Sherri Jackson, Keith Julian, Debra Moore, Rhonda Spidell
Introduction and Background:
Located on the boundary between the Sierra Nevada and Basin and Range Province, the Long Valley Caldera is one of the three most active and dangerous calderas in North America. A catastrophic eruption 760,000 years ago rearranged the landscape of the region by spewing more than an estimated 500 cubic kilometers of ash and lava fragments, some of which was blown as far east as Nebraska (see map inset below).
The caldera currently extends ten miles north south and twenty miles east west with an average depth from rim to floor of 3,500 feet. The caldera is still potentially dangerous as the result of ongoing seismic activity and inflation caused by the underlying magma chamber, the top of which is thought to be located five miles below the surface (see cross-section below).
Source: U.S. Geological Survey
As we drove up to the Conway Grade Overlook, we observed a vast expanse of mountains, valleys, lakes, and snow covered peaks. To the west we saw what appears to be a small mountain range against a background of the higher snow covered mountains. In the expanse before the mountains there was a very large lake.
After crossing Deadman's Summit, located in the “small mountain range”, we saw a large rounded hill that was surrounded by a large green valley with rivers flowing through it. There were some sparkly mountains in the background. Behind the dome were some snow-covered mountains. To the east we saw more mountains lower in elevation than the western mountains. This valley appeared to extend for many miles.
Other features in the valley included a crater with green water in the bottom, and right next door, another crater with turquoise-green water in the bottom. We observed scattered large buff-red rock formations.
Our travel took us past a geothermal plant. We visited Hot Creek where we observed several bubbling hot water springs. We also visited an area in which there were a large number of trees killed even though they were growing close to a lake and were surrounded by green and growing trees.
Based on observations made on July 13–14, 2005, we were able to see physical evidence of the Long Valley Caldera. Our first view of the caldera occurred at the Conway Grade Overlook of Mono Basin, which revealed the northern rim of the caldera (see picture below).
More interpretative evidence occurred as we drove over the caldera rim at Deadman's Summit on Highway 395 and descended onto the caldera floor, approximately 1000 feet below. At this point we were able to see Bishop Tuff, the rhyolitic ash flow from the main eruption 760,000 years ago. From the floor of the caldera near the Casa Diablo geothermal power plant we interpreted the surrounding mountains as the boundaries of the caldera. To the west was 11,000 ft Mammoth Mountain a volcanic remnant, in the opposite direction was the eastern rim of the caldera.
To the north and west of Hot Creek geothermal overlook we were able to see evidence of uplift with in the caldera, which we interpreted as being the resurgent dome formed over an interval of 100,000 years between 580,000 and 680,000 years ago. The Inyo craters north of Mammoth Mountain and the Horseshoe Lake Tree Kill area south of Mammoth Mountain were interpreted as artifacts of recent caldera activity.
View is from near the Glass Creek flow (lower left) toward the northeast caldera rim and Glass Mountain.
View is from above the resurgent dome (lower left) toward the northeast caldera rim and Glass Mountain.
Calderas are not common on Earth or other planets (Mars or Venus). Calderas produce extreme conditions that persist over time. Hydrothermal systems with a wide range of physical conditions associated with calderas provide environments conducive to life and evolution.
For more information, please visit these U.S. Geological Survey Web sites:
Geologic History of Long Valley Caldera and the Mono-Inyo Craters volcanic chain, California
Long Valley Observatory
Living With a Restless Caldera - Long Valley, California
Future Eruptions in California's Long Valley Area - What's Likely?