 Photograph by E. Klett on
27 January 1994;
U.S. Fish and Wildlife Service Snow-covered Kanaga Volcano in Alaska erupts a small column of tephra, gas, and steam. Kanaga is a stratovolcano. View is toward the west. |
Volcano A volcano is a vent at the Earth's surface through which magma (molten rock) and associated gases erupt, and also the cone built by effusive and explosive eruptions. |
Surprisingly, there is no consensus among volcanologists on how to define an active volcano. The lifespan of a volcano can vary from months to several million years, making such a distinction sometimes meaningless when compared to the lifespans of humans or even civilizations. For example, many of Earth's volcanoes have erupted dozens of times in the past few thousand years but are not currently showing signs of activity. Given the long lifespan of such volcanoes, they are very active. By our lifespans, however, they are not. Complicating the definition are volcanoes that become restless but do not actually erupt. Are these volcanoes active?
Active volcano
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 volcano
Dormant volcanoes are those that are not currently active (as defined
above), but could become restless or erupt again.
Extinct volcano
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!
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