Large-scale earth structure - Introducing Tectonics, Rock Structures and Mountain Belts - page 4

Large-scale earth structure
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Large-scale earth structure

If we were to view the Earth from Space
(Figure 2.1), the most obvious large-
scale features would be the continents ,
the oceans , and the rather linear conti-
nental mountain ranges of the Alpine-
Himalayan belt and the circum-Pacific
belt, which includes the Cordilleran-
Andean chain of the western Americas
(Figure 2.2). Were we to imagine the
water of the oceans removed, another

set of mountain ranges would appear
in the form of the ocean ridge network,
which takes up around one-third of
the oceanic surface area. Deep-ocean
trenches would also be apparent,
although these are an order of mag-
nitude smaller in areal extent. They
form a discontinuous system of linear
features parallel to and offshore from
island arcs, as in the NW Pacific, or close

to the continental margin of western
America. These large-scale structures
are not randomly arranged but have a
tectonic significance, being caused by
geological processes that have operated
over long periods of geological time.

Mountain belts, ridges and
trenches
The most prominent and tectonically
active mountain ranges form two linear
belts shown in red in Figure 2.2. The
Alpine-Himalayan belt extends from
Gibraltar, through the Alps, Caucasus,
and Zagros ranges, and culminates
in the huge mass of the Himalayas
and related ranges that sweep around
northern India southwards to the
Indian Ocean. The eastern side of the
circum-Pacific belt chain follows the
western margin of the Americas from
Alaska down through the North Ameri-
can Cordillera to the Andes of South
America. On the western Pacific margin,
it forms a discontinuous chain includ-
ing south-east Siberia, Japan and New
Guinea. These belts vary considerably
both in width and height: the width
of the western American belt is over
600 km in the Canadian Rockies but

Figure 2.1 Earth from space. This photograph
shows clearly the continents of Africa, India and
much of Asia. The Alpine-Himalayan mountain
belt can be faintly seen extending from between
the Black and Caspian seas in the top west
part of the globe running through the Zagros
mountains north of the Persian Gulf and into
Pakistan. The Himalayas are lost beneath cloud.
NASA photo, courtesy Space Images.