Volcanoes are mountains,
but they are very different from other mountains; they are not formed
by folding and crumpling or by uplift and erosion. Instead, volcanoes
are built by the accumulation of their own eruptive products --
lava, bombs (crusted over lava blobs), ashflows, and tephra (airborne
ash and dust). A volcano is most commonly a conical hill or mountain
built around a vent that connects with reservoirs of molten rock
below the surface of the Earth. The term volcano also refers to
the opening or vent through which the molten rock and associated
gases are expelled.
Driven by buoyancy and gas pressure the molten rock, which is
lighter than the surrounding solid rock, forces its way upward
and my ultimately break through zones of weaknesses in the Earth's
crust. If so, an eruption begins, and the molten rock may pour
from the vent as nonexplosive lava flows, or it may shoot violently
into the air as dense clouds of lava fragments. Larger fragments
fall back around the vent, and accumulations of fallback fragments
may move downslope as ash flows under the force of gravity. Some
of the finer ejected materials may be carried by the wind only
to fall to the ground many miles away. The finest ash particles
may be injected miles into the atmosphere and carried many times
around the world by stratospheric winds before settling out.
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Volcano Mount McLoughlin, Oregon |
Some
volcanos occur from hot spots, areas of persistant heat source
in the mantle. They do not have to occur along a plate boundary
in this situation.
However, there are volcanos associated with both subduction and
divergent zones. As the divergent plates separate, magma can create
a volcanic chain. This can be seen most easily in ocean-ocean
diverging plates, commonly called a "mid-oceanic ridge."
In subduction zones, the diving crust reaches higher and higher
temperatures which lead to melting. Eventually the magma reaches
the surface and the lava cools. Build-up of cooled lava creates
a dome or mountain appearance.
Ocean-Continent Subduction Zone
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Earthquakes, Volcanos and Plate Boundaries
Blue lines: Plate boundaries
Yellow dots: Earthquakes
Red triangles: Active Volcanos
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