Geography Reference
In-Depth Information
many volcanoes. The major destructive earthquake resulting in the
tsunami
that struck Indonesia and the eastern shore of the Indian
Ocean on 26 December 2004, causing massive loss of life, occurred
at a subduction zone.
Volcanoes produced around subduction zones can be very
explosive and destructive. The oceanic crust is heated as it is carried
down into the mantle. Water and other materials which are carried
down with the plate are released, producing a mixture that rises to
the surface. If the uppermost plate is oceanic, then basaltic volca-
noes are produced which form into arcs of islands. When an
oceanic plate collides directly with continental crust the oceanic
plate moves under the continent. Water becomes trapped and
causes the basaltic rock to melt under pressure. Rising magma starts
to melt the continental crust of the overlying plate. This magma is
very sticky and can result in a destructive volcanic explosion
destroying large areas and killing many people. Examples of such
volcanoes include Krakatoa (Indonesia), Vesuvius (Italy), Fujiyama
( Japan) and Mount Saint Helens (USA). Sticky, slow-moving lava
builds within these steep-sided volcanoes and once the lava ceases
to flow, it cools producing a plug allowing a considerable pressure
to build up within the volcano ready for the next eruption.
There are around 600 active volcanoes above the land or ocean
surface (but tens of thousands on the ocean floor). On average
around 50 surface volcanoes erupt per year although we tend only
to hear about those that cause major destruction or disruption to
travel. Volcanoes form at the end of a central tube or vent that rises
from the upper mantle. There is often a crater which is a surface
depression at the top of the volcano. Magma within the volcano
slowly rises, building up pressure until conditions are right for
eruption. Often the heat from the magma can also boil water in
the ground resulting in hot springs and
geysers
. Volcanoes do not
always form just at plate boundaries and more information on this
is provided in Box 3.1.
Convergent boundaries compress rocks and deform them. This
causes the rock to fold and rumple as if it were a piece of cloth
being pushed together from both ends. This can produce mountain
ranges which look like ripples when viewed from a plane high
above. The most intense mountain building occurs when two con-
tinents collide. This is because they push into each other and one is
