IS Q&A

Volcanoes form when hot material from below rises and leaks into the crust. This hot material, called magma, comes either from a melt of subducted crustal material, and which is light and buoyant after melting, or it may come from deeper in the interior of a planet and is light and buoyant because it is *very* hot.
Magma, rising from lower reaches, gathers in a reservoir, in a weak portion of the overlying rock called the magma chamber. Eventually, but not always, the magma erupts onto the surface. Strong earthquakes accompany rising magma, and the volcanic cone may swell in appearance, Scientist often monitor the changing shape of a volcano, especially prior to an eruption. The different reasons why a volcano forms are

via plumes or hot spots in the lithosphere
as a result of subduction of the nearby lithosphere

A volcano will remain potentially active as long as it remains within the
active geological environment that gave rise to it. There are three main
types of geological processes that produce volcanism: hotspots, in which
a plume of hot material from deep within the Earth rises to the surface;
constructive plate boundaries at which new ocean crust is created at mid-
ocean ridges; and destructive plate boundaries at which ocean crust is
destroyed by subduction back into the mantle. All three of these
phenomena tend to be long-lived geological features, and so the volcanism
they produce can be long-lived, producing many generations of volcanoes.
Any volcano located within such an active setting is best considered to be
active, even if it has never been observed to erupt within recorded
history!

Having said that, there are many volcanoes that obviously are extinct.
These are often very ancient volcanoes, which are no longer located in an
active volcanic province. A good example is the ancient volcano in which
the city of Edinburgh, in Scotland, is located. The Edinburgh volcano is
the eroded remnant of a volcano that was active during the Carboniferous
Period, about 250,000,000 years ago. It was once located in an active
setting, but the geological action has, so to speak, long since moved
elsewhere. There is no possibility that this volcano is still active.
Less clear-cut is the case of volcanoes that have not erupted for perhaps
many thousands of years, but that are still located in an active volcanic
province such as a volcanic chain which contains active volcanoes. Such
"dormant" volcanoes may be extinct, but it is difficult to state
categorically that they are.