Re: When does a dormant volcano become an extinct volcano?

I would argue that the term "dormant" is meaningless as a geological 
term:  a volcano is either extinct or it is active.  The life cycles of 
volcanoes are measured in hundreds of thousands to millions of years.  
Many active volcanoes tend to erupt infrequently by human standards, which 
can give rise to a mistaken judgement that they are extinct.  Such 
volcanoes would be classed as dormant, which is merely to say active, but 
with a periodicity of eruption that is long by human standards.

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.

A few examples will illustrate the rule that a volcano still part of an 
active volcanic province can never unequivocally be considered extinct:

* The Yellowstone Volcano.  Yellowstone National Park is on the site of a 
huge volcano of a type known as a collapsed caldera.  It has little 
obvious expression at the Earth's surface - no obvious volcanic mountain - 
but instead forms an eroded crater about 75 km across at its widest.  It 
lies above a hotspot, where a rising plume of hot material from deep 
within the Earth fuels a huge magma chamber in the crust.  This magma 
chamber fills very slowly, however, and the last known eruption is dated 
at about 600,000 years ago, since when the volcano has been inactive 
(except for minor surface activity such as the famous Yellowstone 
geysers).  Therefore no human being has ever witnessed an eruption.  In 
all the Yellowstone Volcano is known to have erupted three times, with the 
eruptions spaced at roughly 600,000 year intervals.  So, is the 
Yellowstone Volcano extinct?  The answer is that no one knows, although it 
is safest to assume that it is active because it still lies above the 
hotspot that gave rise to it, and it is obviously still an area with high 
heat flow.  Furthermore it is showing signs that it may be building to 
another eruption!  No one can tell, however, when the next eruption will 
occur:  it may not be for thousands of years, or it might occur within our 
lifetimes.
* The Hawaiian Islands.  The Hawaiian Islands are a chain of volcanoes, 
again built by a hotspot.  The Hawaiian hotspot has been active for at 
least 80,000,000 years.  In this case the movement of the Pacific Plate 
carries the crust of the Pacific Ocean over the hotspot, allowing new 
volcanoes to form and then taking them away from the hotspot, thus making 
them extinct.  The lifetime of these volcanoes is determined by the length 
of time that they are in contact with rising magma originating from the 
hotspot.  Today the Big Island of Hawaii is on the hotspot, and it has two 
very active volcanoes, Kilauea and Mauna Loa, and one "dormant" volcano, 
Mauna Kea, which last erupted about 3,500 years ago.  The next island to 
the northwest, Maui, has two volcanoes, West Maui and East Maui:  West 
Maui is considered extinct, but East Maui last erupted in about 1790 and 
must be considered active.  East Maui is on the road to extinction, as it 
is being carried away from the hotspot, though it may take many thousands 
of years before it finally goes extinct.  All the other islands in the 
Hawaiian chain are extinct.
* Mount Vesuvius.  Vesuvius has erupted more than 50 times since the 
famous eruption of 79 A.D.  It is clearly not extinct, and will not be 
perhaps for hundreds of thousands of years.  The archaeological 
excavations at Pompeii and Herculaneum since the 18th century seem to have 
fostered a view in many people that the devastating eruption of 79 A.D. is 
not likely to be repeated (perhaps on the basis that it was a long time 
ago).  This is a false perspective.  Vesuvius is one of the most dangerous 
volcanoes in the world because of its proximity to major centres of 
population, and will remain dangerous essentially forever, on the 
timescale of human history.  For a fascinating insight into the politics 
surrounding Vesuvius have a look at http://tribeca.ios.com/~dobran
* The Cascades.  The Cascade Mountains are a volcanic chain formed at a 
destructive plate boundary.  Subduction has been in progress here for 
about 12,000,000 years and has yielded many volcanoes, some of which are 
obviously active and others of which have no recorded eruption history.  
Some of these long dormant volcanoes may be extinct.  Mountains which were 
eroded by glacial action during the Late Pleistocene ice age and which 
have not, so to speak, repaired the damage have not erupted seriously in 
at least 10,000 years (Three Fingered Jack in Oregon is a good example), 
and may be extinct.  But they still lie within an active volcanic 
province, so one can never be sure!  Mt. Ranier has experienced numerous 
eruptive cycles spread over the last 730,000 years, the cycles often 
separated by thousands of years of dormancy.  The last major cycle was a 
mere 2500 years ago, and minor activity was reported at various times as 
recently as the 19th century.  Mt. Ranier is an active volcano.

I could go on and on citing examples.  The main point is that no volcano 
in an active volcanic province can ever be written off as certainly 
extinct, even if it has never been observed to erupt.

Sources:

The sources on volcanoes would fill a library.  Below are just a few, 
including a superb website:

* Volcano World, http://volcano.und.nodak.edu/vw.html
* Peter Francis, Volcanoes, A Planetary Perspective (Oxford; Oxford 
University Press, 1993).
* Robert Decker and Barbara Decker, Volcanoes (New York; W.H. Freeman and 
Company, 1997).
* David Alt and Donald W. Hyndman, Northwest Exposures, A Geologic Story 
of the Northwest (Missoula, Montana; Mountain Press Publishing Company, 
1995).
* Stephen L. Harris, Fire Mountains of the West, The Cascade and Mono Lake 
Volcanoes (Missoula, Montana; Mountain Press Publishing Company, 1988).