posted on May, 26 2006 @ 08:25 AM
Definitions of Volcano activity Categories from the USGS:
Scientists usually consider a volcano active if it is currently erupting or showing signs of unrest, such as unusual earthquake activity or
significant new gas emissions. Many scientists also consider a volcano active if it has erupted in historic time. It's important to note that the
span of recorded history differs from region to region; in the Mediterranean, recorded history reaches back more than 3,000 years but in the Pacific
Northwest of the United States, it reaches back less than 300 years, and in Hawai`i, little more than 200 years.
Dormant volcanoes are those that are not currently active (as defined above), but could become restless or erupt again.
Extinct volcanoes are those that scientists consider unlikely to erupt again. Whether a volcano is truly extinct is often difficult to determine. For
example, since calderas have lifespans sometimes measured in millions of years, a caldera that hasn't produced an eruption in tens of thousands of
years is likely to be considered dormant instead of extinct. Yellowstone caldera in Yellowstone National Park is at least 2 million years old and
hasn't erupted for 70,000 years, yet scientists do not consider Yellowstone as extinct. In fact, because the caldera has frequent earthquakes, a very
active geothermal system, and rapid rates of ground uplift, many scientists consider it to be a very active volcano!