Thursday, May 17, 2018

Airliner Chronicles: Do You Know California's Biggest Volcano? You May Be Surprised at the Answer

The summit complex of Medicine Lake Highland, with Glass Mountain at the top, the Medicine flow center-left, and Medicine Lake at center-right.
It's a real problem for a geologist to be flying through cloud cover. One loses track of time and location so when an opening in the clouds occurs, one cannot know for sure where one is. I can pick out the essential geologic details a lot of the time, but without temporal or spatial context, I can't recognize even some of the most familiar places that I've visited many times. That's what happened last Thursday when I was flying to SeaTac. I lost sight of the ground only a few minutes out of Sacramento, and when the ground reappeared again, I was totally fooled by the scenes in these pictures. I thought I might be over Newberry Crater in Oregon, but I couldn't place Paulina Lake with the steep ridge that should have been to the right in the picture above. So I figured it must be some place in the Three Sisters area that I wasn't familiar with. But then I got home and looked at the pictures on the computer instead of the camera. I was wrong.
The Medicine Flow dates to about 2,000 years ago. It is composed of thick dacite lava flows. Frozen Medicine Lake is to the right.

It turns out I was flying right over the top of California's biggest volcano. Mt. Shasta is California's most prominent volcano, rising to 14,162 feet above sea level. It's the largest stratovolcano in the entire Cascades chain, with more bulk than even Mt. Rainier, which we looked at in our previous post.

[At this point there is a bit of uncomfortable shuffling in the back of the room as a few of the geologists look at each other and start mumbling something unintelligible.

"Ahem, is there something you would like to add to the discussion?" I say.

"But we've seen Mt. Shasta, and it looks nothing like that".

"And you would be right" I say, "but give me a chance to finish". They relent.]

But this isn't Mt. Shasta. It may be the tallest volcano in California, and it may be the biggest stratovolcano in the Cascades, but we are forty miles northeast of Shasta, looking at an entirely different kind of volcano. It's a shield volcano called Medicine Lake Highland. Although it is not even 8,000 feet high, it is much wider, with a total volume of around 130 cubic miles, compared to about 108 cubic miles for Mt. Shasta.

The view from the ground shows a gently sloping edifice covered by a variety of cinder cones and plug domes. Medicine Lake Highland (MLH)is shield-shaped like the giant volcanoes that make up the Hawaiian Islands, and is composed in large part of basalt, the fairly non-viscous lava that can flow for long distances before congealing. But MLH is different in some important ways. Some of the magma chambers beneath the volcano originate in the crust rather than the mantle (dacite or rhyolite), and the silica-rich lavas that erupt at the surface are sticky and barely flow at all.
MLH is an active volcano. There have been at least 17 eruptions in the last 11,000 years, and the Glass Mountain flow (below) erupted only about 850 years ago. There are areas of geothermal activity that have been proposed for energy development, but these have been rebuffed so far (thank goodness).
Glass Mountain at the top of Medicine Lake Highland. The flow dates to 850-900 years before present.

The top of the shield complex is a caldera (a sunken crater-like depression caused by the inward collapse of the top of the volcano). It is about the size of the Crater Lake Caldera, but is not nearly as rugged, as it is many thousands of years older and has been partially filled with subsequent lava flows. Medicine Lake sits within the caldera, but is not volcanic in origin. During the ice ages, glaciers scoured the summit area of MLH and left behind some impermeable clay deposits that prevent water from sinking into the ground (there are very few streams or rivers on the volcano, as it is highly porous; massive springs are found around the base of the volcano.).

One of America's most interesting national monuments is found on the north flank of MLH. Lava Beds National Monument preserves more than 75 miles of underground lava tubes, one of the highest concentrations of such caves in North America (and maybe in the world). The park also preserves the memory of the last stand of the Modoc people in their struggle against the U.S. military in the 1870s. They held out for months in the barren lava flows but were finally betrayed and captured. The Modoc people survive, but much of their cultural history was lost when they were banished to Oklahoma.

The clouds soon closed in again and I saw nothing more until we were over Washington.

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