Direct & Indirect Sources for Understanding the Earth's Interior

Importance of understanding the Interior of The Earth

Understanding the structure of the earth’s interior (crust, mantle, core) and various forces (heat, seismic waves) emanating from it is essential to understand

the evolution of the earth’s surface, its current shape and its future.
the geophysical phenomenon like volcanism, earthquakes, etc.
earth’s magnetic field
the internal structure of various solar system objects
the evolution and present composition of the atmosphere
for mineral exploration

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Earth’s surface

Many different geological processes shape the Earth’s surface.
The forces that cause these processes come from both above and beneath the Earth’s surface.
Processes that are caused by forces from within the Earth are endogenous processes (Endo meaning “in”).
By contrast, exogenous processes (Exo meaning “out”) come from forces on or above the Earth’s surface.
The major geological features of the earth’s surface like mountains, plateaus, lakes are mostly a result of endogenous processes like folding, faulting that are driven by forces from inside the earth.

Geophysical phenomenon like volcanism, earthquakes

The forces that cause catastrophic events like earthquakes, volcanic eruptions come from deep below the earth’s surface.
For example, earthquakes occur due to the movement of the tectonic plates and the energy required for this movement is supplied by the conventional currents in the mantle.
Similarly, volcanism occurs through the vents and fissures created by the tectonic movements.

Earth’s magnetic field

Earth’s magnetic field is a result of convection currents in the outer core of the earth.
Life on earth would not have been possible if not for the earth’s magnetic field which protects the earth’s atmosphere from the harmful solar wind.

The internal structure of various solar system objects

The entire solar system was formed from a single nebular cloud, and the process of the formation of every solar system object is believed to be similar to that of the earth.

Evolution and present composition of the atmosphere

For life to flourish on the surface of the earth, the atmosphere needs to have essential components like oxygen for respiration, CO₂ and other greenhouse gases to maintain the temperature on the surface, ozone to protect life from ultraviolet radiation and the right atmospheric pressure.
All these components of the earth’s atmosphere owe their existence to the volcanic eruptions that unlock them from the earth’s interior.

Mineral exploration

Understanding volcanic activity and the nature of rocks is essential for mineral exploration.
Most of the minerals like diamonds (form at a depth of 150-800 km in the mantle) that occur on the earth’s surface are formed deep below the earth’s surface. They are brought to the surface by volcanic activity.

Direct Sources of information about the Earth’s Interior

Deep earth mining and drilling reveal the nature of rocks deep down the surface.
But as mining and drilling are not practically possible beyond a certain depth, they don’t reveal much information about the earth’s interior.
Mponeng gold mine (deepest mine in the world) and TauTona gold mine (second deepest mine in the world) in South Africa are deepest mines reaching to a depth of only 3.9 km.
And the deepest drilling is only about 12 km deep hole bored by the Soviet Union in the 1970s over the Kola Peninsula.

The Kola Peninsula in north-west Russia. (TUBS, from Wikimedia Commons)

Volcanic eruption forms another source of obtaining direct information.

Indirect Sources of information about the Earth’s Interior

Increase in pressure and temperature with depth
Seismic waves
Meteorites
Gravitation
Magnetic field

Increase in pressure and temperature with depth

Gravitation and the diameter of the earth help in estimating pressure deep inside.
Volcanic eruptions and existence of hot springs, geysers etc. point to an interior which is very hot.

Sources of earth’s heat

Radioactive decay

The high temperature below the crust is attributed to the disintegration of the radioactive substances.
The nuclear decay happens primarily in the crust and the mantle.
Scientists believe that uranium could become sufficiently concentrated at the base of Earth’s mantle to ignite self-sustained nuclear fission, as in a human-made reactor.
The new measurements suggest radioactive decay provides more than half of Earth’s total heat.

Nuclear fusion doesn’t occur inside the earth. For nuclear fusion to occur there must be far more pressure and temperature inside the earth. The earth is not massive enough to cause such conditions.

Primordial heat

The rest is the heat left over from Earth’s formation known as the primordial heat.
Primordial heat is the kinetic energy transferred to Earth by external impacts of comets and meteorites and the subsequent effects (friction caused by sinking of heavy elements like Fe, rising light elements like Si) and latent heat of crystallisation released as the core solidified.

Tidal friction

The ocean tides are not the only effect of tidal forces (gravitational influence of the moon and the sun on earth; tides are explained in oceanography). The solid body of the Earth also bulges slightly in this way.
The daily flexing of the Earth (both solid body and the oceans) cause loss of energy of the Earth’s rotation, due to friction.
This energy goes into heat, leading to miniscule increase in the Earth’s internal temperature.
The loss of rotational energy means that the Earth is slowing down in its rotation rate, currently by about 0.002 seconds per century.

Seismic waves

They are the most important source available to understand the layered structure of the earth.
The velocity of seismic waves changes as they travel through materials with different elasticity and density.
The more elastic and denser the material is, the higher is the velocity.
They also undergo refection or refraction when they come across materials with different densities.
Earth’s internal structure can be understood by analysing the patterns of reflection, refraction and change in velocity of the seismic waves when they travel through it.

Meteorites

Meteorites and Earth are born from the same nebular cloud. Thus, they are likely to have a similar internal structure.
When meteoroids they fall to earth, their outer layer is burnt during their fall due to extreme friction and the inner core is exposed.
The heavy material composition of their cores confirms the similar composition of the inner core of the earth.

Gravitation

The gravitation force differs according to the mass of material. The uneven distribution of mass of material within the earth influences this value. Such a difference is called gravity anomaly.
Gravity anomalies give us information about the distribution of mass in the crust of the earth.

Magnetic field

The geodynamo effect helps scientists understand what’s happening inside the Earth’s core. Shifts in the magnetic field also provide clues to the inaccessible iron core.