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Alert systems for volcanic activity
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. The system is now used by the Alaska Volcano Observatory, the California Volcano Observatory (California and Nevada), the Cascades Volcano Observatory (Washington, Oregon and Idaho), the Hawaiian Volcano Observatory and the Yellowstone Volcano Observatory (Montana, Wyoming, Colorado, Utah, New Mexico and Arizona). Under this system, the USGS ranks the level of activity at a U. S. volcano using the terms "normal", for typical volcanic activity in a non eruptive phase; "advisory", for elevated unrest; "watch", for escalating unrest or an eruption underway that poses limited hazards; and, "warning", if a highly hazardous eruption is underway or imminent. These levels reflect conditions at a volcano and the expected or ongoing hazardous volcanic phenomena. When an alert level is assigned by an observatory, accompanying text will give a fuller explanation of the observed phenomena and clarify hazard implications to affected groups.

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.

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. Class Description Green Volcano is in its normal "dormant" state. Yellow Volcano is restless. Seismic activity is elevated. Potential for eruptive activity is increased. Orange Small ash eruption expected or confirmed. Plume(s) not likely to rise above 25,000 feet above sea level. Red Large ash eruptions expected or confirmed. Plume likely to rise above 25,000 feet above sea level. 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. Code Narrative Levels No Alert"Information Statements" may be issued about unusual but non threatening events Alert Level OneNotice of Volcanic UnrestSignificant anomalous conditions are recognized that could be indicative of an eventual hazardous volcanic event. Alert Level TwoVolcano AdvisoryMonitoring and evaluation indicate that processes are underway that have significant likelihood of culminating in hazardous volcanic activity but when the evidence does not indicate that a life or property threatening event is imminent. Alert Level ThreeVolcano AlertMonitoring and evaluation indicate that precursory events have escalated to the point where a volcanic event with attendant volcanologic or hydrologic hazards threatening to life and property appears imminent or is underway.