Geology Reference
In-Depth Information
in the crust above. Thus, when a par-
ticular part of the crust is subjected
to an additional load, by the creation
of a mountain range for example, its
base adjusts by sinking downwards and
displacing mantle material sideways
to form a 'mountain root' as shown in
Figure 2.4B; like an iceberg, much more
of the volume of the thickened crust of
a mountain range lies beneath sea level
than above! The reverse happens when
crust is thinned to form an ocean basin,
for example; the top of the mantle rises
and the surface is depressed, but the
whole is still in gravitational balance.
the occurrence of earthquakes
( seismicity ) and vulcanicity of varying
severity, much of it with disastrous
consequences. This activity is not
random but is concentrated in well-
defined zones (Figure 2.6).
extremes, generally lying above about
300 km. The reason for these differences
will become apparent when we discuss
plate tectonics in the next chapter.
Volcanoes
The distribution of vulcanicity, like
that of earthquakes, is distinctly non-
random; there are three main categories
of occurrence. The majority of active
volcanoes are concentrated along the
circum-Pacific and Alpine-Himalayan
belts, as shown in Figure 2.6; secondly,
there are numerous occurrences along
the ocean ridges and on subsidiary
ridges within the ocean basins, as
well as in Hawaii. A third category is
Earthquakes
Although earthquakes can occur
almost anywhere, the great majority
of them (and all the most severe ones)
are concentrated in narrow zones
along the centres of the ocean ridges
and in a rather wider zone along the
Alpine-Himalayan and circum-Pacific
mountain belts delineated in Figure 2.2.
The earthquakes along the ocean ridge
network are generally quite shallow, less
than ~50 km in focal depth, but those
near the ocean trenches range from
shallow to deep - up to 700 km in depth,
indicating that the tectonic process
responsible for the latter is different. The
earthquakes along the young mountain
belts are intermediate between the two
Asthenosphere and lithosphere
The region of the mantle in which flow
takes place lies at a depth of about
100 km beneath the surface, and is
termed the low velocity layer because
of the fact that seismic waves travel
more slowly through it than would
be expected if it were composed of
normal mantle material, indicating
that its density is lower than that of
the surrounding mantle. It is therefore
regarded as a zone of relative weakness
and is termed the asthenosphere ( see
Figure 2.4B). The region above this level,
composed partly of mantle and partly
of crust, is stronger and is termed the
lithosphere . As we shall see in the next
chapter, it is the lithosphere from which
the tectonic plates are formed, and
their ability to move across the Earth's
surface is due to the relative weakness
of the underlying asthenosphere.
Figure 2.6 Pattern of recent earthquake and
volcanic activity. The majority of earthquakes
follow well-defined narrow zones along the
centres of the ocean ridges and rather broader
zones along the Alpine-Himalayan mountain
ranges, the island-arc network and the western
Americas belt. Most of the volcanoes lie within
these same zones, but some oceanic volcanoes
are situated away from the ridge crests,
especially in the Atlantic ocean.
Lat.
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30 o
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Present-day tectonic effects:
earthquakes, volcanoes and
aseismic movements
The Earth is subject to continuous
tectonic activity, as testified by
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earthquake zone
volcanoes
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