Nearly all volcanic eruptions are preceded by precursory activity (unrest). This unrest typically includes a combination of earthquake swarms, ground deformation, emissions of SO2, CO2 and other volcanic gasses, as well as changes in fumaroles and hydrothermal springs. As magma works its way toward the surface from depth, it must make room for itself by shouldering aside hard, relatively cool rocks that form the Earth's crust. The resulting stresses produce earthquakes and distort the Earth's surface. Gasses dissolved in the magma begin to be released a the magma moves to shallower depths and lower pressures, and the hot magma may begin heating the shallow ground water and generate new hot springs or fumaroles. In general, unrest intensifies as magma moves progressively closer to the Earth's surface.
Not all episodes of unrest culminate in volcanic eruptions, however. In many cases, magma begins moving at depth but never makes it to the surface. The result is a magmatic intrusion accompanied by an episode of magmatic unrest that gradually dies away without an eruption. This is quite common in the case of large, restless calderas and distributed volcanic complexes such as the Mono-Inyo Craters volcanic chain. Of the 60 episodes of magmatic unrest in large calderas worldwide documented by Newhall and Dzurisin (see reading list), for example, only 10 (one in six) have culminated in volcanic eruptions.
As the unrest in Long Valley caldera that began in 1980 has waxed and waned over the past 17 years, so have the short-term odds for an eruption. In 1991, the U.S. Geological Survey in cooperation with the California Division of Mines and Geology and the California Office of Emergency Services established a system of color-coded conditions for the Long Valley Caldera region to provide a graded measure of concern that a given level of unrest might lead to a volcanic eruption. This status system is based on both a global survey of well-documented episodes of pre-eruption unrest and our experience with unrest in Long Valley caldera through the 1980's. (Find out more about the color-coded conditions.)
At no time over the past 17 years has the unrest reached a level that would have triggered a CONDITION ORANGE (WARNING) (the odds of an eruption within the next days to weeks exceed 50%). Had this system been in place in the early 1980's, the unrest would have twice triggered a CONDITION YELLOW (WATCH) -- once during the May 1980 earthquake swarm, which included four M=6 earthquakes, and once during the January 1983 swarm, which included two M=5.3 earthquakes (at this level of activity, the odds of an eruption may exceed one in ten over several months). Since mid-1983, activity has fluctuated from typical background activity to moderate unrest under CONDITION GREEN (no immediate threat). The odds of an eruption occurring during moderate unrest under CONDITION GREEN are only slightly elevated over the long-term odds.
