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.

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.