008 Field Conference, edited by Todd Green

Clynne, M.A., 1999, Complex magma mixing origin of rocks erupted in 1915, Lassen Peak, California: Journal of Petrology, v.40, p. 105-132.

1 comment:

Gaelyn said...

Barren peaks indeed. Great pic of the dime. ;-)