Our Changing Earth Class 7 NCERT: Forces, Landforms and Practice

Overview: Why the Earth Keeps Changing

The Earth changes because it is made of layers and its outer shell is broken into huge pieces called lithospheric plates. These plates move very slowly, but their movement can create big effects over time. When plates press, pull or slide past each other, land may fold, crack or shake. These are internal, or endogenic, forces. At the same time, the surface is attacked by rain, rivers, wind, glaciers and sea waves. They break rocks, carry the broken material and drop it somewhere else. So geography is not only about maps; it is about processes. A valley, delta or sand dune is like evidence left behind by a natural process.

Key Terms You Must Know

Lithospheric plates

Large, rigid pieces of the Earth's crust and upper mantle that move slowly over the softer layer below.

Endogenic forces

Forces that originate inside the Earth, such as movements causing earthquakes, volcanoes and mountain building.

Exogenic forces

Forces acting on the Earth's surface, such as running water, wind, ice and sea waves.

Erosion

The wearing away of the land surface by natural agents like rivers, wind, glaciers and waves.

Deposition

The laying down of transported material such as sand, silt or pebbles when the transporting agent loses energy.

Focus and epicentre

The focus is the point inside the Earth where an earthquake begins. The epicentre is the point on the surface directly above the focus.

Endogenic and Exogenic Forces Compared

How Landforms Are Made: The Three-Step Geography Process

1. Step 1: Weathering breaks rocks — Rocks are weakened and broken into smaller pieces by heat, cold, water, plant roots and chemical action. Weathering mostly happens in the same place, so the material is first loosened before it can be moved.
2. Step 2: Erosion and transportation move material — A river, glacier, wind or sea wave picks up the loosened material. Fast-flowing water can carry larger pieces, while slower water carries fine silt. Wind can lift sand and dust, especially in dry regions.
3. Step 3: Deposition builds new features — When the transporting agent loses energy, it drops its load. A river deposits silt near its mouth to form a delta, wind deposits sand to form dunes, and sea waves deposit material to form beaches.

Worked Geography Examples

• Example 1: How does a river form an ox-bow lake? Step 1: A river flowing on a plain develops bends called meanders because water erodes the outer bank and deposits material on the inner bank. Step 2: The neck of a meander becomes narrower as erosion continues on both outer sides. Step 3: During a flood, the river may cut through the narrow neck and take a straighter course. Step 4: The old loop is separated from the main river by deposition at its ends. Final idea: The cut-off loop becomes an ox-bow lake.
• Example 2: How is a delta formed at a river mouth? Step 1: A river carries sand, silt and clay from mountains and plains. Step 2: Near the sea or lake, the river slows down because the slope becomes very gentle. Step 3: Since slow water cannot carry a heavy load, it deposits fine material at the mouth. Step 4: Over time, layers of deposited silt build a fan-shaped or triangular landform. Final idea: This deposited landform at the mouth of a river is called a delta.
• Example 3: Why do earthquakes cause maximum damage near the epicentre? Step 1: The earthquake starts at the focus inside the Earth. Step 2: Energy waves travel outward from the focus in all directions. Step 3: The epicentre is the point on the surface directly above the focus, so waves reach it strongly and quickly. Final idea: Areas near the epicentre usually feel the strongest shaking and face higher damage.

Exam Tip: Write Cause + Process + Landform

In Class 7 Geography answers, do not write only the name of a landform. Explain the natural agent, the action and the result. For example, instead of writing: A delta is formed by a river, write: A river carries silt, slows down near its mouth, deposits the silt, and forms a delta. Also remember pairs: focus is inside the Earth, epicentre is on the surface; erosion removes material, deposition drops material.

Common Mistakes to Avoid

• Do not confuse weathering with erosion. Weathering breaks rocks in place; erosion removes and carries the broken material.
• Do not say all Earth changes are sudden. Earthquakes are sudden, but river valleys, deltas and dunes often take many years to form.
• Do not mix up focus and epicentre. The focus is below the surface, while the epicentre is on the surface above it.
• Do not write that wind forms deltas. Deltas are mainly formed by rivers; wind forms features such as sand dunes and loess deposits.
• Use correct agents: rivers form meanders and floodplains, glaciers form glacial valleys, sea waves shape cliffs and beaches.

Practice Questions with Solutions

• Q: What are lithospheric plates, and how can their movement cause earthquakes? A: Step 1: Lithospheric plates are large pieces of the Earth's outer shell. Step 2: These plates move slowly, but sometimes their edges get stuck due to friction. Step 3: When the stress becomes too much, the rocks break or slip suddenly, releasing energy as seismic waves. Final answer: Lithospheric plates are moving crustal pieces, and their sudden slipping or breaking can cause earthquakes.
• Q: Explain the difference between erosion and deposition with one example each. A: Step 1: Erosion means wearing away and carrying away material from one place. Step 2: Deposition means dropping the carried material when the agent loses energy. Step 3: Example of erosion: a river cuts a valley in its upper course. Example of deposition: a river deposits silt at its mouth to form a delta. Final answer: Erosion removes material, while deposition lays it down to build new landforms.
• Q: How does running water form a waterfall? A: Step 1: A river may flow over layers of hard and soft rock. Step 2: The softer rock gets eroded faster than the hard rock. Step 3: This creates a steep drop, and water falls from a height. Step 4: The falling water continues to erode the base and deepens the plunge pool. Final answer: A waterfall forms when river erosion creates a sudden steep drop in the river bed.
• Q: Why are floodplains fertile? A: Step 1: During floods, a river spreads over the land beside its channel. Step 2: Flood water carries fine silt and clay from upstream areas. Step 3: When the water slows down, it deposits this fine material on both sides of the river. Step 4: The deposited silt is rich in nutrients and improves soil quality. Final answer: Floodplains are fertile because rivers repeatedly deposit nutrient-rich silt on them.

Frequently Asked Questions