This article
is about the geological feature. For other uses, see Volcano (disambiguation).
Cleveland Volcano in the Aleutian
Islands of Alaska
photographed from the International Space Station, May 2006.
Ash
plumes reached a height of 19 km during the climactic eruption at Mount
Pinatubo, Philippines in 1991.
A volcano is an opening, or rupture, in a planet's surface or crust,
which allows hot magma,
volcanic
ash and gases to escape from below the surface.
Volcanoes are generally found where tectonic
plates are diverging or converging. A mid-oceanic
ridge, for example the Mid-Atlantic Ridge, has examples of volcanoes
caused by divergent tectonic plates pulling apart; the Pacific Ring of Fire has examples of volcanoes
caused by convergent tectonic plates coming together. By
contrast, volcanoes are usually not created where two tectonic plates slide
past one another. Volcanoes can also form where there is stretching and
thinning of the Earth's crust in the interiors of plates, e.g., in
the East African Rift, the Wells Gray-Clearwater volcanic
field and the Rio Grande Rift in North America. This type of
volcanism falls under the umbrella of "Plate hypothesis" volcanism.[1]
Volcanism away from plate boundaries has also been explained as mantle
plumes. These so-called "hotspots", for example Hawaii, are
postulated to arise from upwelling diapirs with magma from the core-mantle boundary, 3,000 km deep in
the Earth.
Erupting volcanoes can pose many
hazards, not only in the immediate vicinity of the eruption. Volcanic ash can
be a threat to aircraft,
in particular those with jet engines where ash particles can be melted by the
high operating temperature. Large eruptions can affect temperature as ash and
droplets of sulfuric acid obscure the sun and cool the Earth's
lower atmosphere or troposphere; however, they also absorb heat radiated up
from the Earth, thereby warming the stratosphere.
Historically, so-called volcanic winters have caused catastrophic famines.
Divergent plate boundaries
Main article: Divergent boundary
At the mid-oceanic ridges, two tectonic
plates diverge from one another. New oceanic
crust is being formed by hot molten rock slowly cooling and solidifying.
The crust is very thin at mid-oceanic ridges due to the pull of the tectonic
plates. The release of pressure due to the thinning of the crust leads to adiabatic
expansion, and the partial melting of the mantle causing volcanism and creating new oceanic
crust. Most divergent plate boundaries are at the bottom of
the oceans, therefore most volcanic activity is submarine, forming new
seafloor. Black smokers or deep sea vents are an example of this
kind of volcanic activity. Where the mid-oceanic ridge is above sea-level,
volcanic islands are formed, for example, Iceland.
