Earthquakes in California's Fish Valley

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.

QUICK TAKE: Fish Valley, CA

DRIVE TIME: The John Muir Wilderness is just west of Mammoth Lakes, 325 miles north of LA (51/2 hours), and 300 miles southeast of San Francisco (51/2 hours).

THE WAY: From US 395 north of Bishop, exit west onto CA 203 toward Mammoth Mountain Ski Resort. Between 7:30 a.m. and 5:30 p.m., you’re required to park and take a shuttle bus ($9) from the resort to Rainbow Falls trailhead. Get there before 7:30 a.m. and you can park at the trailhead.

TRAILS: Leaving Rainbow Falls trailhead, Fish Creek Trail connects with the PCT to form a 31-mile loop through Fish and Cascade Valleys. For a long, strenuous haul, break southwest at Iva Bell Hot Springs and hike 13 miles to Balloon Dome, a Yosemite-size chunk of granite towering above the San Joaquin River.

ELEVATION: The loop bottoms out around Fox Meadow at 6,500 feet and peaks at about 10,500 feet north of Purple Lake.

CAN’T MISS: The Middle Fork of the San Joaquin River tumbling 100 feet over Rainbow Falls.

CROWD CONTROL: Permits are required, and entry is limited to 25 persons per day for Fish Creek and Red Cones. During summer months, go midweek or reserve a mandatory permit in advance by calling the USDA Forest Service information line (760-924-5500).

MAPS AND GUIDES: Tom Harrison’s Mammoth High-Country map covers the entire region (800-265-9090; www.tomharrisonmaps.com; $7.95).

PIT STOP: Kittridge Sports (760-934-7566) in Mammoth stocks any item you left at home, and the friendly staff know their backcountry.

WALK SOFTLY: Iva Bell Hot Springs sees a lot of use, so camp at least 200 feet away and commute to the springs.

MORE INFORMATION: Mammoth Lakes Visitor Center is run by the Forest Service, P.O. Box 148, Mammoth Lakes, CA 93546; (760) 924-5500.