Geoscience Reference
In-Depth Information
Andesitic volcano
Ocean
trench
Ocean crust
Subduction
Origin of volcanism over a subduction zone (after Holmes, 1965).
Fig. 9.5
material is melted and incorporated into the mantle. A
large zone of thrusting, termed the
Benioff zone
, forms
to depths of 700 km with intense heat generation. This
zone builds up heat because of magma extrusion,
friction generated by earthquake activity and radioac-
tive decay of crustal sediments. Beneath the overriding
crust, less dense, heated magma (mainly
andesite
and
rhyolite
) rises from the Benioff zone into the upper
layers of the mantle, before breaking sporadically
through the crust to build volcanic cones. This magma
contains gases derived from contact with groundwaters
or from chemical reactions in dehydrated parts of the
crust that are being subducted. The western Pacific
Ocean ring of fire represents the consumption of the
Pacific Plate by the Eurasian and Indo-Australian
Plates. A chain of volcanoes - an island arc - forms
parallel to the subduction zone. The Japanese, Indo-
nesian, and Aleutian Islands are classic examples of
volcanic island arcs built in this manner.
The Benioff zone is also a major zone of earthquake
activity because it represents thrust faulting of two
plates over a wide area. Deep-focus earthquakes at
70-700 km depth occur mainly beneath the continen-
tal side of the islands surrounding the Pacific and
Indian Oceans. Over 75 per cent of all earthquakes
occur in the plate rim of the Pacific Ocean. The Andes
Mountains can be included in this category as they rep-
resent an uplifted version of an island arc. As the depth
of focus increases in the Benioff zone, the epicenters
of deep earthquakes move further inland towards
the centers of volcanic activity. At times of volcanic
eruptions, earthquakes may occur because of volcanic
explosions, shallow magma movements within the
crust, or sympathetic tectonic earth movements.
The first two factors result from release of pressures
in the crust, or changes due to expansion or deflation
of the magma volume.
Plate collisions can also result in one continental
crust overriding another, as is the case with the
Indo-Australian Plate overriding the Eurasian Plate.
Few if any volcanoes are generated in this situation
because the crust of the Earth generally thickens at
these locations, and fracturing down to the mantle is
rare. Plates can also slide past each other as in the case
of the Pacific and North American Plates along the
San Andreas Fault. Not only earthquakes but also
volcanoes can develop because fractures, called
'transcurrent faults', open up at right angles to the line
of activity. Most of the volcanic activity in the Rocky
Mountains and the Andes is of this nature.
