I live within the Long Valley Caldera!

Mono County can’t boast the tallest mountain (Whitney) or the lowest elevation (Badwater) as does its sister county Inyo to the south. But thanks to a restless caldera and a labyrinthine fault system, it does have some of the most fascinating geology in North America.

“It’s got the volcanos and the volcanic history and all the aspects of that. And then it’s got the mountains, which are basically the result of faulting,” said Terry Wright, an emeritus professor of geology at Sonoma State University who lives on a 10-acre spread in Benton for half of the year.

“Then, within the mountains, the rocks have an extended history that go back 500 million years which are a record of mountain building. There are rocks that first had to be formed to be built into the mountains.”

The main rocks of the Sierra Nevada stretch all the way in bits and pieces across to the White Mountains, which are the same geology as the Sierra Nevada, except the Sierra is more highly “scrunched,” to use Wright’s word, meaning metamorphosed (subject to heat and pressure) and deformed (folds and faults).

As if volcanic and faulting influences were not enough to create a sufficiently mixed bag, the Eastern Sierra from Bishop to Bridgeport is also marked by glacial moraine - visual evidence of streams of ice dating back tens of thousands of years.

Two terrific examples exist near Convict Lake (where, in addition, marine fossils have been found thousands of feet above today’s sea level) and Conway Summit near Bridgeport, where one of the oldest moraines sits on top of granite.

To put geologic time in perspective, Wright says that the span of one million years of geologic time is the equivalent to one week of our time.

The geologic drama begins at the Mono County Line just south of Tom’s Place on Highway 395. There, one drives up along the Bishop Tuff (which sits on top of the Sherwin Till, one of the area’s oldest glacial deposits). The Bishop Tuff is a pink slope formed 760,000 years ago by glowing hot magma that blew from within 4 miles beneath the earth in a cataclysmic volcanic eruption.

What’s left today is the Long Valley Caldera, a 10-mile-wide by 20-mile-long area which can be seen nearly in its entirely at the Crowley Lake Overlook further north. Driving a bit further north, Mammoth Mountain comes into view. It is a resurgent dome which was formed when magma leaked out after the main explosion and marks the western periphery of the caldera. To the east is Glass Mountain, made of obsidian.

View of Crowley Lake in the distance, from within the caldera near where I live in Long Valley!!!
But faults are what downdropped the whole area, Wright emphasizes. “Where the magma blew out, there was nothing left to hold the surface up, so it collapsed along faults.” Airborne ash from the eruption, which was more than 2,000 times that of Mt. St. Helens in 1980, blew as far away as present-day Nebraska.

Scientists have been monitoring geologic unrest in the Long Valley Caldera since a cluster of four earthquakes measuring 6.0 rocked Mammoth Lakes in 1980. At that time, they discovered that the core of the caldera was rising.

“Earthquakes are of two types,” Wright explains. “One is that they are related to faults, which is plate movement, and the front of the Sierra Nevada is sculptured along a series of faults.”

A spectacular example of Eastern Sierra faulting can be seen near McGee Creek, which was the epicenter of the 1980 quakes, near Crowley Lake.

McGee Canyon in the fall, pictured above

“The other thing that causes earthquakes is magma moving,” Wright continued. “That’s been the perennial question in Mammoth: Are we listening to moving magma or are we listening to faults moving?”

A visible geologic remnant from the 1980 quakes can reached along Highway 395 just before the geothermal plant and turn-off into Mammoth Lakes.

“Look over to the right beyond the hatchery and there’s a ridge. Down the front of this ridge as you look carefully, there’s a path, at the bottom of the path there’s a big boulder. This came down - was shaken awake - by the earthquakes in 1980,” Wright explains.

“Basically, it was a perched boulder. When it shakes, these guys are ready to go. This boulder is something that everybody sees if they look hard enough for it.”

The formerly perched boulder is in the area of numerous local hot springs (Whitmore, Hot Creek, and Little Hot Creek) - underground water heated by magma which still underlies the caldera - as well as natural steam vents, which drive the three geothermal powerplants nearby that produce about 40 megawatts of electricity in total.

“The water actually descends along some faults, and ascends along other faults,” Wright clarified. “What happens is water will percolate down, heat up, and come up through other faults. That whole system there is on a series of faults that continues to the north as the Mono Craters. They are basically eruptions of magma that have come up through faults.”

Thousands of feet up to the west, several places at Mammoth Mountain were discovered in the 1990’s to be emitting CO2 gas, including Horseshoe Lake, the site of a large tree kill. This is the southwest edge of the Long Valley Caldera. The gas comes from liquified rock, or magma, most of which is carbon dioxide and fatally poisonous if breathed in high concentrations. Tragically, in 2006, three members of a ski patrol died on the slopes of the dormant volcano while trying to rope off a geothermal vent (called a fumerole) there.

All of these circumstances indicate the presence of a restless caldera and increase the chances of an eruption, but at the same time, this type of unrest could endure for decades or even centuries.

According to a U.S. Geological Survey, an eruption is more likely to occur along the Mono-Inyo Craters chain (north of the Long Valley Caldera) that extends all the way to Mono Lake in the Mono Basin. The Mono Domes can be seen along Highway 395 north of the June Lake turnoff.

These domes, however, are not very explosive because most of the gas in the system erupted with the caldera long ago, making them gas poor. And they are geologically young - within 10,000 years, barely a second’s tick on the geological time scale of 4.6 billion years.

The Panum Dome off Highway 395 provides an especially good example of a combined rhyolite dome and cinder cone. A hike allows visitors straight into the crater where lava ooze can be seen.

Below, view from Panum Crater towards the Eastern Sierra.

The most recent eruption in the Mono-Inyo system occurred at the northern end at Paoha Island about 350 years ago.

Mono Lake, which comes into view near the Highway 120 turnoff into Yosemite, is a terminal lake in the Mono Basin, a watershed area fed by melting runoff from the Sierra Nevada. It has no outlet to the sea and remains geologically active because of faulting at the base of the Sierra Nevada as well as crustal stretching of the Basin and Range Province to the east.

Some scientists have said Mono Lake dates to the caldera eruption 760,000 years ago, but sedimentary evidence indicates that it could be the remnant of a larger, more ancient lake that once covered parts of Nevada and Utah, making it one of the oldest lakes in North America.

Wright agrees with this view. “The lake was being formed and was accumulating sediment as the Bishop Tuff took place. So that eruption doesn’t have to do with the formation of the lake, but it did erupt into the lake.”

Those interested in knowing more about the Eastern Sierra in its greater geologic context should check out two books by John McPhee, a Pulitzer-Prize winning geologist: “Basin and Range,” which explores the terrain from Utah to eastern California, and “Assembling California,” which explores the state’s faultlines from the Sierra Nevada through the central Valley to the coastal ranges and San Francisco. An easy-to read guide that includes eight stops in Mono County is “Geology Underfoot in Death Valley and Owens Valley” by Robert P. Sharp and Allen F. Glazner.

Thanks to Terry Wright, Emeritus Professor of Geology at Sonoma State University, pictured below, for his input on this article.

I wrote this piece for the upcoming special issue of "Portraits in Granite" for the Mammoth Times. I am not sure it will benefit or suffer from the editing process, so I've decided to post my original.