THE EARTH’S
INTERNAL STRUCTURE
Deep
parts of the solid earth are studied indirectly, however, largely through the
branch of geology called geophysics, which is the application of
physical laws and principles to a study of the earth. Geophysics
includes the study of seismic waves, the earth’s magnetic field,
gravity, and heat flow. It was the study of seismic
refraction and seismic reflection that enabled scientists to plot the three
main zones of the earth’s interior. Earth scientists believe that
convection currents occur in the interior of the earth in the zone known as the
mantle, the largest, by volume, of the earth’s three major
concentric zones. the other two zones are the crust
and the core. The crust f the earth is analogous to the skin on
an apple. The thickness of the crust varies. Studies of seismic
waves have shown there are two major types of crust - oceanic crust
and continental crust.
The
crust under the oceans is much thinner (3-8 km). Seismic waves travel
faster in oceanic crust than in continental crust. Because of this
velocity difference, it is assumed that the two types of crust are made up
different kinds of rock. Seismic P waves travel through oceanic
crust at about 7 kilometers per second. It is made of rock that somewhat
denser (3 gram per cm3 ) called basalt.
Samples of rocks are taken from the seafloor by oceanographic ships
verify this. Most of the seafloor samples are basalt, although other
rock types also are found such as gabbro , and peridotite (mantle
rock). Seismic P waves travel more slowly through continental
rocks-about 6 kilometers per second, the same velocity at which they travel
through granite. Continental crust is often referred to be granitic (sial-rocks high
in silicon and aluminum). Some geologists also use the term sima (rocks high in silicon and magnesium) for
oceanic crust. The boundary that separates the crust from the mantle
beneath it is called the Moho
discontinuity.
The
mantle
is solid and probably composed of rock found at the earth’s surface.
The crust and the uppermost part of the mantle (to 100 km) is relatively rigid (strong and brittle). They
make up the lithosphere. The upper mantle underlying the
lithosphere (between 100 and 200 km) behaves plastically (low-velocity
zone: seismic waves travel slowly) and is called the asthenosphere.
The rocks here may have relatively little strength and
therefore are likely to flow. If mantle rocks in the asthenosphere are weaker than they are in the
overlying lithosphere, then the asthenosphere can
deform easily by plastic flow. Convection (small convection
cells) is believed to take place within the asthenosphere
as well as within the lower mantle (large convection cells). The
lithosphere seems to be moving (floating) above the asthenosphere
probably as a result of the underlying mantle convection. Plates of
brittle lithosphere probably move easily over the asthenosphere,
which may act as a lubricating layer below. The new results come from
new seismic studies, known as global seismic tomography (three-dimensional
mapping of seismic speeds in Earth’s mantle) is similar to computerized
axial tomography-CAT, which traces x rays through, say, a person’s head
to take a picture of the brain. The map of wave speed turns out to be a
rough indication of the temperature distribution: Waves travel faster in
regions that are colder.
The
effect of the internal processes on the crust generates the tectonic forces
which cause deformation of the rocks of the earth’s crust. Most
tectonic forces are mechanical forces. The mechanical energy may be
stored (an earthquake is a sudden release of stored mechanical energy), or
converted to heat energy (rocks may melt, resulting in volcanic eruptions).
The way the machinery of the solid earth works is called plate
tectonics.
Data
from seismic reflection and refraction indicate several concentric layers in
the mantle, with prominent boundaries at 400 km and 670 km. It is
doubtful that the layering is due to the presence of several kinds of rock.
But, most geologists think that the chemical composition of the
mantle rock is about the same. Because pressure increases with depth into
the earth, the boundaries between the mantle layers possibly represent the
depths changing in mineral composition. For example, at the depth
of 400 km, the mineral olivine should transform into a denser mineral
called spinel.
Seismic-wave
data provide the primary evidence for the existence of the core of the earth.
Seismic waves do not reach certain areas on the opposite side of the
earth from a large earthquake. The region between 103° and 142°, which
lacks P waves, is called the P-wave shadow zone. This zone can be
explained by the refraction of P waves when they encounter the core boundary
deep within the earth’s interior. In other words, P waves are
missing within the shadow zone because they have been bent (refracted) by the
core. While P waves can travel through solids and fluids, S waves can
travel only through solids. An large S-wave
shadow zone also exists. S waves are not recorded in the entire
region more than 103° away from the epicenter. Thus seems to indicate that
S waves do not travel through the core at all. If it is true, it implies
that the core is a liquid, or at least acts like a liquid. The way in
which P waves are refracted within the earth’s core suggests that the
core has two parts, a liquid (molten) outer core and a solid
inner core. Calculations show that the core has to have a density about 10 gr/cm 3 at the
core-mantle boundary, increasing to 12-13 gr/cm3 at the center of
the earth. This great density would be enough to give the earth an average
density of 5. 5 gr/cm.
Iron-nickel alloy mixed a small amount of a lighter element (such as sulfur,
oxygen, hydrogen and silicon) would have the required density.
Geophysical
studies have been suggested that a region of magnetic force -a magnetic
field- surrounds the earth. The field has north and south
magnetic poles, displacing about 11.5° from the geographic poles. The
rate of poles’ changes in position and strength of the magnetic field
suggests that the magnetic field is generated within the liquid metal of outer
core. A widely accepted hypothesis suggests that the geomagnetic field
is created by electric currents within the slowly circulating liquid part of
the core. This requires the core to be an electrical conductor.
This is evidence that the core is metallic. The earth’s magnetic
field has periodically reversed its polarity. Such a change in the past
is called magnetic reversal. During a time of normal polarity (the
present position), magnetic lines of force leave the earth near the geographic
south pole and reenter near the geographic north pole. During a time of
reversed polarity magnetic lines run the other way. In other words,
during a magnetic reversal the magnetic poles exchange positions. Many
seafloor rocks contain magnetic minerals (for example, magnetite).
During the crystallization in a cooling lava flow, the atoms within the
crystals respond to the earth’s magnetic field and form magnetic
alignments that point toward the north magnetic pole. As the rock solidifies,
this magnetic record is permanently trapped in the rock. Unless the rock
reheated again this magnetic signature is changed. The study of ancient
magnetic fields is called paleomagnetism.
The
temperature increase with depth into the earth is called the geothermal gradient. The
geothermal gradient can be measured on the seafloor by droping
specially designed probes into the mud (or on land in abandoned wells).
The average temperature increase is 25°C per kilometer of
depth. Earth scientists believe that gradient could not continue very
far into the earth. If they did, the temperature would be 2500°C at the
shallow depth of 100 kilometers. Seismic evidence seems to indicate a
solid mantle at this depth, so the gradient must drop to values as low as
1°C/kilometer within the mantle. Calculations extrapolating from the
surface downward showed that the temperature at the boundary between the inner
core and the outer core was about 3700°C (3 million-atmosphere pressure found
there). The temperature at the center of the earth was assumed to be
about 4000°C. A measurable amount of heat from the earth’s
interior being lost through the earth’s surface is called the heat
flow. It is the product of the increase in temperature with depth
times the thermal conductivity. Some regions on earth have high heat
flow. High heat flow is caused by the presence of magma body (on
seafloor) or still-cooling pluton near the surface
(on land). The average heat flow from continents is the same as the
average heat flow from the seafloor, but the origin of the heat differs from
the ocean to continents. Oceanic crust generates little heat, so oceanic
heat flow must come from the mantle. Heat flow from continent is largely
generated within continents by radioactive decay.
(Note: I recognize that
this is a large amount of reading. Don't feel like you have to do all of
it at once; you have at least two more lectures - really 5 days+ to read it in.
Take it slowly, as this is important stuff. )
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Nilgün Okay
okayn@itu. edu. tr
Last modified on Fri Oct. 26 17:35:47 PST 2000.