Interior of the Earth

Understanding the internal structure of the Earth is essential to grasp various geological phenomena like earthquakes, volcanoes, mountain formation, and plate tectonics. Although direct access to the earth’s core is impossible, geologists and geophysicists have developed a fairly accurate model of the earth’s interior using various scientific tools and observational data.

Sources of Knowledge about Earth’s Interior

Knowledge about the Earth’s interior comes from both direct and indirect sources:

Direct Sources

• Mining and drilling operations allow us to study the earth’s surface and crustal materials. However, the deepest drill has barely scratched 12 km of the crust.
• Volcanic eruptions eject magma and gases from deep within the Earth. The composition of these materials offers clues about subsurface layers.

Indirect Sources

These form the primary basis of our understanding:

• Seismic waves: The speed and path of primary (P) and secondary (S) waves during earthquakes help identify the nature and composition of layers. Shadow zones offer clues about the state of matter in different zones.
• Gravitational variations: Changes in gravity across the earth’s surface suggest differences in density and material mass within.
• Magnetic field data: The Earth’s magnetic properties hint at metallic materials in the outer core.
• Meteors: Since meteorites share composition with early Earth, their study provides indirect evidence of Earth’s interior.
• Temperature and pressure gradients: The increasing temperature and pressure with depth help estimate the physical and chemical states of Earth’s layers.

Structure of the Earth’s Interior

The Earth’s internal structure is layered, composed of three concentric shells:

1. Crust

• The outermost and thinnest layer of the Earth, with a thickness ranging from 5 km under oceans to 70 km beneath mountain ranges.
• Composed primarily of igneous and metamorphic rocks, with a surface layer of sedimentary rocks.
• The continental crust is thicker and granitic in composition (rich in silica and aluminium – Sial), whereas the oceanic crust is thinner and basaltic (rich in silica and magnesium – Sima).
• The average density of the crust ranges from 2.7 to 3.0 g/cm³.
• Crust forms only about 0.5% of Earth’s volume but plays a crucial role in sustaining life.
• The Conrad Discontinuity separates the upper and lower crust.
• The Mohorovičić Discontinuity (Moho) marks the boundary between the crust and the mantle.

2. Mantle

• Extends from the Moho discontinuity to about 2900 km depth.
• Constitutes about 82% of the Earth’s volume and 65% of its mass.
• Composed primarily of silicate rocks rich in iron and magnesium, such as olivine and pyroxene.
• Divided into:
• Upper Mantle (up to 660 km)
• Includes the asthenosphere (around 100–400 km), a partially molten, ductile layer crucial for plate movement.
• Above the asthenosphere is the lithosphere, which includes the crust and the rigid uppermost mantle.
• Lower Mantle (from 660 km to 2900 km)
• Known for high pressure and temperature but remains in a solid state due to intense pressure.
• Repetti Discontinuity separates the upper and lower mantle.
• Gutenberg Discontinuity at 2900 km separates the mantle from the outer core.

3. Core

The Earth’s core is made mostly of iron and nickel and is divided into two distinct zones:

Outer Core

• Extends from 2900 km to 5100 km.
• Composed of liquid iron and nickel.
• The movement of the liquid outer core generates Earth’s magnetic field.
• It is the only layer that is truly liquid.

Inner Core

• Extends from 5100 km to 6371 km, the Earth’s center.
• Despite the extreme temperatures (comparable to the surface of the Sun), the inner core is solid due to immense pressure.
• Composed mainly of solid iron with some nickel.
• The Lehmann Discontinuity separates the inner and outer cores.
• Recent studies suggest the inner core may be slowly growing as the outer core cools.

Chemical Composition of the Earth (By Weight)

Element	Percentage (%)
Oxygen	46.60%
Silicon	27.72%
Aluminium	8.13%
Iron	5.00%
Calcium	3.63%
Sodium	2.83%
Potassium	2.59%
Magnesium	2.09%
Others	1.41%

Oxygen and silicon together constitute nearly 75% of Earth’s crust, forming silicate minerals that dominate rock composition.

The Four Domains of the Earth

The Earth’s surface represents a unique zone where major environmental components interact and overlap:

1. Lithosphere

• The solid portion of the Earth, including the crust and the rigid upper mantle.
• Comprises rocks, mountains, plains, and soil that support terrestrial life.

2. Atmosphere

• The gaseous envelope surrounding Earth, held by gravity.
• Extends up to about 1600 km and divided into five layers:
  • Troposphere
  • Stratosphere
  • Mesosphere
  • Thermosphere
  • Exosphere
• Composed primarily of nitrogen (78%) and oxygen (21%), with trace amounts of carbon dioxide and argon.
• Carbon dioxide, although present in small quantity (~0.03%), plays a vital role in heat retention and photosynthesis.

3. Hydrosphere

• Covers about 71% of Earth’s surface and includes oceans, seas, rivers, lakes, glaciers, groundwater, and atmospheric water vapor.
• 97% of Earth’s water is in oceans (too saline for consumption), with the remainder in ice caps, underground reservoirs, and as freshwater.

4. Biosphere

• The life-supporting zone where land (lithosphere), water (hydrosphere), and air (atmosphere) interact.
• Includes all forms of life, from microorganisms to complex plants and animals.
• The biosphere is fragile, constantly influenced by both natural and anthropogenic activities.

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Subject: Geography

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