Our Changing Earth Class 7 Chapter Notes

Key points

• The Earth is not fixed; its surface changes slowly through external forces and suddenly through internal forces.
• Endogenic forces act from inside the Earth and may cause earthquakes and volcanoes.
• Exogenic forces act on the Earth surface and include weathering, erosion, transportation and deposition.
• An earthquake starts inside the Earth at the focus; the point on the surface just above it is the epicentre.
• A seismograph records earthquake vibrations; earthquake intensity is commonly measured on the Richter scale.
• Rivers form features such as waterfalls, meanders, ox-bow lakes, flood plains, levees and deltas.
• Sea waves form sea caves, sea arches, stacks and beaches by erosion and deposition.
• Glaciers are rivers of ice; they erode valleys and deposit moraines.
• Wind creates mushroom rocks by erosion and sand dunes or loess by deposition in dry regions.
• Exam memory chain: Weathering breaks, Erosion carries, Deposition drops.

Important definitions

Endogenic forces

Forces that originate inside the Earth and cause sudden movements like earthquakes and volcanic eruptions.

Exogenic forces

Forces that act on the Earth surface, mainly through air, water and ice, causing erosion and deposition.

Weathering

Breaking up of rocks into smaller pieces due to heat, cold, water, wind, plants or animals.

Erosion

Wearing away and removal of rock, soil or sediments by running water, wind, glaciers or sea waves.

Deposition

Laying down of eroded material when the transporting agent loses speed or energy.

Focus

The point inside the Earth where earthquake vibrations begin.

Epicentre

The point on the Earth surface directly above the earthquake focus; shaking is usually strongest near it.

Volcano

An opening in the Earth crust through which molten material, gases and ash come out.

Meander

A loop-like bend formed by a river flowing through a plain.

Delta

A triangular or fan-shaped deposit formed at a river mouth when sediments are dropped.

Why is the Earth called changing?

The Earth looks stable, but its surface is continuously shaped by internal forces and external forces. Internal forces come from deep inside the Earth. They can be sudden and powerful, causing earthquakes, volcanoes, uplift and sinking of land. External forces work from outside on the surface. Rivers, wind, sea waves and glaciers break rocks, carry material and deposit it elsewhere. This creates new landforms and destroys old ones. For example, a river may cut a valley in the mountains, form loops in plains and finally build a delta near the sea. The main idea to remember is that landscapes are not permanent; they are the result of continuous erosion, transportation and deposition over long periods.

Endogenic forces vs Exogenic forces

How external forces make landforms

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Earthquake notes: causes, terms and safety

Landforms made by rivers, sea waves, glaciers and wind

Quick landform comparison for exams

Mini examples to remember the chapter

• Example 1: River bend becomes ox-bow lake A river flowing in a plain makes a big loop. During floods, it cuts across the narrow neck of the loop. The old loop is separated and becomes an ox-bow lake.
• Example 2: Delta formation A river slows down when it reaches the sea. It cannot carry all its load, so it deposits sand and silt at the mouth. Over time, a delta is formed.
• Example 3: Mushroom rock In deserts, wind carrying sand erodes the lower part of a rock more than the upper part. The rock gets a narrow base and broad top, like a mushroom.

Exam tip: avoid these common mistakes

Do not mix up weathering, erosion and deposition. Weathering only breaks rocks; erosion removes and carries them; deposition drops them. In earthquake questions, write both focus and epicentre correctly: focus is inside Earth, epicentre is on the surface. For landform questions, mention the agent and the process. Example: Delta is formed by river deposition at the river mouth. For safety questions, write practical points such as taking cover, moving to open areas and avoiding lifts.

Quick revision checks

• Q: What are endogenic forces? A: Forces that originate inside the Earth and cause changes such as earthquakes and volcanoes.
• Q: What is the difference between focus and epicentre? A: Focus is the point inside Earth where an earthquake begins; epicentre is the point on the surface directly above it.
• Q: Name two landforms formed by river deposition. A: Delta and flood plain.
• Q: How are sand dunes formed? A: Wind deposits sand in heaps in deserts, forming sand dunes.

Frequently Asked Questions