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distance offshore. The Sunda arc forming the Indonesian
islands between Malaysia and northern Australia repre-
sents a transitional phase, present on a smaller scale in
South Island, New Zealand.
During convergence, B-subduction magmas erupt
through continental crust as terrestrial volcanoes. Recent
major eruptions along the Pacific coast of the Americas
include Katmaï (erupted 1912) and Mount St Helens
(1980) in North America and El Chichón (1982) and
Nevado del Ruiz (1985) in Central and South America
( Plate 10.2 ). Magma depleted of its more volatile
components also crystallizes as huge granitic batholiths ,
intruded at depth from deep magma reservoirs. More
than 2 M km 3 of granite intruded in the roots of the Andes
now lie exposed by erosion over a surface area of nearly
500,000 km 2 .
In due course, subduction of remnant oceanic crust
marks the death of the ocean and leads eventually to
collision between converging continental plates. Despite
the acquisition of denser rocks, lighter continental crust
remains buoyant and influences the development of
different continental subduction processes. Convergence
rates fall sharply along the intracontinental suture but
driving forces are still sufficient to cause crustal
shortening. Since neither slab of lightweight continental
crust is capable of significant B-subduction, crustal
shortening must be compensated by crustal thickening .
This is achieved by complex thrusting of slivers of crust
into each other which does not proceed to sufficient
depths/temperatures for crustal recycling ( Figure 10.15 ).
Instead, downward displacement of light crust, known as
Ampferer or A-subduction , is compensated by isostatic
elevation of the developing pile to form thick continental
plate. Basal material may extend deep enough to
experience remagmatization at moderate temperatures
to form granitic cores. Intercontinental collision tectonics
in the suture zone are active along almost the entire
Alpine-Himalayan systems, also known collectively as the
Tethyan orogen , after the ocean which spawned them.
Transform faulting is a variation of general subduction.
Different rates and directions of ocean spreading are
transmitted onshore as strike-slip faulting, with horizontal
displacement along the strike or fault axis. Plates may
move past each other with nothing more than seismic
Plate 10.2 Mount St Helens stratovolcano in the Cascade Range Washington State, USA, prior to its explosive eruption in May
1980.
Photo: US Geological Survey
 
 
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