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: What do you do with a used forest?

Humans have cast a long shadow over Sierra National Forest (oh, sorry about literal metaphors).

The Sierra Nevada is a world-class mountain range. Scenic, in turns rugged and visitor-friendly, spectacular, destination of millions of travelers from around the planet, the mountains host one of the earth's most precious parks in Yosemite, and the only slightly lesser-known (but no less incredible) Sequoia/Kings Canyon National Park. Regular readers of this blog will certainly know of the explorations we have covered in those parks, but my Other California series is about those significant places that aren't quite so famous. They are the places that don't always show up on the postcards...

There is a huge swath of mountain territory between the south boundary of Yosemite, and the northern reaches of Kings Canyon. This is the drainage of the San Joaquin River, and to most travelers it is terra incognita, a blank spot on the map.

Structurally the Sierra Nevada is a 400-mile-long block of mostly granitic rock that is tilted towards the west, with a long gentle western slope and a very steep eastern flank. Most of the major rivers start at the crest and flow west through deep canyons to the Central Valley. Due to a geological glitch of sorts, the San Joaquin has its headwaters more or less east of what by all rights should be the Sierra Crest. The high rugged ridge of Mts. Ritter and Banner and the jagged Minarets extends south from Yosemite, and reaches elevations exceeding 13,000 feet. This would seem to be the crest, but the river flows in a deep canyon around the south end of the Minarets, and the actual Sierra Crest off to the east is a somewhat uninteresting ridge of volcanic rock that barely exceeds 10,000 feet in elevation. Mammoth Mountain (that of the ski area) is the high point on that part of the crest, and from the west it doesn't look like much. Even the John Muir Trail skips this part of the crest and winds along the lakes and ridges below Ritter, Banner and the Minarets. The headwaters of the San Joaquin is a spectacular area, and Devils Postpile National Monument draws many visitors. But that's not the region I want to talk about today.

The blank spot I speak of is downstream, west of the Minarets, and between the national parks. The land is administered by the Department of Agriculture as Sierra National Forest, and there aren't generally a vast number of tourists to be seen there. That's not to say that the land is not known. It is known quite well by some. It is a landscape that has been fully utilized under the doctrine of multiple-use, the operating principle used by the Forest Service to justify any money-making enterprise that anyone can think of. The vast conifer forests have been logged heavily, huge dams have been constructed, herds of cattle graze the meadows, and mining took place, especially in the metamorphic rocks near the Minarets. In land-use terms, this is a used-car lot of a natural environment.
So what do you do with a used forest? What do you do when the biggest trees have been cut down, and the mills shut down? The mines have been abandoned for years even though they were probably the reason the Minarets were left out of Yosemite's boundaries in the first place (although a desire to build another paved trans-Sierra highway might have been another motivation). It starts with the understanding that time is a healer. Trees are growing back, and logging has left a network of roads through the region, some paved, others not, but somebody got creative and linked a series of utility roads into a rather marvelous route that explores some very scenic and interesting country. The public relations people labeled it the Sierra Vista Scenic Byway. We explored the road last fall, and I can give it a thumbs-up as a great place to explore.
Much of the region is a densely forested plateau pockmarked by green meadows and the occasional granitic dome. The San Juaquin River has carved a deep gorge through the bedrock, which is mostly an intrusion of granitic rock called the Mt. Givens granodiorite. It is one of the largest single intrusions of the Sierra Nevada batholith, with a length of about 50 miles and a width of 10 to 20 miles. It was emplaced about 90 million years ago (Bateman, P., 1992, Plutonism in the Central Part of the Sierra Nevada Batholith, California; USGS Professional Paper 1483). Glaciers carved the rugged peaks of the Ritter Range, and extended across much of the area betwen the parks.
The whole loop is more than 70 miles, and is paved road for the most part. Several sections were once paved, but they sort of gave up trying about 40 years ago, so there are a few bumps here and there, but most cars will have no problems. The road is closed during the winter and spring until the snow melts. The Forest Service has a handy map and guide online here, and America's Byways has a guide here. A number of interpretive displays have been constructed, and there are a few chemical/pit toilets at the picnic stops, but don't look for much in the way of amenities or gas stations. Supplies can be had down the hill at North Fork, Bass Lake, and Oakhurst.
Mammoth Pool Reservoir (above) impounds the waters of the San Joaquin River. A side road provides access. A second side road near the southern end of the loop road visits the geographic center of California. The Eagle Peaks (below) are exfoliating slopes of granitic rock seen at the Mile High Overlook, one of the best view points on the road. The Overlook provides the spectacular views of the Minarets seen in the pictures above as well (photos courtesy of Susan Hayes; first photo is mine, standing in Jackass Meadow, appropriately enough). Globe Rock, a huge balanced boulder, was discussed in a previous post.
Check it out! In a few months, that is. I didn't get a chance to visit the Nelder Sequoia Grove on our last trip out, so we might run into each other this summer...