Our knowledge of the frequency of volcanic eruptions in any given area depends on careful studies of the ages and distribution of volcanic products from earlier eruptions (see geologic map). We can use the frequency of previous eruptions to estimate the long-term odds that another eruption might occur during some future time interval (sometime during the next year or the next decade, for example). Of course increased unrest can temporarily increase the odds of an eruption above these long-term estimates.

In the case of Long Valley Caldera and the Mono-Inyo Craters volcanic cahin, geologic studies indicate that:

• During the past 35,000 years, volcanic activity in the region has been confined to the Mono-Inyo Craters volcanic chain. Some 20 small to moderate eruptions have occurred somewhere along the chain in the last 5,000 years (diagram). Some of these eruption were closely clustered in time and space (for example, first the Mono Craters eruptions and then the Inyo Craters eruptions about 600 years ago).

• The intervals between eruptions or eruption clusters range from 250 years to 750 years. Assuming that this pattern has not suddenly stopped for our benefit, the long-term odds of another eruption in any given year are roughly one in a few hundred. These are comparable with odds for a great (magnitude 8) earthquake occurring on the San Andreas fault in coastal California. They are smaller than the odds for a major (magnitude 7 or greater) earthquake in the Los Angeles basin or the San Francisco Bay Area.

View of Mono Craters looking east from Highway 395. Crater Mountain is on the left and South Coulee is on the right.

Mammoth Mountain was built up by a series of some 20 separate eruptive episodes between 220,000 and 50,000 years ago. These eruptions were fed by a separate magma body located somewhere beneath Mammoth Mountain. The average interval between eruption episodes was about 8,000 years, and the most recent magmatic eruptive episode was 50,000 years ago (steam blast, or phreatic eruptions formed several craters on the north flank of Mammoth Mountain 550 to 600 years ago during the Inyo Domes eruption sequence). The long-term odds of another magmatic eruption from the original Mammoth Mountain magma chamber are less than one in 10,000 in any given year.

View of Mammoth Mountain looking north from the shore of Horseshoe Lake in the Mammoth Lakes Basin.

During the same time interval that Mammoth Mountain was active, a series of effusive (Hawaiian type) eruptions from vents in the west moat of the caldera fed fluid, basaltic lava flows that covered the west moat and flowed around the resurgent dome into the north and south moat areas of the caldera. We have yet to obtain reliable information on the frequency of these effusive eruptions, but it seems reasonable that the odds of another of these effusive eruptions are comparable to those for an eruption on Mammoth Mountain (less than one in 10,000 per year).

The magma chamber under the central part of the caldera (the resurgent dome) has produced just three eruption sequences over the past 600,000 years. These moderately large, explosive eruptions occurred some 500,000, 300,000, and 100,000 years ago (the most recent of these eruptions produced the Mammoth Knolls volcanic domes located just north of Mammoth Lakes). The interval between these eruptions is roughly 200,000 years, and thus the long-term odds of another eruption from the central magma chamber from the Long Valley magma chamber are roughly one in 200,000 in any given year.