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.

Scenes from a Shield: Traversing California's Biggest Volcano

Continuing on with our late September exploration of the California Cascades, we prepared to leave Lava Beds National Monument. The main paved access is from the north at Tulelake, but our interests lay to the south over the top of the biggest volcano in California: Medicine Lake Highland. MLH is a shield shaped edifice, but the history and composition of volcano suggests a complicated history that is quite distinct from shields like those in Hawaii and elsewhere. Lava Beds National Monument covers about 10% of the shield complex on the north flank. We headed up the good gravel road to the highest corner of the park where we could investigate the source of the lava flows and tubes at Mammoth Crater.

Before reaching Mammoth Crater, we stopped at Heppe Caves to catch an unusual sight: water! Very few sources of open water exist at the Lava Beds. Fracturing of the basalt allows water to seep into the ground quickly and no streams or canyons can be seen in the park. At Heppe Cave, a deep layer of ice allows a small pool of water to persist in the lowest part of the huge cavern in most years. Birds and other animals depend on the pool, despite its diminutive size.

We had gained several hundred feet from the vicinity of the park campground and visitor center, and the cooler wetter climate allowed for the growth of ponderosa pines, and to my surprise, a small aspen grove! These stunted trees are not a common sight in the area.

A few hundred yards up the road brought us to Mammoth Crater, the source of the lava flows that cover about 70% of Lava Beds National Monument (and which produced many of the lava tubes). During the eruptions, the crater would have been a lake of molten lava, spilling over in several directions. In the waning phases of the eruption, the lava drained away, leaving a pit a quarter of a mile across and several hundred feet deep.

People often think that volcanoes erupt only from their summit, but we were quite a few miles short of the summit of Medicine Lake Highland. The mountain is covered with cinder cones and lava flows that originated from vents on the flanks of the volcano.

 It took a few false starts, but I finally located the obscure track that left the main "highway" and took us to these unusual holes in the lava. Does anyone want to speculate what they might be??

The two holes above joined about two feet down. People can reach in and hold hands underground. A few of the mysterious holes are quite large and deep. The deepest sometimes have snow all summer.

Another one or two miles through the thick forest brought us to the edge of the Medicine Lake Highland caldera. It is about as wide as the Crater Lake, some four or five miles, but beyond the horizontal distance, there are few similarities. Crater Lake has cliffs that drop straight into the lake, where MLH has rounded glaciated knobs and peaks. Crater Lake has a nearly 2,000 foot deep lake, where MLH as a lake that barely makes it past 100 feet in depth. Mt. Mazama, the mountain that existed prior to Crater Lake erupted just 7,000 years ago. The eruptions that produced the caldera on MLH took places tens of thousands of years earlier.

Medicine Lake technically shouldn't exist on a mountain that notoriously sucks up all the water. It seems that the glaciers that formed on the mountaintop produced and scraped up enough clay to seal the bottom of the lake and keep the water from draining away.

It was in the low to mid forties while we stood on the shoreline, and it was drizzling. But the sign said "swimming beach", so my students, who always follow what the signs say...

 ...did just that: went swimming in a subalpine lake. I make no judgements here, but I had a bone-chilling cold day, and it didn't help to see them swimming away!

Our next stop: the last thing you would expect to see on a basaltic shield!