The USGS continues to monitor volcanoes around the country; four of them are now on a heightened ORANGE / WATCH level owing to activity. In Alaska, Kilauea on Hawaii's Big Island and Great Sitkin, Pavlof, and Semisopochnoi are all displaying symptoms of unrest or activity.

Hawaii's Kilauea Volcano Erupts Forcing Evacuations
(Photo : Photo by Mario Tama/Getty Images)

Except for Hawaii's Mauna Loa, which remains at a YELLOW / ADVISORY status, all other volcanoes monitored by the USGS today are GREEN or unrated.

Monitoring Hundreds of Volcanoes

The USGS monitors 169 potentially active volcanoes in the United States, most of which are in Alaska. However, Alaska has a lot of volcanoes; there have been over 130 active volcanoes and volcanic fields in the last 2 million years. Since the mid-1700s, 50 have been active, and AVO investigates them as well. Hawaii is another area known for its volcanoes; on the Big Island of Hawaii, Kilauea, Mauna Loa, and Hualalai are all considered active and potential dangers, although only Kilauea is now erupting.

The United States is home to just a small percentage of the world's volcanoes: the USGS estimates that there are around two dozen active volcanoes around the globe at any one moment. According to the USGS, there are roughly 1,500 potentially active volcanoes globally, with about 500 of them erupting in recent history.

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Volcanoes All Over the US

Active volcano
(Photo : Getty Images)

The Hawaii Volcano Observatory (HVO) monitors Hawaii volcanoes, while the Alaska Volcano Observatory (AVO) monitors Alaska volcanoes. Other observatories include the California Volcano Observatory, Cascades Volcano Observatory, Yellowstone Volcano Observatory, and Northern Mariana Islands Volcano Observatory. Each of the USGS's different volcanic observatories keeps track of volcanoes in their particular areas. None of the other observatories are reporting any unusual activity or evidence of anything other than background noise at this time.

Aviation Codes

Alert-Level Terms
(Photo : From U.S. Geological Survey (USGS))

The USGS and volcanic observatory units are issuing Aviation Codes and Volcanic Activity Alert Levels in the United States. Green, yellow, orange, or red is the many types of aviation codes. Ground-based monitoring fails to show that a volcano is operating at an average background level of activity. It is labeled "unassigned."

Yellow indicates that a volcano is displaying symptoms of high unrest above known background levels, whereas green indicates usual activity associated with a non-eruptive condition. The color of a volcano changes from red to orange when it experiences significant or growing instability with an increased risk of eruption. Finally, the code becomes red when an eruption is imminent, with sizeable volcanic ash emissions predicted in the atmosphere, or when an eruption is underway, with major volcanic ash emissions expected in the atmosphere. The alert levels for volcanic activity include normal, advisory, watch, or warning.

When data is lacking, the volcano is designated as "unassigned," as with aviation codes. When the volcano is at regular background activity in a non-eruptive condition, it is deemed normal. An advisory is issued if the volcano shows evidence of heightened disturbance above the background level. A watch is issued when a volcano shows signs of increased or growing disturbance, whereas a warning is issued when a dangerous eruption is imminent.

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