The area of eastern California that includes Long Valley Caldera and the Mono-Inyo Craters volcanic chain has a long history of geologic activity that includes both earthquakes and volcanic eruptions. This activity is likely to continue long into the future. Geologic processes proceed at a ponderous pace, however, and when measured in the time-scale of a human lifetime, volcanic eruptions or destructive earthquakes are infrequent events. How does this ongoing geologic activity affect those who live in or visit this area of spectacular eastern Sierra scenery?

The best guide to the future behavior of a volcano or volcanic system is its past behavior. Geological studies of Long Valley Caldera and the Mono-Inyo Craters volcanic chain indicate that:

• Future eruptions are more likely to occur somewhere along the Mono-Inyo Craters volcanic chain than from the resurgent dome or south moat area within the caldera.
• In the absence of unrest (earthquake swarms, ground deformation, gas emissions, and fumarole activity), the odds of an eruption occurring in any given year along the chain are one in a few hundred (comparable to the odds for a great [magnitude 8] earthquake along the San Andreas fault in coastal California).
• Unrest can temporarily increase the odds of an eruption, depending on the nature, intensity, and location of the unrest. Current, relatively low levels of unrest increase the odds of an eruption only slightly.
• Future eruptions are likely to be explosive in style but small to moderate in size (Click here for a note on eruption sizes.) Effusive (non-explosive), Hawaiian style eruptions are also possible but somewhat less likely.
• The odds that a small eruption somewhere along the chain will have a significant impact on any specified place along the chain are roughly one in a thousand in a given year.
• Larger eruptions are possible but less common (and thus less likely) than smaller ones (true for most volcanoes).
• Massive eruptions of the size that accompanied formation of Long Valley Caldera 760,000 years ago are extremely rare (none have occurred during the period of written human history). Scientists see no evidence that an eruption of such catastrophic proportions might be brewing beneath Long Valley caldera

Looking at the history of volcanic eruptions at any volcano helps scientists understand what might happen in the future at any volcano. Find out more about the prehistoric eruptions at Long Valley Caldera and how these eruptions affect the odds of a future eruption.

All but three of the 20 or so eruptions over the past 5,000 years at Long Valley Caldera have been explosive in nature. Those three were of the effusive, Hawaiian type (the Red Cones eruptions south of Mammoth Mountain about 5,000 year ago, the Negit Island eruption about 2,000 years ago, and the Paoha Island eruption just 250 years ago). All have been small to moderate in scale. So what can we expect if an eruption does occur at Long Valley Caldera?

Intense unrest can temporarily increase the odds that an eruption will follow. The U.S. Geological Survey's monitoring program is designed to recognize the sort of unrest that indicates a significant increase in the chances of an imminent eruption. Find out how unrest affects the chances of another volcanic eruption at Long Valley.