Well this is exciting – as we have moved north up the eastern side of the Sierras past Yosemite, we have entered the Long Valley Caldera, the remnants of a supervolcano eruption 760,000 years ago that blanketed the entire region with pyroclastic flows and volcanic ash as far east as Kansas. Yellowstone gets all the press, but this eruption was of similar magnitude, and the Long Valley Caldera was formed when the magma pool was exhausted and the ground subsided above it. It's 20 miles east to west, 11 north to south, and 3000 feet deep – one of the largest calderas in the world. And we're camping in it.
What's the risk? Well, none of us live long enough to make much of a dent in geologic time scales, but California has eight areas of volcanic activity, and the USGS predicts a 16% chance of something blowing up somewhere in the next 30 years. The only community of any size here is Mammoth Lakes a dozen miles from where we're camped. Mammoth Mountain, the famed ski area where Mammoth Lakes is located, is a giant volcano on the western side of the caldera. Authorities there were sufficiently concerned that they build an additional road leading out of town in case everyone had to leave in a hurry. It was originally called the Mammoth Escape Road, but people fretted that this name might discourage tourism, so it's now the Mammoth Scenic Route. If this baby blows, though, I doubt people will be looking at the scenery.
An eruption the size of the formative event is very unlikely – typical activity is smaller eruptions, so there are craters, cones, and domes that have been formed in the last few thousand years all over the area. The magma is still down there, and it's still hot enough to let you know it's there. Water percolates down into the ground from the western side where the Sierras contribute snowmelt to the aquifer. It gets heated to 400 degrees or so, and flows eastward across the caldera, cooling down as it goes. The Casa Diablo geothermal plant utilizes this thermal energy to generate enough electrical power for 22,000 homes.
Further east is Hot Creek, which started out as an ice-cold snowmelt stream flowing down the eastern slope of the Sierras past Mammoth Lakes, but as it crosses the caldera it's supplemented by hot water, plus actual geysers, coming up from the superheated groundwater. It used to be open to swimming but has now been closed off – as at Yellowstone, the authorities were forced to implement regulations designed to keep people from cooking themselves. The water temperature is unpredictable because the geysers in the creekbed periodically go off, raising the water temperature to 193 degrees Farenheit, the boiling point of water at this altitude, which has caused injuries and some deaths. You can still visit the creek, of course, but don't bring your bathing suit.
There are safer alternatives, though – look around and you will find those remarkably democratic institutions in hot springs areas, pools people have constructed to allow locals – and visitors – to soak in the hot water. Many are available so just google around and pick one which matches your cultural ambience preferences. Just watch out for the naked people. We didn't hit any of these but I can imagine how relaxing it must be to soak in the hot water and admire the scenery.
Glass Mountain on the east side of the caldera is the only geological feature that survived the original eruption, and gets its name from the obsidian at its summit. I thought that maybe there were a few pieces lying around or something, but i pulled up the geological quadrant map, and large swathes of the exposed bedrock are obsidian. There are massive deposits all over the place. A more recent eruption, Obsidian Dome, is right off 395, the main (and only) highway through the area, and there's a trail through the dome so you can see it without climbing a 10,000 foot mountain. Sharon has pestered me for years to find some obsidian, but our drive by the dome conflicted with her and Fiona's sleep schedule so I guess we'll have to wait some more. The native Americans in this area have used obsidian to fashion projectile points and knives since they first came to the area 10,000 years ago – because of its non-crystalline structure, obsidian produces edges so sharp that surgeons use it.
We camped at Crowley Lake BLM Campground, and I climbed up the valley leading to Mount Morgan far enough to get a view out across the caldera. That's the campground down there – Crowley Lake is actually a reservoir created by damming the Owens River where it conveniently goes into a narrow gorge leading south out of the caldera, and all this water belongs to the City of Los Angeles. Fishing is big here, and they have to keep restocking the lake with trout because the fishermen are catching them faster than they can reproduce. Glass Mountain is the nearer mountain off to the left, and the mountains on the horizon are the White Mountains, the eastern boundary of the Owens Valley. The Nevada state line is right on the other side of them. This view is only the western part of the caldera, it extends around to the left toward Mammoth.
We also camped for a few days just southeast of the caldera at Tuff Campground, where the eastern side of the campground is a 150 foot high wall of the Bishop Tuff. Tuff is volcanic ash and chunks of lava blown out during eruptions, and is usually welded because when it lands it's still semi-molten and fuses into rock. This tuff covers hundreds of square miles around the caldera, and is named after the town of Bishop, about 20 miles southeast of here. The Owens River Gorge is cut through this deposit. We cut our visit at Tuff Campground short, even though it's right on Rock Creek with beautiful Jeffreys pines and aspens, because we were down in the creek bed and couldn't really see the horizon. This is the kind of place where you want to be able look out across the countryside and enjoy the view.
We're going to continue to push our luck and visit two other volcanic hotspots. Lassen and Shasta, on our way out to the coast, before it gets too hot inland. We usually just head straight out to the Pacific Coast Highway but we've done that so many times that we decided to see the interior of California for a change of pace. If these posts stop suddenly, you'll know we visited the wrong volcano at the wrong time 😉