Deformation, Mountain Building, and the Continents - The Changing Earth: Exploring Geology and Evolution - page 260

seismicity along South America's
west coast indicate that the An-
des Mountains are still forming.

Orogenies at Continental-
Continental Plate Boundaries
The best example of an orogeny
along a continental-continental
plate boundary is the Himalayas of
Asia. The Himalayas began form-
ing when India collided with Asia
about 40 to 50 million years
ago. Prior to that time, India
was far south of Asia and sepa-
rated from it by an ocean basin
( Figure 10.22a). As the Indian

plate moved northward, a sub-
duction zone formed along
the southern margin of Asia
where oceanic lithosphere was
consumed. Partial melting
generated magma, which rose
to form a volcanic arc, and large
granite plutons were emplaced
into what is now Tibet. At this
stage, the activity along Asia's
southern margin was similar to
what is now occurring along the
west coast of South America.
The ocean separating India from Asia continued to close
and India eventually collided with Asia (Figure 10.22a). As
a result, two continental plates became welded, or sutured,
together. Thus, the Himalayas are now within a continent
rather than along a continental margin. The exact time of In-
dia's collision with Asia is uncertain, but between 40 and 50
million years ago, India's rate of northward drift decreased
abruptly from about 15-20 cm per year to about 5 cm per
year. Because continental lithosphere is not dense enough to
be subducted, this decrease seems to mark the time of col-
lision and India's resistance to subduction. Consequently,
the leading margin of India was thrust beneath Asia, caus-
ing crustal thickening, thrusting, and uplift. Sedimentary
rocks that had been deposited in the sea south of Asia were
thrust northward, and two major thrust faults carried rocks
of Asian origin onto the Indian plate. Rocks deposited in the
shallow seas along India's northern margin now form the
higher parts of the Himalayas (Figure 10.22b). Since its colli-
sion with Asia, India has been thrust horizontally about 2000
km beneath Asia and now moves north at several centime-
ters per year.
Other mountain systems also formed as a result of col-
lisions between two continental plates. The Urals in Russia
and the Appalachians of North America formed by such col-
lisions. In addition, the Arabian plate is now colliding with
Asia along the Zagros Mountains of Iran.

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Figure 10.17 The Teton Range in Wyoming The Teton Range is one of many mountain ranges
in the Rocky Mountain system. It began forming about 10 million years ago as uplift took place
on normal faults that parallel the range front. As uplift of the Teton Range proceeded, it was
eroded by running water, glaciers, and gravity-driven processes, giving it its rugged aspect.

Orogenies at Oceanic-Continental Plate Boundaries The
Andes of South America are perhaps the best example of con-
tinuing orogeny at an oceanic-continental plate boundary.
Among the ranges of the Andes are the highest mountain peaks
in the Americas and many active volcanoes. Furthermore, the
west coast of South America is an extremely active segment of the
circum-Pacifi c earthquake belt, and one of Earth's great oceanic
trench systems, the Peru-Chile Trench, lies just off the coast.
Prior to 200 million years ago, the western margin of
South America was a passive continental margin where sedi-
ments accumulated much as they do now along the East Coast
of North America. However, when Pangaea split apart along
what is now the Mid-Atlantic Ridge, the South American plate
moved westward. As a consequence, the oceanic lithosphere
west of South America began subducting beneath the continent
(

Figure 10.21). Subduction resulted in partial melting of the
descending plate, which produced the andesitic volcanic arc of
composite volcanoes, and the west coast became an active con-
tinental margin. Felsic magmas, mostly of granitic composition,
were emplaced as large plutons beneath the arc (Figure 10.21).
As a result of the events just described, the Andes Moun-
tains consist of a central core of granitic rocks capped by
andesitic volcanoes. To the west of this central core along
the coast are the deformed rocks of the accretionary wedge.
And to the east of the central core are intensely folded sedi-
mentary rocks that were thrust eastward onto the continent
(Figure 10.21). Present-day subduction, volcanism, and

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