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The Earth’s Interior
Earth is a called a terrestrial planet as it is a rocky body as compared to other planets that consist
of gaseous structure. There are four terrestrial planets in Solar system and Earth is largest and
densest amongst these. The other distinguished features of Earth are its highest surface gravity,
rapid rotation, stronger magnetic field and its active plate tectonics.
The lithosphere of Earth is divided into many segments; these segments are called tectonic
plates. These tectonic plates keep on migrating over a period of several million years. The poles
of earth are mostly covered with ice. Water comprises of almost 70% of the surface of Earth.
Rest of 30% consists of continents and islands. The other sources of water are in the form of
lakes and they also contribute to hydrosphere.
The interior of earth has inner and outer cores and it remains active. The inner core is solid;
however outer core is a liquid that generates the magnetic field. This is followed by a thick solid
layer called mantle. The crust is separated from the mantle by Mohorovicic discontinuity. The
thickness of crust varies between a range of six kilometers to fifty kilometers being thin under
the oceans and thick underneath the continents. The crust and upper mantle is collectively called
lithosphere. This is followed by a less viscous layer called asthenosphere. There is a transition
zone located at 410-660 km beneath the surface of Earth. This separates the upper and lower
mantle. Beneath the mantle is a liquid outer core, and then solid inner core
There has been a hypothesis named “The Hollow Earth Hypothesis.” According to this, Earth is
either hollow from inside or contains an interior space. However, this has been disapproved by
the scientific community since the 18th century. In the mythology in ancient times, there has been
a concept of subterranean land. This was mentioned as after life, Greek underworld, Budhism
believe and Christian Hell concepts previously.
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Later on in 17th and 18th centuries, the Edmond Halley's hypothesis was introduced in 1692. This
Hypothesis stated that earth is a hollow shell approximately 800 km thick having two inner shells
and the innermost core that is comparable to diameters of the planets Venus, Mars, and Mercury.
Each sphere has individual magnetic pole and rotates at a different speed. This concept was built
to explain the varying compass readings. The atmosphere inside was supposed to be luminous
and inhabited as well. On the other hand, De camp and Ley proposed that there is an inner earth
civilization having an interior sun that provides light. Sir John Leslie suggested two central suns,
Pluto and Proserpine. In 1781 Le Clerc Milfort journeyed to several caverns near Red River. He
proposed that Creek Indian Ancestors emerged from caverns to surface of Earth.
John Cleves Symmes Jr proposed the idea that Earth is a hollow shell having 4 inner shells, each
of these opening at both poles. He gave the idea of North Pole hole. Several authors later on
wrote books on his concept. One of these is Symmes’ Theory of Concentric Spheres (1826).
In twentieth century, William Reed wrote Phantom of the Poles. He said earth being hollow but
without inner shells or suns. One of the Spiritualist writer lady Paget predicted in her book that in
21st century, entrance to the subterranean kingdom will be discovered. William Fairfield Warren
wrote in his book paradise found that the human life arise from Hyperborea, a continent in
Arctic. On the contrary, Marshel Gardner believed that the Mongolians and Eskimos came from
interior of Earth from the North Pole and supported the idea of interior sun.
In 1940, George Papashvily wrote a novel Anything Can Happen. He mentioned that there are
caverns in the Caucasus Mountains containing human skeletons with heads as big as bushel
baskets." And there is a tunnel leading to center of earth. Other novelists also mentioned that
interconnected underground chambers exist beneath the Himalayas of Tibbet. Dr. Raymond
Bernard (1964) wrote about UFOs coming from inside the earth. Bernard said that after the
bombings of Hiroshima and Nagasaki, radioactive air might have entered into interior of earth
and flying saucers came out for their defense. There has also been an idea called Shaver Mystery
proposing honeycomb of caves and tunnel systems inside the earth.
In 21st century (2011) Hontariovalens and Paul Veneti made a video Lazeria Map Collection
about older maps of Arctic region and North pole North pole was said to have a 100 miles wide
canyon where north-flowing rivers are drained into central Earth.
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In contrary to above mentioned concepts, the seismic wave picture of Earth is quite different
from the hollow earth theory. Proposing that interior of earth (mantle and core) has many layers
of various elements and molten rocks. Also another concept against hollow earth being the
gravity saying that due to force of gravity the larger objects clump together and create solid, nonhollow spheres. The hollowness doesn’t go in favor of energetic sense. If the earth would have
been a hollow object, it could not have been able to maintain its hydrostatic equilibrium and
would have collapsed. The person on inside of earth would be almost weightless and won’t be
able to stand. As demonstrated by Newton in his Shell Theorem. Though Earth isn’t a perfect
sphere, a small gravitational force may arise. Also forces from moon and earths itself rotation
may play pull a person outwards from interior of earth. In addition, the mass of Earth and extent
of its gravitational force doesn’t go in favor of hollow earth theory. If the earth would have been
hollow, its gravity might not have been of this extent. The Sg-3 borehole is the deepest one
drilled to date. It is 12.3 km deep and is part of Soviet kola superdeep borehole project.
The Interior of the Earth
The study of the Earth's surface and interior is the domain of geology. We know little directly
about the interior of the Earth. Most of our information in that regard has come from seismic
waves, which are vibrations in the body of the Earth. There have been different indirect
evidences to give us information about the interior of earth as discussed below.
1. Rocks lying beside the magma chambers and the conduits mostly lie loosen up and in a
frozen state within the igneous rocks. These are called as Xenoliths. Sometimes Xenoliths
arise from the mantle as a result of volcanic eruptions. And in this way we can directly
visualize the mantle.
2. Since the 19th century, there have been sverel surveys conducted in India. These surveys
showed the reaults that there have been unexpectedly larger errors noted due to the influence
of the gravitational force of Himalayas. So the gravitational anamolies can be used to look
for tha variation in density within the earth.
3. Third evidence is the meteorites. Three types of materials that fall on earth include
chondrites, stony meteorites and iron meteorites. Chondrites are an undifferentiated material
from the solar nebula. It comprises of rocky as well as metallic substances. Most of the
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meteorites are classifies as chondrites. Next class, stony meteorites are the differentiated
material and comprise of silicate matter. Another type, iron meteorite mostly comprise of
metals like iron and nickel. They usually have the interlocked crystals of these. We can
conclude from this that there would have been an event leading to formation of rocky and
metallic bodies from the planetary bodies. Sometimes we also find the stony iron meteorite
that might have arisen from the mantle and core of the planetary body.
We can conclude that Xenoliths arise from the moderate depth in the mantle. However, the
meteorites don’t have their origin from earth. The seismic waves do allow the direct
exploration of the interior of earth and can be used as a direct evidence to study interior of
Earthquake waves were studied earlier as P and S waves. These arrive at a given location at
different timings. Possible reason for the difference in timings would be the reflection and
refraction phenomena while there passage from various media having variable properties.
Quite a large number od extra seismic waves were studied. This indicates that earth
comprises of several regions with variable physical properties. Initially with the help of these
waves, crust and mantle were differentiated.
The shadow zone was discovered next, that is the liquid core. The S and P waves are
1. S waves emerge between the 0-104 degrees angles measured from the center of earth.
They are unable to pass between 104-180 degree angles. As liquids don’t allow the
passage of S waves, so in the area of liquids, shadow is cast.
2. P waves moves faster from solids as compared to liquids. These cast shadow between
105-140 degree angles. Thus indicating their refraction or slowing down at a deeper
interface. Thus proving that the core of earth is liquid in nature.
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The depth and radius of the outer core can be assessed by geometrical formula. Depth is
calculated to be 2880 km. It has been observed that the P waves speed up at the depth of 5100
km thus indicating that core comprises of outer liquid part and inner solid part.
Figure 1: Showing Earth internal Structure Observed by Seismic Waves
The reflected waves are used to calculate the depth. If we know the velocity, we can calculate the
depth by measuring the time taken by reflected wave to reach to the surface.
The relationship of depth and velocity:
Charts have been constructed through observations. The relationship between depth and speed of
seismic waves has been studied by these charts. Increase in velocity is related to increase in
depth in the mantle. However in the outer core it decreases in terms of P waves. In this way it
approaches the speed comparable to outer mantle in the inner core.
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there are higher number of igneous rocks is seen in the crust. According to laboratory studies,
speeds of seismic waves through various materials have been observed. The rocks rich in silicon
and aluminum comprise most of the rocks. Here the P waves usually propogate at approx. 7km/s.
The seismologist, Andrija Mohorovicic in 1910 calculated the depth at which sudden
discontinuity of seismic waves is observed. Here the waves rapidly accelerate at the speed of
8km/s. this is called Mohorovic discontinuity. This transition zone lies between the crust and
mantle. This seismic discontinuity is now known as the Moho. It is the boundary between the
felsic/mafic crust with seismic velocity around 6 km/sec and the denser ultramafic mantle with
seismic velocity around 8 km/sec. The depth to the Moho beneath the continents averages around
35 km but ranges from around 20 km to 70 km. The Moho beneath the oceans is usually about 7
km below the seafloor (i.e., ocean crust is about 7 km thick).
Seismic stations within about 200 km of a continental earthquake (or other seismic disturbance
such as a dynamite blast) report travel times that increase in a regular fashion with distance from
the source. But beyond 200 km the seismic waves arrive sooner than expected, forming a break
in the travel time vs. distance curve. Mohorovicic (1909) interpreted this to mean that the seismic
waves recorded beyond 200 km from the earthquake source had passed through a lower layer
with significantly higher seismic velocity.
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With the help of this information, seismic waves can be used to plot the thickness of the crust.
Ocean is assesses to be 5km, continental region average 40 km, larger mountain ranges approx.
Regarding mantle, following points should be considered. The upper part of lithosphere
comprises of mantle. Here there is rapid propogation of waves. The asthenosphere lies below it
and it is the area of partial melting and ductile flow. In this region, waves are comparatively
slowed down. Below this region, pressure and the density further rises, that leads to acceleration
of the waves.
Seismic velocities tend to gradually increase with depth in the mantle due to the increasing
pressure, and therefore density, with depth. However, seismic waves recorded at distances
corresponding to depths of around 100 km to 250 km arrive later than expected indicating a zone
of low seismic wave velocity. Furthermore, while both the P and S waves travel more slowly, the
S waves are attenuated or weakened. This is interpreted to be a zone that is partially molten,
probably one percent or less (i.e., greater than 99 percent solid). Alternatively, it may simply
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represent a zone where the mantle is very close to its melting point for that depth and pressure
that it is very "soft." Then this represents a zone of weakness in the upper mantle. This zone is
called the asthenosphere or "weak sphere."
The asthenosphere separates the strong, solid rock of the uppermost mantle and crust above from
the remainder of the strong, solid mantle below. The combination of uppermost mantle and crust
above the asthenosphere is called the lithosphere. The lithosphere is free to move (glide) over
the weak asthenosphere. The tectonic plates are, in fact, lithospheric plates.
Below the low velocity zone are a couple of seismic discontinuities at which seismic velocities
increase. Theoretical analyses and laboratory experiments show that at these depths (pressures)
ultramafic silicates will change phase (atomic packing structure or crystalline structure) from the
crystalline structure of olivine to tighter packing structures. A discontinuity at around 670 km
depth is particularly distinct. The 670 km discontinuity results from the change of spinel
structure to the perovskite crystalline structure which remains stable to the base of the mantle.
Perovskite (same chemical formula as olivine) is then the most abundant silicate mineral in the
Earth. The 670 km discontinuity is thought to represents a major boundary separating a less
dense upper mantle from a denser lower mantle.
Core Mantle Boundary:
At the bottom enigmatic zone (20 km) of the mantle there is partial melting. This is proved by
the deceleration of the S waves in this region. There might be a source of hot-spot volcanism
caused by the plumes of hot rocks. Seismic waves recorded at increasing distances from an
earthquake indicate that seismic velocities gradually increase with depth in the mantle
(exceptions: see Low Velocity Zone and 670 km Discontinuity above). However, at arc distances
of between about 104° and 140° no P waves are recorded. Furthermore, no S waves are record
beyond about 104°. Gutenberg (1914) explained this as the result of a molten core beginning at a
depth of around 2900 km. Shear waves could not penetrate this molten layer and P waves would
be severely slowed and refracted (bent).
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Inner and Outer Core Boundary:
Between 143° and 180° from an earthquake another refraction is recognized (Lehman, 1936)
resulting from a sudden increase in P wave velocities at a depth of 5150 km. This velocity
increase is consistent with a change from a molten outer core to a solid inner core.
There is an outer core (liquid) and inner core (solid). While keeping in view the mass of earth,
one can assess its contents to be heavy. Also possibly these are among the commonly seen in
Solar system. The contents should also be compatible, compact ions that have the tendency to
sustain so much pressure. Uranium being a larger ion, has the tendency to be forced towards
surface,thus crust is rich in it. However, iron being the heaviest element can be assessed by tests
of wave propagation via compressed iron and study of iron meteorites.
Since the core makes up about one-third of the Earth's mass it must be a material that is common
in the solar system. It must account for the observed seismic velocities. It should also be a
material with magnetic properties to account for the Earth's magnetic field. Iron is the obvious
Magnetic Properties of Earth:
The liquid outer core of earth is responsible for the magnetic field around the earth. The currants,
orientation and the intensity of magnetic fields vary over time. A more serious hypothesis
considered the Earth or some solid layer within the Earth to be made of iron or other magnetic
material forming a permanent magnet. There are two major problems with this hypothesis. First,
it became apparent that the magnetic field drifts over time; the magnetic poles move. Second,
magnetic minerals only retain a permanent magnetism below their Curie temperature (e.g.,
580°C for magnetite). Most of the Earth's interior is hotter than all known Curie temperatures
and cooler crustal rocks just don't contain enough magnetic content to account for the magnetic
field and crustal magnetization is very heterogeneous in any case.
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The discovery of the liquid outer core allowed another hypothesis: the geodynamo. Iron, whether
liquid or solid, is a conductor of electricity. Electric currents would therefore flow in molten iron.
Moving a flowing electric current generates a magnetic field at a right angle to the electric
current direction (basic physics of electromagnetism). The molten outer core convects as a means
of releasing heat. This convective motion would displace the flowing electric currents thereby
generating magnetic fields. The magnetic field is oriented around the axis of rotation of the Earth
because the effects of the Earth's rotation on the moving fluid (coriolis force).
The deepest hole drilled to date is the SG-3 borehole which is 12.3 km (7.6 mi) deep, part of
the Soviet Kola Superdeep Borehole project; thus, visual knowledge of the Earth's structure
extends that far.