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The best warning of a volcanic eruption is one that specifies when and where an eruption is most likely to occur and what type and size eruption should be expected. Such accurate predictions are sometimes possible but still rare in volcanology. The most accurate warnings are those in which scientists indicate an eruption is probably only hours to days away based on significant changes in a volcano's earthquake activity, ground deformation, and gas emissions. Experience from around the world has shown that most eruptions are preceded by such changes over a period of days to weeks. A volcano may begin to show signs of unrest several months to a few years before an eruption. In these cases, however, a warning that specifies when it might erupt months to years ahead of time are extremely rare. |
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The strategy that we use to provide volcano warnings in the United States involves a series of alert levels that correspond generally to increasing levels of volcanic activity. As a volcano becomes increasingly active or as our monitoring data suggest that a given level of unrest is likely to lead to a significant eruption, we declare a corresponding higher alert level. This alert level ranking thus offers the public and civil authorities a framework they can use to gauge and coordinate their response to a developing volcano emergency.
We currently use different alert levels (also referred to as status levels, condition levels, or color code) for providing volcano warnings and emergency information regarding volcanic unrest and eruptions. These levels are different for Long Valley caldera in California and for volcanoes in Alaska, the Cascade Range in the Pacific Northwest, and Hawai`i for several reasons:
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