INTERIOR OF THE EARTH

Introduction

The interior of the Earth cannot be directly observed because no human can reach the centre of the Earth. Most knowledge regarding the Earth’s interior is obtained through indirect evidences, estimates and scientific inferences. The surface configuration of the Earth is mainly shaped by both endogenic processes and exogenic processes. Understanding the interior is important to explain phenomena such as earthquakes, tsunamis, volcanic eruptions, and landscape development.The Earth has different layers extending from the crust to the core, each having distinct characteristics.

SOURCES OF INFORMATION ABOUT THE INTERIOR OF THE EARTH

The Earth’s radius is approximately 6,370 km. Since direct access to the deep interior is impossible, scientists depend upon both direct and indirect sources of information.

DIRECT SOURCES OF INFORMATION

Surface Rocks and Mining

The most accessible materials are the surface rocks and rocks obtained from mining areas.

• Gold mines in South Africa extend to depths of about 3–4 km.
• Beyond such depths, temperature becomes extremely high.
• Mining activities help scientists understand crustal conditions.

Deep Drilling Projects

Scientists conduct deep drilling projects to study crustal layers.Major projects include:

• The deepest drilling at Kola in the Arctic Ocean has reached about 12 km depth.
• Materials collected from various depths are analysed in laboratories.

Volcanic Eruptions

Volcanic eruptions provide direct information regarding Earth’s interior.

• During eruptions, magma reaches the surface.
• The erupted materials are analysed scientifically.
• However, the exact depth of magma origin remains difficult to determine.

INDIRECT SOURCES OF INFORMATION

Temperature, Pressure and Density

Mining and drilling activities show that:

• Temperature increases with depth.
• Pressure increases with depth.
• Density of materials also increases towards the interior.

Scientists estimate the rate of increase and infer the conditions prevailing inside the Earth.

Meteors

Meteors provide important indirect information.

• Meteoric materials are similar to Earth materials.
• Their structure and composition resemble Earth’s internal materials.
• Hence, they help in understanding the Earth’s interior composition.

Gravitation

The gravitational force (g) varies across Earth.

• Gravity is greater near the poles.
• Gravity is lower near the equator.

This variation occurs because:

• The equatorial radius is larger than the polar radius.
• Mass distribution within Earth is uneven.

Gravity Anomaly

The difference between expected and observed gravity values is called gravity anomaly.It provides information regarding:

• distribution of mass,
• density variations,
• crustal structure.

Magnetic Surveys

Magnetic surveys help identify:

• distribution of magnetic materials,
• crustal composition,
• variation in rock properties.

Seismic Activity

Seismic activity is the most important indirect source of information regarding Earth’s interior.The study of seismic waves provides a detailed understanding of the layered structure of the Earth.

EARTHQUAKES

Meaning

An earthquake is the shaking of the Earth caused by sudden release of energy within the Earth.This energy generates waves travelling in all directions.

Cause of Earthquakes

Earthquakes occur mainly due to movement along faults.

Fault

A fault is a sharp break in crustal rocks.

• Rocks on opposite sides tend to move.
• Friction prevents movement initially.
• Pressure and stress gradually deform rocks.
• When stress exceeds frictional resistance, rocks suddenly slide.
• This sudden movement releases energy.

Focus and Epicentre

• The epicentre experiences earthquake waves first.

EARTHQUAKE WAVES

• Earthquakes mainly occur within the lithosphere.
• A seismograph records earthquake waves.

The recorded waves show distinct sections indicating different wave types.

STRUCTURE OF THE EARTH

Lithosphere

• Includes the crust and uppermost mantle.
• Thickness ranges from 10–200 km.
• It is the rigid outer layer of the Earth.

Mantle

• Located below the crust.
• Has higher density than crust.

Asthenosphere

The mantle contains a weaker zone called the asthenosphere.

• Magma originates from this zone.
• It allows movement of molten materials.

Lower Mantle

• Extends below the asthenosphere.
• Exists in solid state.

THE CORE

The existence of the core was understood through seismic wave studies.

Core-Mantle Boundary

• Located at about 2,900 km depth.

Outer Core

• Exists in liquid state.

Inner Core

• Exists in solid state.

Composition of Core

The core mainly contains:

• Nickel
• Iron

Hence, it is also called the Nife Layer.

VOLCANOES

Meaning

A volcano is an opening through which:

• gases,
• ashes,
• molten rock material (lava)

escape onto the Earth’s surface.

Active Volcano

A volcano is termed active if eruptions are occurring currently or occurred in the recent past.

Magma and Lava

Materials Released During Eruptions

Volcanic eruptions release:

• lava flows,
• pyroclastic debris,
• volcanic bombs,
• ash,
• dust,
• gases.

Important Gases

• Nitrogen compounds
• Sulphur compounds
• Chlorine
• Hydrogen
• Argon

TYPES OF VOLCANOES

Shield Volcanoes

• Largest volcanoes on Earth.
• Hawaiian volcanoes are classic examples.
• Mainly composed of basaltic lava.
• Lava is highly fluid.
• Slopes are gentle and not steep.
• Generally show low explosivity.
• Water entering vents may increase explosiveness.

Cinder Cone

Lava fountains throw material around the vent forming a cinder cone.

Composite Volcanoes

Characteristics:

• Eruptions involve cooler and viscous lava.
• Highly explosive eruptions occur.
• Large quantities of ash and pyroclastic materials are emitted.
• Materials accumulate in layers near vents.

This layered structure gives them a composite appearance.

Calderas

• Among the most explosive volcanoes.
• Eruptions may collapse the volcanic structure itself.
• The resulting depression is called a caldera.
• Indicates presence of a huge nearby magma chamber.

Flood Basalt Provinces

These volcanoes release highly fluid lava over extensive areas.

Features

• Lava spreads over thousands of square kilometres.
• Individual flows may extend hundreds of kilometres.
• Thickness of flows may exceed 50 m.

Example: Deccan Traps

• Located mainly over the Maharashtra Plateau.
• Originally covered a much larger area.

Mid-Ocean Ridge Volcanoes

• Occur in oceanic regions.
• Associated with mid-oceanic ridges extending over 70,000 km.
• Frequent eruptions occur in ridge centres.

VOLCANIC LANDFORMS

Intrusive Forms

When lava cools below the Earth’s surface, it forms intrusive igneous rocks.

Types of Igneous Rocks

BATHOLITHS

• Large dome-shaped intrusive bodies.
• Formed by cooling of magma at great depths.
• Usually granitic in composition.
• Become exposed after denudation removes overlying rocks.
• Represent cooled magma chambers.

LACOLITHS

Characteristics:

• Dome-shaped intrusive bodies.
• Have flat bases.
• Connected through pipe-like conduits below.

Example

• Granitic domal hills of the Karnataka Plateau.

LAPOLITHS

• Saucer-shaped intrusive bodies.
• Concave towards the sky.

PHACOLITHS

• Wavy intrusive masses.
• Occur at:
  • base of synclines,
  • top of anticlines.
• Connected to magma chambers below.

SILLS AND SHEETS

DYKES

• Formed when lava solidifies within cracks and fissures.
• Usually vertical or nearly vertical structures.
• Appear as wall-like formations.

Importance

• Common in western Maharashtra.
• Considered feeders of the Deccan Trap eruptions.