Volcano warning schemes of the United States
Prior to October 2006, three parallel Volcano warning schemes were used by the United States Geological Survey and the volcano observatories for different volcano ranges in the United States. They each have a base level for dormant-quiescent states and three grades of alert. Developed in 1997 to replace a previous 5-level system devised in 1991. The Alaska Volcano Observatory (AVO) used the following color-coded system to rate volcanic activity. It was originally established during the 1989-90 eruption of Redoubt Volcano. All five classifications are spelled as proper nouns, i.e., Level of Concern Color Code Orange not Level of concern color code Orange or any other variation. On its website the AVO spells the alert color in all capitals, but this is not otherwise necessary outside their system. Introduced following the May 18, 1980, eruption of Mount St. Helens.
Earlier volcano warning schemes for the United States
In October 2006, the United States Geological Survey (USGS) adopted a nationwide alert system for characterizing the level of unrest and eruptive activity at volcanoes.