Super Volcanoes in the United States: Long Valley Caldera

In May of 1980, a series of earthquakes struck the Long Valley Caldera near Mammoth Mountain in Eastern California. Four of the largest earthquakes were greater then magnitude 6 on the Richter scale. The earthquakes served as a reminder that the Long Valley Caldera is still a geologically active area, where a super volcano could erupt at any time.

Shortly after the earthquakes, the United States Geologic Survey set up a full time observation program in 1982. With the observatory at the caldera, advance warning can be provided if a large volcanic eruption were to occur in eastern California.

The Long Valley Caldera ranks right up there with some of the largest calderas on earth, measuring about 20 miles long and 11 miles wide. It is not quite as large as its big brother, the Yellowstone Caldera. The Long Valley Caldera is about half the size of the Yellowstone Caldera.

Since the 1980s earthquake swarm, geologic activity of the Long Valley Caldera his tapered off. However, weekly reports from the US Geologic Survey indicate that several small earthquakes are still occurring each week.

The Long Valley Caldera was created by an huge volcanic eruption that occurred about 760,000 years ago. The blowout was so massive that the huge magma chamber collapse beneath the site, leading to an elongated depression in the ground that forms the caldera. Subsequent eruptions have occurred in about 200,000 year intervals. The last eruption was estimated to be about 100,000 years ago.

The recent earthquake swarms of the 1980s lead to the creation of a "resurgent dome" near the center of the caldera. This resurgent dome may be an indication of magma expanding beneath the caldera, a precursor for an upcoming eruption. During the 1980s it was noted that the dome was expanding at a rate of several inches per year.

In the 1990s scientists discovered large amounts of carbon dioxide seeping from the ground from magma beneath Mammoth Mountain. This led to a large number of tree dying on the south face of the mountain. It is believed that these are further signs that volcanic unrest is increasing at Long Valley Caldera. Symptoms of volcanic unrest could last for decades for at large calderas like Yellowstone and Long Valley so longer term monitoring will be needed.

Although not quite as dramatic as the features in Yellowstone National Park, there some interesting geologic phenomena to see in and around the Long Valley Caldera. There are numerous hot springs, steam vents, fumaroles, and mineral deposits. There is a composite volcano, Mammoth Mountain, which is a popular ski area. Nearby Mono Lake is also a popular attraction.