<P> A popular way of classifying magmatic volcanoes is by their frequency of eruption, with those that erupt regularly called active, those that have erupted in historical times but are now quiet called dormant or inactive, and those that have not erupted in historical times called extinct . However, these popular classifications--extinct in particular--are practically meaningless to scientists . They use classifications which refer to a particular volcano's formative and eruptive processes and resulting shapes, which was explained above . </P> <P> 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 eruption . Given the long lifespan of such volcanoes, they are very active . By human lifespans, however, they are not . </P> <P> Scientists usually consider a volcano to be erupting or likely to erupt if it is currently erupting, or showing signs of unrest such as unusual earthquake activity or significant new gas emissions . Most scientists consider a volcano active if it has erupted in the last 10,000 years (Holocene times)--the Smithsonian Global Volcanism Program uses this definition of active . Most volcanoes are situated on the Pacific Ring of Fire . An estimated 500 million people live near active volcanoes . </P> <P> Historical time (or recorded history) is another timeframe for active . The Catalogue of the Active Volcanoes of the World, published by the International Association of Volcanology, uses this definition, by which there are more than 500 active volcanoes . However, the span of recorded history differs from region to region . In China and the Mediterranean, it reaches back nearly 3,000 years, but in the Pacific Northwest of the United States and Canada, it reaches back less than 300 years, and in Hawaii and New Zealand, only around 200 years . </P>

Where are most volcanoes above sea level located