Interior Structure of Earth

Interior Structure of the Earth – UPSC Geography Notes

Understanding the Earth’s interior through direct observation poses significant challenges due to the immense size of the planet and the dynamic composition within. Accessing the Earth’s core, situated approximately 6,370 kilometers beneath the surface, remains an unattainable feat for human exploration. Consequently, our direct observations of the Earth’s interior are confined to a shallow depth of only a few kilometers, primarily facilitated by mining and drilling activities.

The formidable escalation in temperature beneath the Earth’s crust serves as a predominant deterrent, creating a boundary that restricts direct explorations into the Earth’s depths. Despite these constraints, scientists have developed a comprehensive understanding of the Earth’s internal structure through a combination of direct and indirect data sources.

Sources of Information about the interior of the earth

Direct Sources

  • Rocks from the mining area
  • Volcanic eruptions

Indirect Sources

  • Examination of temperature and pressure gradients from the Earth’s surface towards its core
  • Meteorites, composed of materials akin to those found on Earth
  • Variances in gravitational force, stronger near the poles and weaker at the equator
  • Discrepancies in gravity measurements, denoted as gravity anomalies, providing insights into the composition of the Earth’s interior
  • Magnetic phenomena
  • Seismic waves, particularly the shadow zones of primary and secondary body waves, offering crucial data concerning the state of materials within the Earth.

Structure of the Earth’s Interior

The structure of the earth’s interior is fundamentally divided into three layers – crust, mantle and core.


  • The Earth’s crust, typically 8-40 kilometers thick, forms the planet’s outer solid layer.
  • It exhibits a brittle nature.
  • Approximately 1% of the Earth’s volume and 0.5% of its mass constitute the crust.
  • Oceanic crust measures about 5 kilometers thick, while continental crust is approximately 30 kilometers thick.
  • The primary constituents of the crust are Silica (Si) and Aluminium (Al), giving rise to the term SIAL (sometimes used to refer to the lithosphere).
  • The average density of crustal materials is 3 grams per cubic centimeter.
  • The boundary between the hydrosphere and the crust is termed the Conrad Discontinuity.


  • Beyond the crust lies the mantle, approximately 2900 kilometers thick.
  • The Mohorovich Discontinuity, or Moho, marks the boundary between the crust and the mantle.
  • The mantle constitutes around 84% of the Earth’s volume and 67% of its mass.
  • Silicon and Magnesium are the primary constituents of the mantle, leading to its alternative name, SIMA.
  • The mantle’s density ranges from 3.3 to 5.4 grams per cubic centimeter, higher than that of the crust.
  • The lithosphere comprises the uppermost solid part of the mantle and the entire crust.
  • Beneath the lithosphere, the asthenosphere, ranging from 80 to 200 kilometers, is a highly viscous and mechanically weak region of the upper mantle, crucial for plate tectonics and magma generation.
  • The Repetti Discontinuity marks the boundary between the upper mantle and the lower mantle.
  • The Mesosphere refers to the mantle section located just below the lithosphere and asthenosphere but above the core.


  • Encircling the Earth’s center, the core is its innermost layer.
  • Guttenberg’s Discontinuity separates the core from the mantle.
  • The core, predominantly composed of iron (Fe) and nickel (Ni), is also known as NIFE.
  • Approximately 15% of the Earth’s volume and 32.5% of its mass consist of the core.
  • With a density ranging from 9.5 to 14.5 grams per cubic centimeter, the core is the densest layer of the Earth.
  • The core comprises two sub-layers: the solid inner core and the liquid (or semi-liquid) outer core.
  • The Lehmann Discontinuity denotes the boundary between the upper core and the lower core.
  • Barysphere is a term occasionally used to refer to either the Earth’s core or the entire interior.

Temperature, Pressure and Density of the Earth’s Interior


  • Mines and deep wells demonstrate a temperature rise with increasing depth.
  • Molten lava eruptions and other evidence from the Earth’s interior support the notion of increasing temperatures towards the Earth’s center.
  • Observations indicate that the temperature increase rate is non-uniform from the surface to the Earth’s center, varying in speed at different locations.
  • Initially, the temperature increases by an average of 10 degrees Celsius for every 32-meter depth increase.
  • In the upper 100 kilometers, the temperature rises at a rate of 120 degrees Celsius per kilometer, followed by 200 degrees Celsius per kilometer in the subsequent 300 kilometers. However, at greater depths, this rate diminishes to a mere 100 degrees Celsius per kilometer.
  • This suggests that the temperature increase rate beneath the surface decreases towards the center, distinct from the increasing temperature gradient.
  • The estimated temperature at the center ranges from 30,000 to 50,000 degrees Celsius, potentially even higher due to chemical reactions under high-pressure conditions.
  • Despite such extreme temperatures, materials at the Earth’s core remain in a solid state, owing to the immense pressure exerted by the overlying materials.


  • Similar to the temperature, pressure also intensifies from the Earth’s surface towards its center, attributed to the substantial weight of overlying materials such as rocks.
  • Deeper within the Earth, pressure reaches incredibly high levels, estimated to be approximately 3 to 4 million times greater than atmospheric pressure at sea level.
  • Elevated temperatures could cause materials to liquefy towards the Earth’s center. However, the extreme pressure conditions enable these molten substances to assume solid-like properties, likely rendering them in a plastic state.


  • The Earth’s layers experience an increasing density towards the center, owing to heightened pressure and the prevalence of heavier elements such as Nickel and Iron.
  • From the crust to the core, the average density progressively increases, reaching approximately 14.5 grams per cubic centimeter at the very center.


Q. What is the composition of the Earth’s inner core?

A. The Earth’s inner core is primarily composed of iron and nickel.

Q. How does the pressure and temperature change as one moves from the outer core to the inner core?

A. Both pressure and temperature increase with depth, leading to the solidification of the inner core under immense pressure despite high temperatures.

Q. What evidence supports the understanding of the solid inner core and the semi-liquid outer core?

A. Seismic wave studies indicate the behavior of these layers, with the propagation of S-waves revealing the solid nature of the inner core and the liquid outer core’s properties inferred from the behavior of P-waves.

Q. How do scientists estimate the temperature of the Earth’s inner core?

A. Scientists use models based on the Earth’s composition, behavior of seismic waves, and laboratory experiments to estimate the temperature of the Earth’s inner core.

Q. What are the main layers that make up the Earth’s structure?

A. The main layers include the crust, mantle, outer core, and inner core.

Q. What processes are responsible for the creation and movement of tectonic plates?

A. Tectonic plate movement is primarily driven by the convective currents in the mantle, caused by the heat generated from radioactive decay and residual heat from the Earth’s formation.

Q. How do the Earth’s layers interact to create the planet’s magnetic field?

A. The Earth’s magnetic field is generated by the movement of molten iron in the outer core, creating electric currents that produce the magnetic field.

Q. How do the properties of the Earth’s layers change with depth?

A. With increasing depth, the pressure and temperature increase, leading to changes in the state of materials from solid to semi-solid and liquid.

Q. What role do the Earth’s layers play in geological events such as earthquakes and volcanic eruptions?

A. Earthquakes and volcanic eruptions are often the result of interactions between the Earth’s layers, including the movement of tectonic plates, convection currents in the mantle, and the release of pressure and heat from the Earth’s interior.

Q. How does the Earth’s crust differ beneath the ocean and continents?

A. Oceanic crust is thinner and denser, primarily composed of basalt, while continental crust is thicker and less dense, composed mainly of granite.

Q. What are the primary elements that make up the Earth’s crust?

A. The Earth’s crust primarily consists of oxygen, silicon, aluminum, iron, calcium, sodium, and potassium.

Q. How does the Earth’s geography impact weather patterns and climate?

A. The Earth’s geography influences weather patterns and climate by affecting atmospheric circulation, moisture distribution, and the formation of weather systems.

Q. What are the major geographical features of the Earth, such as mountains, valleys, and plains?

A. Major geographical features include mountains such as the Himalayas, valleys such as the Grand Canyon, and plains such as the Great Plains.

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