The current scientific knowledge base with regards to the structural composition of the
Earth is based around the facts that scientists have observed, these scientific observations have
lead to the development of fundamental theories regarding the structure and composition of the
Earth, these theories represent a relationship between scientific models and scientific
observations which underpin and form the basis for these models.
Current day geologists have a very detailed understanding of the complex structural
composition of our planet. The utilisation of seismic tomography has allowed the scientific
community to agree and catagorise the layers of the Earth, those being; the Lithosphere which is
the rocky oceanic and continental crustal layer on which humans inhabit. The Moho below this is
set at the boundary of the Lithosphere and the Asthenosphere which is part of the upper most
section of the Mantle. The Mantle is the largest part of the Earth, it is set below the Lithosphere
and above the Earth's core which is segmented into an inner and outer layer.
The average density of the Earth is 5.5 t/m3, however when scientists take samples of the
lithosphere around us we find that continental crust has an average density of 2.7 t/m3 and even
the denser Oceanic Crust only has an average density of 2.9 t/m3. When scientists take samples
from deep in the Earth itself samples from the Mantle have shown values on an average of
3.3t/m3, this must mean to get an average of 5.5 t/m3 it must be extremely dense below the
mantle for this value to be correct. The currently accepted model of the Inner Core is inferred to
be composed of predominantly Iron and Nickel with an average density of 12t/m3.1
The Temperature of the core of the earth is much too hot for a permanent magnet to exist
within the Core of the Earth as the Maximum temperature at which Iron can be magnetic is
770°C and 580°C which is known as the Curie temperature for Magnetite. Due to this and the
presence of Earth’s magnetic field it means the Earth must produce a very powerful magnet by
another means, and it harness’ electric currents made by the Liquid metal Outer Core moving
around the solid Iron rich Inner Core.2
Seismic Tomography from earthquakes gives conclusive evidence for the structure and
composition of the core of the Earth. After an Earthquake occurs, two subsurface body Waves
known as P and S Waves travel from the focus of the quake throughout the Earth. We can
observe that P "Primary" waves are the first to arrive after an Earthquake, these are very fast and
are compression waves. These waves move through liquids slower due to the decreased rigidity
of the material. S "Secondary" waves are next to arrive and they move in a shearing motional
wave. S waves are completely dissipated when they meet liquids as they cannot travel through a
substance without rigidity. 3
The velocity of these waves increases the higher the rigidity of the material they travel
through. Body waves are slowed the higher the density of the material, which would make it
seem as though the waves would slow down as they travel further down into the planet, this isn't
so because the deeper into the planet we go the more rigid the rock becomes.
The same way in which we observe visible light refracting as it passes into water, do
body waves refract as they pass into denser rock within the earth. Scientists have used this
observation to theorize the nature and boundaries within the Earth, so after an earthquake we can
use this fact to observe and identify the paths that these body waves take. 4
When Seismic Tomography is used shadow zones can be observed where S waves are
completely blocked from passing through the Core of the planet while two large P wave shadow
zones are formed from where they are refracted by the liquid Outer Core but still manage to pass
through the core of the planet which means that P and S waves must either be slowed or stopped
by a layer of rock in a liquid state.5
In the past before more complex geological technologies such as seismography or deep
drilling were developed or readily available many theories of what the composition of the Earth
was. Many were made purely on observed geological phenomena and possible reasons for their
occurrence.
One such model for the structural composition of the Earth was proposed that the Earth
was hollow much like an empty egg shell with more inner shells with vast empty cavernous
spaces between them. This theory was put forward to account for the great discrepancy between
the stated density of the earth being so much higher than the surface density they would have
measured against within the lithosphere. This is a logical argument what took observed facts and
put forward a theory in an attempt to explain it.
The present day model of the Earth is that the Lithosphere is the uppermost layer of the
Earth which contains the hydrosphere and cryosphere as well as the planet’s crust which is
mainly comprised of silicates and is catagorised as either Oceanic or continental crusts, of the
two the oceanic crust being of higher density as it is cools as it moves away from the mid ocean
ridge before being reabsorbed to the mantle. The temperature within the Lithosphere increases
with depth, where it’s been observed that mines reach 50°C and deep drilling can even reach up
to 250°C 6
The chemical composition of the Lithosphere changes at a point known as the Moho, at
this point the velocity of body waves are greatly increased. The Moho also introduces the
uppermost part of the mantle which contains the Asthenosphere.
The Asthenosphere, also known as the Low velocity zone which is called this because
body waves, especially P waves travel much faster above and below this zone which can range
from 100km to 300km in depth, S waves can travel though it which proves is not a continuous
liquid but does slow them down. This observation has allowed scientists to conclude that the
Astheneosphere is solid but just below the beginning of melting temperature all of these features
are what enable it to act as the soft layer upon which the ridged tectonic plates are able to move
on. 7
The Mantle is the largest layer of our planet which contains up to 80% of the Earths
overall volume and up to 65% of its mass. The Mantle maintains temperatures that range from
1200°C to 1600° and even in deep parts in excess of 1800°C. The Mantle is understood to be
solid and made up of peridotitic rock such as olivine which is a pale yellow mineral that is often
carried up by lava, as well as containing many silicates that have minerals such as Iron and
Magnesium. The Mantle itself has been identified to reach a depth of close to 2900km.
The Outer Core of the planet is so hot that the rock has melted into a highly viscous
liquid that flows around the solid inner core and produces the magnetic field of the Earth that
protects all living life from deadly solar radiation. 8
The inner Core is extremely hot, even more so than the outer core however unlike the
Outer Core it is not a liquid, the gravitational pressure acting upon the Inner Core is a far greater
force than the heat energy acting upon it, forcing it to maintain a super dense, super heated, solid
state. The center of the Inner Core is 3671km from the surface of the Earth.
The current theoretical model of the structural composition of the Earth can be seen to
have been developed in conjunction with the facts that can be observed such as Seismic
Tomography with respect to the travel velocity and range of Seismic body waves through the
Earth itself as well as the presence of the Earth’s magnetic field requiring flowing metallic liquid
producing electrical currents and finally the need for a super dense Inner core required to make
up for the discrepancy observed between the density of the Lithosphere and Mantle compared
with the average density of the Earth.
1
Origin and evolution of earth : principles of historical geology / Kent C. Condie, Robert E. Sloan / Upper
Saddle River, NJ / Prentice Hall / 1998
2
Historical geology : evolution of earth and life through time / Reed Wicander, James S. Monroe / Pacific
Grove, CA / Brooks & Cole / 2000
3
Planet Earth : an encyclopedia of Geology / consultant editor, A. Hallam / Oxford / Elsevier Phaidon /
1977
4
The Earth, its origin, structure, and evolution / edited by M.W. McElhinny / London / New York /
Academic Press / 1979
5
An introduction to geology : illustrative of the general structure of the earth / Blakewell, Robert /
London / Harding Printers / 1973
6
The changing earth : exploring geology and evolution / James S. Monroe, Reed Wicander / St. Paul, MN
/ West / 1994
7
The dynamic earth : an introduction to physical geology / Brian J. Skinner, Stephen C. Porter / New York
/ Wiley / 1992
8
Structure of the earth / [by] Sydney P. Clark / Englewood Cliffs, N.J. / Prentice Hall / 1971