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) are 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.   A 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 dropping 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.