Geology Reference
In-Depth Information
Crust
Mantle
Liquid
outer
core
Solid
outer
core
Early Earth probably had a
uniform composition and
density throughout.
The temperature of early Earth reached the
melting point of iron and nickel, which, being
denser than silicate minerals, settled to Earth's
center. At the same time, the lighter silicates
flowed upward to form the mantle and the crust.
In this way, a differentiated Earth formed,
consisting of a dense iron-nickel core, an
iron-rich silicate mantle, and a silicate crust
with continents and ocean basis.
a
c
b
◗
Figure 1.9
Homogeneous Accretion Theory for the Formation of a Differentiated Earth
AND EVOLVING PLANET
Earth is a dynamic planet that has continuously changed
during its 4.6-billion-year existence. The size, shape, and
geographic distribution of continents and ocean basins have
changed through time, the composition of the atmosphere
has evolved, and life-forms existing today differ from those
that lived during the past. Mountains and hills have been
worn away by erosion, and the forces of wind, water, and ice
have sculpted a diversity of landscapes. Volcanic eruptions
and earthquakes reveal an active interior, and folded and
fractured rocks are testimony to the tremendous power of
Earth's internal forces.
Earth consists of three concentric layers: the core, the
mantle, and the crust (
what eventually became Mars and Jupiter in much the same
way that other planetesimals formed the terrestrial planets.
The tremendous gravitational fi eld of Jupiter, however, pre-
vented this material from ever accreting into a planet. Comets,
which are interplanetary bodies composed of loosely bound
rocky and icy materials, are thought to have condensed near
the orbits of Uranus and Neptune.
The solar nebula theory for the formation of the solar
system thus accounts for most of the characteristics of the
planets and their moons, the differences in composition
between the terrestrial and Jovian planets, and the presence of
the asteroid belt. Based on the available data, the solar nebula
theory best explains the features of the solar system and pro-
vides a logical explanation for its evolutionary history.
Figure 1.10). This orderly divi-
sion results from density differences between the layers as
a function of variations in composition, temperature, and
pressure.
The
core
has a calculated density of 10-13 grams per
cubic centimeter (g/cm
3
) and occupies about 16% of Earth's
total volume. Seismic (earthquake) data indicate that the core
consists of a small, solid inner region and a larger, apparently
liquid, outer portion. Both are thought to consist mostly of
iron and a small amount of nickel.
The
mantle
surrounds the core and comprises about 83%
of Earth's volume. It is less dense than the core (3.3-5.7 g/cm
3
)
and is thought to be composed mostly of
peridotite
, a dark,
dense igneous rock containing abundant iron and magne-
sium. The mantle can be divided into three distinct zones
based on physical characteristics. The lower mantle is solid
and forms most of the volume of Earth's interior. The
as-
thenosphere
surrounds the lower mantle. It has the same
composition as the lower mantle, but behaves plastically and
fl ows slowly. Partial melting within the asthenosphere gen-
erates
magma
(molten material), some of which rises to the
surface because it is less dense than the rock from which it
◗
Some 4.6 billion years ago, various planetesimals in our solar
system gathered enough material together to form Earth and
the other planets. Scientists think that this early Earth was
probably cool, of generally uniform composition and density
throughout, and composed mostly of silicates, compounds
consisting of silicon and oxygen, iron and magnesium
oxides, and smaller amounts of all the other chemical ele-
ments (
Figure 1.9a). Subsequently, when the combination
of meteorite impacts, gravitational compression, and heat
from radioactive decay increased the temperature of Earth
enough to melt iron and nickel, this homogeneous compo-
sition disappeared (Figure 1.9b) and was replaced by a series
of concentric layers of differing composition and density,
resulting in a differentiated planet (Figure 1.9c).
This differentiation into a layered planet is probably the
most signifi cant event in Earth's history. Not only did it lead
to the formation of a crust and eventually continents, but it
also was probably responsible for the emission of gases from
the interior that eventually led to the formation of the oceans
and atmosphere.
◗
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