What is a volcano? According to Dictionary.com, the word means “burning mountain” and comes from the Roman God Vulcan. It also says that a volcano is a vent in the Earth’s crust that ejects ash, lava, and gases. However, there doesn’t seem to be a clear definition of an active, or potentially active, volcano.
Around 1500 active volcanoes exist on Earth today. Of those, approximately 800 have erupted in human history and the other 700 have erupted within the last 10,000 years. The Pacific’s “Ring of Fire” has two-thirds of the world’s active volcanoes.
So what is an active or potentially active volcano? There’s no quick and easy answer. Short descriptions of some volcanoes and their eruptions might help in understanding this. The “active” or “potentially active” labels for any particular volcano should be placed after sufficient study of its history, current seismic activity, the nature of its gas emissions, and ground temperatures near and away from vent areas. Many volcanoes totally lack seismicity, heat anomalies, or gas emissions and can be considered extinct.
A great example of an active volcano is Mt. Kilauea in Hawaii (see http://www1.newark.ohio-state.edu/Professional/OSU/Faculty/jstjohn/Volcano%20stuff/Kilauea.htm for more information). It is erupting right now, and has been continuously since 1983. Besides actual eruptions, indications of an active volcano would be frequent, small to large earthquakes, gases being released into the air, and temperature changes in the ground. Active volcanoes will usually show at least mild signs of these.
Some volcanoes can be considered “potentially active”. This is a bit harder to define. A potentially active volcano may not have any obvious, physical signs that an eruption is near. The volcano may not erupt for hundreds or thousands of years. Geologists can determine the general histories of volcanoes to determine whether a particular inactive, or dormant, volcano will likely erupt again in the future. Good examples of potentially active volcanoes are Mt. Spurr in Alaska (see http://www1.newark.ohio-state.edu/Professional/OSU/Faculty/jstjohn/Volcano%20stuff/Spurr.htm) and Mt. Vesuvius in Italy (see http://www1.newark.ohio-state.edu/Professional/OSU/Faculty/jstjohn/Volcano%20stuff/Vesuvius.htm). Neither is currently erupting, but certainly will in the future. Many other volcanoes can be classified as such, including Yellowstone. Although Yellowstone’s current activity is subject to varying interpretations, studies of its history tell us that it will definitely erupt again in the future.
An extinct volcano has no possibility of erupting again. An example would be Mount Kenya in eastern Africa (see http://www1.newark.ohio-state.edu/Professional/OSU/Faculty/jstjohn/Volcano%20stuff/Mt.-Kenya.htm). Extinct volcanoes are distinctively eroded, sometimes down to the size of small hills. They have not erupted for thousands of years or more. Longer-scale eruption patterns can occur, but the history of any particular volcano can tell geologists whether it could be potentially active.
Volcanoes are quite interesting and deserve to be studied in much detail. People often watch the news and hear about eruptions that totally shock them. The shock could be eased by widespread awareness of the nature of volcanoes and the common warning signs preceding eruptions. Many currently active volcanoes have been indicating to us for many years that an eruption is coming.
For more information about some specific volcanoes, feel free to examine the site put together by geologist James St. John. Volcanoes are listed in the “Cool Rocks” section.