Spend an evening camped in the John Muir Wilderness southwest of Mammoth Lakes, and in the morning you can ask your tentmate with a straight face, “Did the earth move for you last night?” Chances are it did, geologically speaking. The Mammoth Lakes region rides a magma dome whose every burp and gurgle sends shivers to the Earth’s crust, making it the most active volcanic hot spot in the continental United States south of Washington’s Mt. St. Helens. A record 1,000-plus mini earthquakes were detected in a single day in 1997.
Now and again the situation in the eastern Sierra Nevada really heats up, as it did 760,000 years ago when the Long Valley Caldera blew its top, carrying 150 square miles of real estate skyward and leaving behind some of the most diverse scenery in the Sierra. Evidence of that cataclysm, along with other signs of the area’s volcanic activity, is on display along a weekend-length trek up Fish Valley.
The trip starts with a bang, or more accurately a roar, at the 1-mile mark at Rainbow Falls, where the San Joaquin River plunges more than 100 feet. From there, it’s typically gorgeous Sierra scenery-rolling forests of lodgepole and Jeffrey pine, stunning watchtowers of 12,000-foot peaks, lush meadows-all the way to Iva Bell Hot Springs. Soak in the thermal pools and contemplate your options: turn around and head back; spend a few days exploring the steep-walled, Shangri La-like Cascade Valley; or press on to complete a 31-mile, three- to four-day loop through this hyperactive Sierra terrain.
Go with option three and the geologic sideshow begins in earnest. You’ll come across ice-smoothed granite slabs peppered with fist-size lava bombs (glaciation was the other dominant geologic force in the Mammoth area), and cones of red cinder poke from hillsides like fiery anthills. Not far from the trailhead, whole swaths of forest have been strangled by noxious gases seeping up through the ground.
To complete the loop, climb from Cascade Valley to Purple Lake, then head back to the trailhead on the Pacific Crest Trail (PCT). If the trip leaves your knees wobbling, relax. It’s probably just the ground shaking.