A volcano is a geological landform usually caused by the eruption through a vent（火山口） in a planet's surface of magma（巖漿），molten rock welling up from the planet's interior.（1）
G
G
A popular way of judging volcanoes goes by their frequency（頻率） of eruption.（2）
B
B
Those that have erupted in historical times but are now quiet called dormant（休眠的） and those that have not erupted in historical times called extinct.
（3）
F
F
The lifespan of a volcano can vary from months to several million years，making such a distinction（辨別） sometimes meaningless when compared to the lifespans of humans or even civilisations.For example，many of the 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.（4）
A
A
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.（5）
E
E
It is 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 Hawaii，little more than 200 years.The Smithsonian Global Volcanism programme's definition of "active" is having erupted within the last 10，000 years.
A.By our lifespans，however，they are not.
B.Those that erupt regularly called active.
C.One way of classifying volcanoes is by the type of material erupted.
D.Obviously，such eruptions tend to be destructive mainly in one direction.
E.Many scientists also consider a volcano active if it has erupted in historic time.
F.There is no real agreement among volcanologists on how to define an "active" volcano.
G.Volcanoes of various types are found on other planets and their moons as well as on the earth.
