Long Valley Caldera

A Paper For Geology 212

By Kim Patrick Noyes

There are multiple types of volcanoes in the Long Valley Caldera area but the most notable volcanic feature is the caldera itself. It is not well-understood why this feature exists where it does but what is known is that it is not formed by a hot spot below it nor is it in a subduction setting. Indeed, the area is known for its divergence, NOT convergence.

In any case, volcanism associated with what would become the Long Valley Caldera began between 3.1 and 2.5 million years ago. Over time the type of ejecta changed from first, rhyodacite, then rhyolite and later obsidian in the case of Glass Mountain. This volcano has the distinction of having produced BOTH silicic AND basaltic lavas from the same source.

About 760,000 years ago there was a catastrophic explosive eruption that emptied a vast magmatic chamber. Once the magma chamber became sufficiently emptied the mountain above it collapsed into it resulting in an even larger explosion that created the Bishop Tuff and formed a caldera where the mountain had been. Not only was there a large collapse of the rising column of ash but monstrous pyroclastic flows flowed up and over the Sierra Nevada Mountains and down into the San Joaquin River drainage to the west and down the Owens Valley to the south as well as in other directions.

Since that time smaller but nonetheless significant rhyolitic eruptions have occurred from various locations around the resurgent dome within the caldera about every 200,000 years or so with the last one occurring about 100,000 years ago. More frequent smaller eruptions also occur within the caldera (some Rhyolitic domes within the caldera are younger than 1,000 years old) that would be a problem for human beings were one to occur while human beings lived in the area.

Currently, the City of Mammoth Lakes, CA, (population 7-8,000 full-time residents with 1,000's more additional visitors daily year-round) is the only community located within the caldera. Nearby to the south is the town of Bishop, CA, (with about half the population of Mammoth Lakes but likewise regularly occupied by many tourists). There is really only one way in and out of the caldera and that is along US395 which is located adjacent to numerous geologically recent volcanic features. Were this road to be cut off those located in the caldera would be in a world of hurt in the event of an eruption of any significant size.

In 1980 a series of M6.0 quakes rattled the area and along with earthquake swarms and ominous ground swelling near the resurgent dome it was feared an eruption was eminent at that time. Scientists fresh on the heels of what happened at Mount Saint Helens some weeks earlier sounded the alarm. Fortunately, nothing else happened but the locals were fairly ugly to the scientists over lost tourist income and overall disruption of life. Since that time numerous other earthquake swarms have occurred but none notable over the past several years. However, there are locations in the caldera and on its flanks where hydrothermal springs and CO2 degassing from the ground have altered the landscape by creating dangerously hot streams that have had to be closed off to the public and areas of mass die-offs of trees due to oxygen-depletion in the soils from which they grew.

In 2006, six Mammoth Mountain Ski Patrol members died from CO2 poisoning when one fell into a depression and was overcome and his companions in turn died trying to rescue him when they entered the depression.

The Long Valley Caldera is an officially recognized volcanic hazard and is closely monitored by the Long Valley Observatory for any ominous changes and contingencies are in place in the advent of an event.

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