Environmental Engineering Reference
In-Depth Information
Abyssal
hills
Abyssal
floor
Oceanic
ridge
Abyssal
floor
Trench
Folded mountain belt
Craton
Volcanoes
Continental
rise
Oceanic crust
(lithosphere)
Continental
slope
Continental
shelf
Abyssal plain
Continental crust
(lithosphere)
Mantle
(lithosphere)
Mantle (lithosphere)
Mantle (asthenosphere)
Figure 12-2
Natural capital:
major features of the earth's crust and upper mantle. The
lithosphere,
com-
posed of the crust and outermost mantle, is rigid and brittle. The
asthenosphere,
a zone in the mantle, can be
deformed by heat and pressure.
12-2 INTERNAL AND EXTERNAL
GEOLOGIC PROCESSES
Throughout the earth's history, continents have
split and joined as tectonic plates have very slowly
drifted thousands of kilometers back and forth across
the planet's surface (Figure 4-8, p. 72).
As these plates collide, break apart, and slide by
one another over millions of years, they produce
mountains on land (such as the Himalayas and the Ap-
palachian Mountains of the eastern United States),
huge ridges and trenches on the ocean floor, and other
features of the earth's surface (Figures 12-2 and 12-3).
These movements and geological processes continue
today. Natural hazards such as earthquakes and volca-
noes are likely to occur at plate boundaries. Science
Supplement 7 at the end of this topic discusses earth-
quakes and volcanoes.
The theory of plate tectonics also helps explain
how certain patterns of biological evolution occurred.
By reconstructing the course of continental drift over
millions of years, scientists can trace how species mi-
grated from one area to another when continents that
are now far apart were still joined together. As the con-
tinents separated, populations became geographically
and reproductively isolated, and speciation occurred.
In other words, tectonic plates have played a major but
mostly unnoticed role in the drama of life that contin-
ues to unfold on our planetary home.
Science: Plate Tectonics
Huge volumes of heated and molten rock
moving around within the earth's interior form
massive solid plates—called tectonic plates—
that move extremely slowly across the earth's
surface.
We tend to think of the earth's crust, mantle, and core
as fairly static. In reality, geologic processes taking
place within the earth and on its surface, mostly over
thousands to millions of years, bring about changes in
these components (Figure 12-3, p. 272).
The flows of energy and heated material in the
mantle convection cells cause about 15 huge rigid
plates, called
tectonic plates,
to move extremely
slowly across the earth's surface (Figures 12-3, p. 272,
and 12-4, p. 273). These thick plates are composed of
the continental and oceanic crust and the rigid, outer-
most part of the mantle (above the asthenosphere), a
combination called the
lithosphere.
The tectonic plates move constantly, supported by
the slowly flowing asthenosphere. They are somewhat
like large icebergs floating extremely slowly on the
surface of an ocean or like the world's largest and
slowest-moving surfboards. A typical speed is about
the rate at which fingernails grow. You ride or surf on
one of these plates throughout your entire life, but the
motion is too slow for you to notice.
Witness what happens when the earth's tectonic plates pull
apart or collide at Environmental ScienceNow.
