Interior of the Earth Class 11 Geography Chapter 3 CBSE Notes 2025-26

The interior of the Earth cannot be observed directly, but it affects many natural processes and features on the Earth’s surface. Our understanding of Earth's internal structure is mainly built with the help of indirect evidences and a few direct observations, obtained from rocks, drilling operations, and some volcanic materials. The surface of the Earth, as well as the distribution and nature of continents and oceans, is strongly shaped by the activity inside the Earth.

Sources of Information About the Earth’s Interior

Scientists use both direct and indirect sources to learn about the Earth’s interior. Direct sources cover rock samples obtained from the surface and deep mines, with gold mines like those in South Africa reaching up to 3–4 km deep. Deep ocean drilling, such as the Kola Superdeep Borehole, has managed to drill as far as 12 km. Volcanic eruptions also bring up magma and rocks from inside the Earth, giving clues about deeper layers.

Indirect sources are much more significant for understanding Earth’s composition. With increasing depth, temperature, pressure, and density rise steadily, as seen in mining and drilling data. Meteorites, though extraterrestrial, have similar composition to Earth's interior. The variation in gravity at different locations on the Earth's surface indicates mass and density distribution. Magnetic surveys help reveal the distribution of magnetic materials, while seismic waves generated during earthquakes are the most important tool for mapping the Earth's internal structure.

Earthquakes and Seismic Waves

An earthquake is the sudden shaking of the ground caused by the release of energy from rocks breaking or moving along faults. The origin point inside the Earth where this energy is released is called the focus (or hypocentre), and the location directly above it on the surface is the epicentre. When rocks move past each other along faults, friction first prevents movement; when stress builds to overcome friction, a sudden movement occurs, creating seismic waves that travel in all directions.

There are two main types of earthquake waves—body waves and surface waves. Body waves start from the earthquake focus and travel through the Earth's interior, and are divided into Primary (P) waves and Secondary (S) waves. P-waves move fastest, are compressional, can travel through solids, liquids, and gases, and cause material particles to move in the direction of the wave. S-waves travel slower, can only move through solids, and move particles perpendicular to the direction of travel, creating crests and troughs. Surface waves are created when body waves reach and interact with surface rocks; they travel along the Earth’s crust and cause the most destruction during earthquakes.

Earthquake waves can reveal much about Earth's structure because of phenomena like the "shadow zone". S-waves do not pass through liquids, so their absence in certain regions on the globe (beyond 105° from the epicentre) proves that the outer core of the Earth is liquid. For P-waves, a shadow zone appears between 105° and 145° from the epicentre as they are refracted by the liquid outer core.

Types and Measurement of Earthquakes

There are various types of earthquakes:

• Tectonic earthquakes – caused by movement along faults and are the most common.
• Volcanic earthquakes – happen near volcanoes due to magma movement.
• Collapse earthquakes – due to roof collapse in underground mines.
• Explosion earthquakes – result from chemical or nuclear explosions.
• Reservoir induced earthquakes – result from the filling of large dams and reservoirs.

Earthquakes are measured using two main scales. The Richter Scale measures the magnitude, or the amount of energy released, and typically ranges from 0 to 10. The Mercalli Scale measures the intensity, that is, the effects and damage caused at specific locations, from I to XII.

Effects and Occurrence of Earthquakes

Earthquakes can cause ground shaking, differential settlement of ground, landslides, mudslides, soil liquefaction, ground lurching, avalanches, and ground displacement. Secondary effects like floods (from dam failure), fires, building collapse, falling objects, and tsunamis can also be caused. The first six primarily alter the Earth's surface, while the rest mostly cause property damage or loss of life. Not all earthquakes are of the same strength—very powerful earthquakes (magnitude 8 or more) are rare, while minor earthquakes occur almost all the time in some part of the world.

Structure of the Earth: Crust, Mantle, and Core

Earth’s interior consists of three main layers: the crust, the mantle, and the core. The outermost layer is the crust, which is brittle and thin. The oceanic crust is about 5 km thick, while the continental crust is around 30 km thick and up to 70 km below high mountains like the Himalayas.

The mantle lies beneath the crust and extends to a depth of almost 2,900 km. The upper part of the mantle contains the asthenosphere, a semi-molten, weak layer up to about 400 km deep, which acts as a source of magma. The lithosphere includes the crust and the uppermost part of the mantle and can be between about 10 to 200 km thick. The lower mantle is more solid due to higher pressure.

The core is separated from the mantle by the core-mantle boundary at a depth of about 2,900 km. The outer core is liquid, proven by the absence of S-waves, while the inner core is solid. The core contains mainly nickel and iron, giving rise to what is called the “nife” layer.

Volcanoes and Volcanic Landforms

A volcano is any opening in the Earth’s crust from which molten rock (lava), gases, and ash can escape. When a volcano is active or has erupted in recent history, it is called an active volcano. Most magma comes from the upper mantle just beneath the crust.

• Shield volcanoes are broad, gentle-sloped, and created by fluid basaltic lava; usually not explosive unless water interacts with magma.
• Composite volcanoes have steep sides and are formed by alternating layers of lava and ash; eruptions are explosive due to viscous lava.
• Calderas are large depressions formed when a volcano collapses after a massive eruption.
• Flood basalt provinces occur when huge quantities of basaltic lava pour out, forming large plateaux like the Deccan Traps in India.
• Mid-ocean ridge volcanoes are found along the ocean floor at mid-ocean ridges, with continuous undersea eruptions.

Volcanic Intrusive Landforms

When magma cools and solidifies within the crust, it forms various types of intrusive (plutonic) rocks. Batholiths are large dome-shaped masses formed deep within the crust, and are sometimes exposed at the surface by erosion. Lacoliths are large, dome-shaped bodies with a flat base, connected to magma sources by pipes. Lapoliths are saucer-shaped bodies which are concave upwards. Phacoliths appear as wavy masses along folded regions. Sills (or sheets) are horizontal intrusions, while dykes are vertical wall-like features where magma has filled cracks; dykes are common in regions like Maharashtra, feeding the lava flows of the Deccan Traps.

Practice and Application

A strong grasp of these fundamentals will help in understanding questions related to sources of information about Earth’s interior, different earthquake and volcanic features, and the structure and effects of major geophysical events. Students should revise the types and impacts of earthquakes and volcanoes, and remember key differences between surface and body waves, as well as intrusive and extrusive igneous landforms.

Class 11 Geography Chapter 3 Notes – Interior of the Earth: Quick Revision and Key Points

These concise notes for Class 11 Geography Chapter 3 cover the essentials of the Earth's internal structure, from crust to core, as explained in the NCERT book. Mastering topics like earthquake waves, volcanoes, and the nature of Earth's layers becomes easier with organized points, examples, and clear language.

This revision guide is ideal for last-minute prep, clarifying how scientists study the Earth's interior and the effects caused by endogenic processes. Focusing on key definitions and structures helps students answer textbook and exam questions confidently.