INTERIOR OF THE EARTH

Evolution of the Earth’s Layered Structure

Earth is composed of multiple layers each with distinct physical and chemical properties. Our understanding of Earth interior is based on direct sources like mining and volcanic eruptions as well as indirect sources such as seismic waves, gravity anomalies, magnetic field studies.

The structure of the Earth as we know it today evolved over billions of years through a series of key geological and chemical processes. This evolution can be divided into several major phases:

1. Formation of Earth (4.6 Billion Years Ago)

• The Earth was formed from the solar nebula a rotating cloud of gas and dust left over from the Sun’s formation.
• Gravity pulled this material together leading to accretion of planetesimals(small rock-like bodies).
• Over time these planetesimals collided and merged generating enormous amounts of heat due to:
  • Kinetic energy of collisions
  • Compression under gravity
  • Radioactive decay of short lived isotopes(e.g., aluminum-26)
• The result was a molten homogeneous Earth with no distinct layers.

2. Differentiation (4.5 Billion Years Ago)

• As Earth remained in a molten state due to intense heat materials began to separate based on density through a process called gravitational differentiation:
  • Heavier elements (Iron & Nickel) sank to the center, forming the core.
  • Lighter silicate materials (oxygen, silicon, aluminum, magnesium) rose to form the mantle and crust.
• This differentiation created a layered Earth with a dense metallic core and a less dense rocky exterior.
• This stage also established the early magnetic field as liquid iron in the outer core started moving generating a dynamo effect.

3. Volcanic Outgassing (4 Billion Years Ago)

• As the Earth’s surface began to cool volcanic activity became intense due to the trapped heat inside the planet.
• These volcanoes released gases(CO₂, H₂O vapor, methane, ammonia, nitrogen, hydrogen) leading to the formation of the early atmosphere.
• There was no oxygen at this stage making the atmosphere toxic for modern life.
• Water vapor condensed over time leading to the formation of oceans as Earth cooled.

4. Crust Formation (3.8 Billion Years Ago)

• The outermost layer of the Earth cooled and solidified forming the first crust.
• However this early crust was highly unstable and frequently destroyed and reformed by volcanic activity.
• Earth surface was dominated by magma oceans and early landmasses were small scattered volcanic islands.

5. Tectonic Activity and Continental Growth (3 Billion Years Ago – Present)

• Plate tectonics began as convection currents in the mantle moved the lithosphere.
• Small landmasses collided and merged forming the first stable continental crust.
• Over time supercontinents formed and broke apart in cycles (e.g. Rodinia (~1.1 billion years ago), Pangaea (~300 million years ago)).
• The process of subduction, volcanic activity and crustal recycling shaped the modern continents.

6. Oxygenation and Stabilization (2.5 Billion Years Ago – Present)

• Around 5 billion years ago cyanobacteria began producing oxygen through photosynthesis.
• This led to the Great Oxygenation Event (GOE) which transformed Earth’s atmosphere.
• The crust thickened and stabilized supporting the emergence of complex life.

Temperature Profile of Earth’s Interior

The temperature inside Earth increases with depth due to residual heat from planetary formation, radioactive decay and heat from the core. This temperature gradient known as geothermal gradient varies across different layers of the Earth.

Temperature Variation Across Earth’s Layers

Layer	Depth (km)	Temperature (°C)
Crust	0 – 35 km	~0°C to 900°C
Upper Mantle	35 – 660 km	~900°C to 1600°C
Lower Mantle	660 – 2900 km	~1600°C to 3700°C
Outer Core	2900 – 5150 km	~3700°C to 5000°C
Inner Core	5150 – 6371 km	~5000°C to 6000°C

Layers of Earth Surface explained by APTI PLUS

1. Crust

The crust is the Earth’s outermost layer comprising both oceanic and continental crusts. Though it represents only 1% of Earth’s total mass it holds nearly all known life. The crust is dynamic constantly shaped by geological processes like plate tectonics which both form and destroy crustal materials.

• Composition: The crust is primarily composed of igneous, metamorphic, sedimentary rocks.
  • Igneous rocks like granite (continental crust) and basalt (oceanic crust) form when magma cools.
  • Sedimentary rocks are formed by the accumulation of particles.
  • Metamorphic rocks are created by the alteration of existing rocks under high pressure and temperature.
• Density: Typically less than 2.7 g/cm³ making it the least dense layer of the Earth.
• Subtypes:
  • Oceanic Crust: Composed of mafic rocks (rich in magnesium and iron). It is thinner typically about 5-10 km thick and denser than continental crust.
  • Continental Crust: Made of felsic rocks (rich in aluminum and silicon) typically about 30-40 km thick and less dense than the oceanic crust enabling it to float higher on the mantle.
• Conrad Discontinuity: The boundary between the oceanic and continental crust.

Differences between Oceanic and Continental Crust

Feature	Oceanic Crust	Continental Crust
Composition	Mafic (Silicate + Magnesium, SiMa)	Felsic (Silicate + Aluminum, SiAl)
Mineralogy	Rich in iron and magnesium	Rich in silicon and oxygen
Thickness	~5-10 km	Up to 40 km
Density	2.9 g/cm³	2.7 g/cm³
Formation	Formed by magma cooling at the ocean floor	Formed by accumulation of sediments and rock melting
Dominant Rock Types	Basalt, Gabbro	Granite, Diorite, Sedimentary Rocks
Age	Less than 200 million years ago	Up to 3.5 billion years

 

• Seismic Behavior:
  • P-waves travel at 6 km/s in the crust.
  • S-waves confirm that the crust is solid.
• Temperature Range: 200–400°C at the base.

Major Discontinuity – Mohorovicic Discontinuity (Moho)

• Depth: ~30–50 km
• Separation: The crust from the mantle
• Seismic Effect: The P-wave velocity increases from 6 km/s to 8 km/s, indicating a sudden increase in density due to a change from silicate rocks to peridotite.

2. Mantle

The mantle extends from the Mohorovičić Discontinuity (Moho) marking the boundary with crust to about 2,900 km below the surface. It accounts for 84% of Earth’s volume and 67% of its mass.

• Composition: The mantle is mainly composed of silicate minerals like olivine, garnet, and pyroxene with significant amounts of magnesium oxide. It also contains aluminum, iron, calcium, sodium and potassium.
• Density: The mantle has density of around 9 g/cm³ increasing with depth due to increased pressure.
• Layer Structure:
  • Upper Mantle: Extends from the Moho to 410 km deep, characterized by solid rocks with semi plastic regions enabling tectonic movements.
  • Lower Mantle: Spans from 660 km to 2,900 km. This region is hotter, denser, less malleable compared to the upper mantle.
  • Transition Zone (410-660 km): The mantle’s transition zone has abundant water stored in crystal structures holding as much water as all Earth’s oceans combined.
• Key Discontinuities:
  • Mohorovičić Discontinuity (Moho): The boundary between the crust and the mantle where seismic wave velocities change.
  • Repetti Discontinuity: Marks the boundary between the upper and lower mantle.
  • D’’ (D-double prime) Layer: The boundary between the lower mantle and the outer core, which features complex material behaviors including iron-silicate mixtures.

General Properties

• Thickness: ~2,900 km
• Composition:
  • Upper Mantle: Olivine, Pyroxene, Garnet
  • Lower Mantle: Bridgmanite(Perovskite) and Ferropericlase
• Density:
  • Upper Mantle: 3.3–4.4 g/cm³
  • Lower Mantle: 4.4–5.6 g/cm³
• Temperature: 400°C (top) to ~4,000°C (base)
• Seismic Behavior: Both P-waves and S-waves increase in speed due to increasing rigidity.

Subdivisions of the Mantle

(a) Lithosphere (Rigid Upper Mantle & Crust):

• Depth: 0–100 km
• State: Solid and rigid
• Includes: Tectonic plates

(b) Asthenosphere (Plastic Layer, part of Upper Mantle):

• Depth: 100–410 km
• State: Semi-molten (partial melting)
• Seismic Effect: S-waves slow down indicating low rigidity.
• Importance: Responsible for plate tectonics due to convection currents driving plate movements.

(c) Transition Zone (410–660 km):

• Depth: 410–660 km
• Seismic Effect: A significant increase in P-wave and S-wave
• Mineral Changes:
  • Olivine transforms to Wadsleyite at 410 km.
  • Wadsleyite transforms to Ringwoodite at 520 km.
  • Ringwoodite transforms to Bridgmanite & Ferropericlase at 660 km.

(d) Lower Mantle (Mesosphere):

• Depth: 660–2,900 km
• Composition: Bridgmanite(Perovskite) and Ferropericlase
• Density: 4.4–5.6 g/cm³
• Seismic Effect: P-waves and S-waves speed up due to high pressure.

Gutenberg Discontinuity (~2,900 km):

• Seismic Effect: P-waves slow down (13.7 km/s → 8 km/s)and S-waves disappear confirming that the outer core is liquid.

3. Lithosphere

The lithosphere comprises crust and uppermost mantle forming the rigid outer shell of the Earth. It is rigid and cool compared to deeper layers.

• Thickness: Varies from 50 km under oceanic crust to about 300 km beneath orogenic (mountain forming) regions.
• Structure: The lithosphere is segmented into tectonic plates that float over the more ductile asthenosphere below.
• Role in Plate Tectonics: Lithospheric plates are in constant motion driven by forces like mantle convection. These plates interact at boundaries leading to phenomena like earthquakes, volcanoes and mountain formation.

4. Asthenosphere

The asthenosphere lies below the lithosphere and extends from 100 km to 350-650 km deep. It consists of semi molten rock that behaves plastically allowing the lithosphere to move over it.

• Properties: The asthenosphere is mechanically weak and ductile acting as a lubricating layer for the tectonic plates. It is also called Low Velocity Zone (LVZ) due to the slow seismic wave velocities observed in this region.
• Composition: Primarily composed of peridotite rock a mineral mix rich in olivine and pyroxene.
• Thickness: The asthenosphere’s thickness ranges from 180 km to 220 km depending on location.

5. Core

The Earth’s core is its innermost region composed almost entirely of iron (Fe) and nickel (Ni) making it extremely dense and hot. It is divided into two parts: the outer core (liquid) and the inner core (solid).

• Composition: The core consists primarily of iron and nickel but also contains sulfur, oxygen and silicon. The outer core is mainly liquid while the inner core is solid.
• Volume: The core makes up about 33% of Earth’s mass and 16% of its volume.

Outer Core

• Properties: The outer core is composed of molten iron and nickel and it generates Earth’s magnetic field via the dynamo process. The liquid state of the outer core causes the magnetic field by convection currents.
• Thickness: It extends from 2,900 km to 5,150 km below the surface and its temperature reaches around 6,000°C(comparable to the Sun’s surface temperature).
• Density: The outer core has a lower density than the inner core around 9 to 12.2 g/cm³.

Inner Core

• Properties: The inner core despite having a temperature greater than the melting point of iron, remains solid due to the immense pressure. It is composed mostly of iron with some nickel.
• Rotation: The inner core rotates slightly faster than the rest of the Earth contributing to its generation of Earth’s magnetic field.
• Growth: The inner core grows slowly over time as the planet cools. As it solidifies it releases latent heat and could contribute to the melting of parts of the outer core forming a convection cycle.
• Thickness: The inner core extends from 5,150 km to 6,370 km beneath the Earth’s surface.
• Density: The inner core has a density of around 8 to 13.1 g/cm³ significantly higher than the outer core.

Outer Core (Liquid)

• Depth: 2,900–5,100 km
• Composition: Liquid iron and nickel
• Density: 9.9–12.2 g/cm³
• Temperature: 4,000–5,700°C
• Seismic Effect: P-waves slow down due to the liquid state while S-waves do not travel confirming it’s liquid.
• Importance: The outer core generates the Earth’s magnetic field(Dynamo Effect).

Inner Core (Solid)

• Depth: 5,100–6,371 km
• Composition: Solid iron and nickel
• Density: 12.6–13.0 g/cm³
• Temperature: ~5,700°C
• Seismic Effect: P-waves speed up from 8 km/s to 11 km/s and S-waves reappear confirming that the inner core is solid.

Lehmann Discontinuity (~5,100 km)

• Seismic Effect: The P-waves accelerate and S-waves reappear suggesting that the inner core is rigid.

Discontinuities Inside the Earth

Discontinuities are boundaries inside the Earth where there is a sudden change in density, composition or state of matter. These discontinuities were discovered through the study of seismic waves, as they change speed and direction when passing through different layers.

Discontinuity	Depth (km)	Location	Seismic Effect	Discovery
Conrad	15–20	Upper & lower crust	Slight P-wave increase	Not globally present
Moho (Mohorovičić)	30–35 (continents) 5–10 (oceans)	Crust & Mantle	P-wave jumps from 6 km/s to 8 km/s	Andrija Mohorovičić (1909)
Repetti	660	Upper & Lower Mantle	P-waves & S-waves speed up	Transition zone
Gutenberg	2,900	Mantle & Outer Core	P-wave slows down, S-wave disappears	Beno Gutenberg (1913)
Lehmann	5,100	Outer Core & Inner Core	P-wave speeds up, S-wave reappears	Inge Lehmann (1936)

 

The above detailed explanation by APTI PLUS best IAS coaching in Bhubaneswar covers the structure of the Earth, discontinuities and  evolution of Earth layered structure.

Earth interior is a complex and dynamic system that plays crucial role in shaping geological processes like plate tectonics, volcanism and earthquakes. Understanding the composition and behavior of Earth layers is essential for subjects like Geography, Geology, Environmental Science.