Large-scale earth structure - Introducing Tectonics, Rock Structures and Mountain Belts - page 5

Figure 2.2 Main topographic features of the
continents and oceans. The young mountain
ranges form two distinct belts, the Alpine-
Himalayan and circum-Pacific, meeting in
Indonesia. The western part of the circum-Pacific
belt consists of partly submerged mountain
ranges. The ocean ridges, which are almost
entirely submerged, form a continuous network
extending through the mid-Atlantic into the
Antarctic where it meets the Indian Ocean ridge,
and continues into the Pacific, where it ends
against the coast of Mexico. The much narrower
trenches lie offshore of western America and
also parallel to a series of island arcs around the
north and west Pacific Ocean and off Indonesia
in the Indian Ocean. Smaller arcs are located in
the Caribbean and south of S. America.

Young mountain range

Island arc
Ocean trench

2

Ocean ridge

4 5

Alpine-Himalayan
mountain belt

Aleutian
arc

J

West
Pacific
arc-trench
system

Western
American
Cordilleran-
Andean
mountain
range

NG

mid-
Atlantic
ridge

East
Pacific
ridge

Earth's internal structure
A considerable amount is known about
the internal structure of the Earth,
much of it from the study of earthquake
waves. On the scale of the whole Earth
(Figure 2.3) the crust represents only a
thin outer shell with an average thick-
ness of around 20 km. The bulk of the
Earth is made up of the mantle, the
base of which lies at a depth of 2900 km
below the surface, and is composed

Indian
Ocean
ridge

East
Pacific
ridge

Circum-Antarctic
ridge

only around 400 km in the southern
Andes. The Alpine- Himalayan belt
is even more variable: it divides into
several branches around the Mediter-
ranean, including the Atlas and Apen-
nine ranges; in central Asia, the belt
broadens to around 1500 km where it
surrounds the high plateau of Tibet.
Ongoing tectonic activity in these
belts is expressed by sporadic volcan-
ism and frequent earthquakes. Other
prominent mountain belts, such as
the Urals, Caledonian and Appala-
chian ranges, are no longer tectonically
active, being the result of processes
in more remote geological periods.
The ocean ridges , shown in blue
on Figure 2.2, are even more promi-
nent features than the mountain belts
in terms of their volume, although of
course almost entirely hidden below
sea level; they are typically 500-1000 km
wide and elevated by as much as
2-3 km from the deep ocean floor.

The ridge network consists of three
main strands: the Mid-Atlantic, Indian
Ocean and circum-Antarctic ridges,
the latter extending through the south-
east Pacific Ocean to end against the
North American coast off Mexico.
The deep-ocean trenches (thin
green lines on Figure 2.2) are much
smaller features than the ridges, being
only about 100-150 km across, but they
attain depths of up to 11 km below
sea level. They fall into two groups:
those of the first group lie offshore
from, and parallel to, the western con-
tinental margin of the Americas, and
those of the second group form a dis-
continuous series of curved features
parallel to the volcanic island arcs
of the western and northern Pacific
Ocean and south-west of Indonesia
in the Indian Ocean. Smaller arcs are
located in the Caribbean and between
South America and Antarctica.

crust

mantle

inner
core

outer core

Figure 2.3 The cutaway model shows the main
regions of the Earth's interior: solid inner core,
liquid outer core, mantle and thin crust.