Geology Reference
In-Depth Information
The
crust
, Earth's outermost layer, consists of two
types.
Continental crust
is thick (20-90 km), has an average
density of 2.7 g/cm
3
, and contains considerable silicon and
aluminum.
Oceanic crust
is thin (5-10 km), denser than
continental crust (3.0 g/cm
3
), and is composed of the dark
igneous rocks
basalt
and
gabbro
.
Oceanic
crust
Continental
crust
Upper mantle
Asthenosphere
Lower mantle
6380-km radius
The recognition that the lithosphere is divided into rigid
plates that move over the asthenosphere forms the founda-
tion of
plate tectonic theory
(
Mantle
Crust
Outer
core
(liquid)
Figure 1.12). Zones of volca-
nic activity, earthquakes, or both mark most plate boundaries.
Along these boundaries, plates separate (diverge), collide
(converge), or slide sideways past each other (
◗
Inner core
(solid)
Figure 1.13).
The acceptance of plate tectonic theory is recognized
as a major milestone in the geologic sciences, comparable
to the revolution that Darwin's theory of evolution caused
in biology. Plate tectonics has provided a framework for in-
terpreting the composition, structure, and internal processes
of Earth on a global scale. It has led to the realization that
the continents and ocean basins are part of a lithosphere-
atmosphere-hydrosphere system that evolved together with
Earth's interior (Table 1.3).
A revolutionary concept when it was proposed in the
1960s, plate tectonic theory has had far-reaching conse-
quences in all fi elds of geology because it provides the basis
for relating many seemingly unrelated phenomena. Besides
being responsible for the major features of Earth's crust,
plate movements also affect the formation and occurrence
of Earth's natural resources, as well as the distribution and
evolution of the world's biota.
The impact of plate tectonic theory has been particularly
notable in the interpretation of Earth's history. For example,
the Appalachian Mountains in eastern North America and
the mountain ranges of Greenland, Scotland, Norway, and
Sweden are not the result of
unrelated mountain-building
episodes, but, rather, are part
of a larger mountain-building
event that involved the closing
of an ancient Atlantic Ocean
and the formation of the su-
percontinent Pangaea approxi-
mately 251 million years ago.
◗
◗
Figure 1.10
Cross Section of Earth Illustrating the Core, Mantle,
and Crust The enlarged portion shows the relationship between the
lithosphere (composed of the continental crust, oceanic crust, and solid
upper mantle) and the underlying asthenosphere and lower mantle.
was derived. The upper mantle surrounds the asthenosphere.
The solid upper mantle and the overlying crust constitute
the
lithosphere
, which is broken into numerous individual
pieces called
plates
that move over the asthenosphere, par-
tially as a result of underlying
convection cells
(
Figure 1.11).
Interactions of these plates are responsible for such phenom-
ena as earthquakes, volcanic eruptions, and the formation of
mountain ranges and ocean basins.
◗
Mid-oceanic ridge
Trench
Ocean
Subduction
Oceanic
lithosphere
Continental
lithosphere
Cold
A
rock
is an aggregate of
miner-
als
, which are naturally occur-
ring, inorganic, crystalline solids
that have definite physical and
chemical properties. Minerals
are composed of elements such
as oxygen, silicon, and alumi-
num, and elements are made up
of atoms, the smallest particles
Convection
cell
Upwelling
Hot
Outer
core
Mantle
Inner
core
◗
Figure 1.11
Movement of Earth's Plates Earth's plates are thought to move partially as a result
of underlying mantle convection cells in which warm material from deep within Earth rises toward
the surface, cools, and then upon losing heat, descends back into the interior as shown in this
diagrammatic cross section.
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