The Sierra Beyond Yosemite: Another Caldera near Mammoth Lakes? Don't Worry About This One...

In the 1980s and unfortunate series of events led to conflict in Mammoth Lakes between geologists concerned about a possible volcanic eruption, and civic leaders and business people worried about their economy and profits. The conflict was exacerbated by irresponsible reporting in the national media. The problems would have been trivial had the volcano actually erupted, but in the end the magma cooled to solid rock at a depth of 7,000 feet or so.

Volcanic events of the last 8,000 years in the eastern Sierra Nevada, from interpretive display at the Mammoth Lakes National Forest Visitor Center

The possibility of volcanic activity in the Mammoth Lakes/Inyo Craters/Mono Lake area is very real. Studies show a dozen or more volcanic events in the last thousand years, and the region is constantly shaken by earthquakes, and riddled with hot springs and steam vents. Things have been mostly calm in recent years, and the warning stage is currently green. The Long Valley Caldera is currently at rest, but hot magma still exists at a depth of three or four miles.

As noted in some of my previous posts, the Long Valley Caldera exploded in apocalyptic fury about 760,000 years ago, releasing some 150 cubic miles of ash into the atmosphere. As the eruption evolved, the Earth's crust collapsed into a massive hole 20 miles long, 10 miles wide, and as much as three miles deep. Probably the last thing anyone would want to find out is that a second caldera exists in the region. But it's not anything to worry about. It went extinct around 100 million years ago in the middle Cretaceous period.

Try to imagine what it is like at a depth of two miles or so in the present day Long Valley Caldera. When the crust collapsed during the climactic eruption a hundred million years ago, huge chunks of rock hundreds, even thousands of feet across fell into the mix of magma and ash. And a great many dinosaurs perished (now that's a thought, but this was a much more local event, not the K/T extinction). A great deal of time passed. Plutons of molten rock intruded the old caldera, cooling slowly to form a granite-like rock, and metamorphosing the older volcanic rocks into slate, schist and greenstone. Even more time passed, and tectonic activity caused the crust to rise many thousands of feet. Erosion stripped away the overlying rock, and the depths of the ancient caldera were exposed to the air, water, and eventually, glacial ice. What had once been the deep interior of a gigantic caldera now stood as high castle-like cliffs. That is the story of the Minarets, seen in spectacular fashion from Minaret Summit just a few miles up the hill from the Mammoth Lakes ski resort.

The rugged ridge of the Minarets and the high peaks of Mt. Banner and Mt. Ritter are not actually on the eastern crest of the Sierra Nevada. They lie a bit to the west of Mammoth Mountain and the headwaters of the west-flowing San Joaquin River lie between the two ridges. The dark peaks of metamorphic rock have always contrasted strongly with the "usual" granite of the Sierra high country.

I once had a marvelous adventure in the Minarets. Decades ago, I was looking for a post-high school adventure, and ended up walking most of the John Muir trail. We tried to follow a few alternate routes in some places, including an attempt to make a cross-country climb over the pass above Iceberg Lake at the base of the Minarets. Unfortunately the lake was living up to its name, and was still frozen over. The pass above was covered in snow, and we had no equipment to surmount the obstacle. We had to backtrack quite a few miles to catch an alternate route. Just about then, our mosquito repellent ran out, so we more or less ran, arms waving at the swarming insects, ten miles or so to Reds Meadow and Devils Postpile. The Minarets had totally rebuffed our efforts to pass through. We took an alternate route that involved hitchhiking to Mammoth Lakes, consuming hamburgers, and following a much easier trail past Duck and Purple Lakes.

I've wondered why the Minarets were never incorporated into nearby Yosemite National Park. I strongly suspect that the metamorphic rocks had mineral plays and potential mining claims. I've heard (but can't confirm) that a large body of magnetite somewhere in the Minarets screws up attempts at orienteering with compasses. In any case it's a protected wilderness today, and a spectacular place to view some of the more unusual rocks to be seen in the high country of the Sierra Nevada. And there aren't any active magma chambers underneath.

The Other California: Why worry about Yellowstone? We've got our own "Supervolcanoes" to worry about

First off, let's get something straight. As far as geologists are concerned, there are no "supervolcanoes". There are stratovolcanoes, plug domes, cinder cones, and shields, but no "supervolcanoes". What the tv documentaries call a "supervolcano" wouldn't technically be a volcano at all, since it is a giant hole in the ground, not a mountainous feature produced by lava flows.

The volcanic features referred to as "supervolcanoes" are actually calderas, large, generally round depressions in the crust that form when huge eruptions of hot ash empty out magma chambers. The destablilized crust collapses inward to fill the void. Calderas form in a number of circumstances, and are not always catastrophic events. On the Big Island of Hawaii, for instance, the calderas on the summit of the vast shield volcanoes are just a few miles across, and are stable enough that the Hawaii Volcano Observatory is perched on the edge of one. On the other hand, the formation of some calderas is a catastrophe of almost unimaginable proportions. The eruption that produced the caldera at Crater Lake involved the extrusion of something like 25 cubic miles of ash, and turned the 11,000-12,000 foot high mountain into a gaping crater 4,000 feet deep, with a crater rim elevation of only 8,000 feet or so. The eruption was around 7,000 years ago, and human witnesses related stories of the eruption that have survived as myths into the present day. A similar-sized eruption in 1815 at Tambora in Indonesia caused food shortages in the northern hemisphere by blocking sunlight. Snow fell during the summer months, disrupting agriculture.

The ultimate in a caldera eruption, though, is one that forms over a rhyolite magma chamber. Rhyolite is a silica-rich magma that can be highly explosive. It is also pasty (highly viscous), and the magma chambers can grow to massive proportions, sometimes tens of miles across. Rhyolite magma underlies Yellowstone National Park, and eruptions produced 185 cubic miles of ash 640,000 years ago, 67 cubic miles of ash 1.3 million years ago, and a simply unimaginable 585 cubic miles of ash 2.1 million years ago. An eruption of that size in the present day would be a civilization killer by way of a more or less instant ice age and massive disruptions of agriculture worldwide.

Everyone (at least everyone on cable tv channels) seems worried that the Yellowstone "supervolcano" is going to explode and kill us all. But if you live in California, you have other volcanic catastrophes to worry about. But that's not what I'm writing about today. I want to talk about a giant caldera that is part of California's past.

Rhyolite and granite are the same rock, at least as far as composition is concerned. The difference between them is crystal size; granite has visible crystals of quartz, feldspar and mica, while the crystals in rhyolite are too small to see. The difference arises from the place where the rock cooled. Granite cools slowly over many years deep in the crust, while rhyolite cools in a matter of hours or days following an eruption at the earth's surface. The Sierra Nevada is justly famous for the extensive exposures of granitic rock that form the sharp pointed alpine peaks and deep glacial valleys. The range is essentially a 400 mile-long tilted block of the speckled rock. What intrigues me is that while all that granite was intruding into the crust 100 million or so years ago (actually from about 140 to 85 million years), there must have been quite a collection of volcanoes and volcanic features on the surface above, features that would have been removed long ago by erosion. Except in a few spots...

The Minarets are probably the most beautiful and spectacular mountain ridge in the state that is not protected in the boundaries of a national park or monument (It is however protected as a National Forest Wilderness Area). The peaks are familiar to travelers headed to Devils Postpile National Monument, as they pass a pullout called Minarets Vista as they cross the crest of the Sierra Nevada. Far fewer actually walk among the pinnacles and towers in the depths of the wilderness.
The rocks of the Minarets are clearly different than your "average" Sierra. Granitic rock doesn't usually have the dark tones that are characteristic of the Minarets and is not usually as "spiky" (is that a geology word?). A closer inspection of the rock reveals that they are metamorphic, that is, baked and pressurized. Usually such rocks are older than the surrounding granite, but research reveals that this is not the case here. The metamorphic rocks are roughly the same age as some of the surrounding granitic rocks. The rocks include ash flow tuffs and large chunks of various kinds of volcanic rocks. Chunks, as in pieces as much as a mile wide. Big pieces of rock that collapsed into an evacuated magma chamber. The rocks of the Minarets are the remains of giant caldera, as seen from inside the crust of the earth.

So...Yellowstone National Park is a really neat place to see an active caldera, and if the cable news channels are right, it will probably kill us all by erupting, I dunno, maybe in 2012, but what a cool idea to be able to explore a caldera from the inside out! And it has the added virtue of being absolutely extinct, so it won't destroy us and our society in a cataclysmic explosion (just 15 miles to the east, the news isn't so good; more on that later).

As noted before, the Minarets are easily seen from a distance at places like Minaret Vista, and a somewhat less visited view can be had from the Sierra Vista Scenic Byway on the west slope of the Sierra Nevada. The only good way to see the rocks themselves is to hike in. I did some hiking there in my pre-geology days, and would love to return!

R. S. Fiske and O. T. Tobisch, 1994, Middle Cretaceous ash-flow tuff and caldera-collapse deposit in the Minarets Caldera, east-central Sierra Nevada, California: GSA Bulletin; v. 106; no. 5; p. 582-593 (abstract here)