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 head of time are extremely rare.
USGS Volcano- Warning Scheme
for the United States
The U.S. Geological
Survey (USGS) has adopted a common system nationwide for characterizing the
level of unrest and eruptive activity at volcanoes. The new volcano alert-level
system is now used by the Alaska Volcano Observatory, the Cascades Volcano
Observatory, the Hawaiian Volcano Observatory, the Long Valley Observatory, and
the Yellowstone Volcano Observatory.
Summary of Volcanic-Alert Levels
| Normal | Typical background activity of
a volcano in a non-eruptive state After a change from a higher level: Volcanic activity considered to have ceased, and volcano reverted to its normal, non-eruptive state. |
| Advisory | Elevated unrest about known
background activity After a change from a higher level: Volcanic activity has decreased significantly but continues to be closely monitored for possible renewed increase. |
| Watch | Heightened/escalating unrest with increased potential for eruptive activity (timeframe variable) OR a minor eruption underway that poses limited hazards. |
| Warning | Highly hazardous eruption underway or imminent. |
Aviation Color Codes
| Green | Volcano is in
normal, non-eruptive state. or, after a change from a higher level: Volcanic activity considered to have ceased, and volcano reverted to its normal, non-eruptive state. |
| Yellow | Volcano is exhibiting signs of
elevated unrest about known background levels. or, after a change from a higher level: Volcanic activity has decreased significantly but continues to be closely monitored for possible renewed increase. |
| Orange | Volcano is
exhibiting sings of elevated unrest about known background levels. or, Volcanic eruption underway with no or minor ash emission |
| Red | Eruption is
forecast to be imminent with significant emission of ash into the
atmosphere likely. or, Eruption is underway with significant emission of ash into the atmosphere |
