Shaping of the earth's SURFACE: NCERT Solutions (2026)

The earth's surface is constantly changing due to two primary types of forces:

• Endogenic Forces: These forces act in the interior of the earth. They are primarily land-building forces because they create new relief features like mountains. Can cause sudden movements or slow, long-term movements. Examples: Earthquakes, Volcanism, Mountain formation (Folding).
• Exogenic Forces: These forces act on the surface of the earth. They are primarily land-wearing forces because they degrade and level the existing features. Continuous, slow, and gradual processes. Examples: Weathering, River erosion, Wind deposition.

The Theory of Plate Tectonics states that the earth's solid outer crust (lithosphere) is broken into several large and small pieces called tectonic plates. These plates constantly float on a semi-fluid layer of the mantle beneath them. The movement of these plates is driven by intense heat and convection currents inside the earth.

There are three distinct ways these plates move relative to one another:

1. Convergent Boundary: When two tectonic plates move toward each other. They can collide, causing the crust to crumple up into fold mountains (e.g., the formation of the Himalayas).
2. Divergent Boundary: When two plates move away from each other. This creates gaps or rifts through which magma can rise to form new crust (e.g., Mid-Atlantic Ridge).
3. Transform Boundary: When two plates slide horizontally past each other. This movement doesn't create or destroy landforms but causes massive earthquakes (e.g., San Andreas Fault).

Running water is one of the most powerful exogenic agents of graduation. A river shapes the earth's surface in three sequential stages:

• Upper Course (Erosion): In mountainous areas, the river flows rapidly down steep slopes. It cuts deeply into the valley floor, creating sharp features like V-shaped valleys, gorges, and breathtaking waterfalls.
• Middle Course (Transportation): As the river reaches flat plains, its speed decreases. It begins to deposit heavier rocks but transports finer sediment, carving out broad curves known as meanders.
• Lower Course (Deposition): Right before entering the sea, the river becomes extremely slow. It splits into channels and deposits its entire load of silt and sand, forming a fertile, triangular landform called a delta.

Though they sound similar, weathering and erosion are two distinct steps in landform transformation:

• Weathering is an in-situ (on-site) process. It means rocks crack, split, or decay right where they are, without being moved. For example, water freezing inside a rock crack expands and breaks it apart.
• Erosion is a dynamic process that involves movement. Once rocks are broken down by weathering, agents like wind, running water, or moving ice actively pick up those pieces and carry them away to distant places.

Answer: C) Convergent. Explanation: When two plates collide, they squeeze the sedimentary rocks between them upward to form mountains.

Answer: C) Wind. Explanation: In deserts, wind erodes the lower section of a standing rock faster than the top, leaving a mushroom shape.

Answer: B) Physical weathering.

Answer: C) Meander.

Answer: C) Volcanic eruption.

Answer: Block mountains are formed when large areas of the earth's crust are broken and displaced vertically due to tensional forces (faulting). The uplifted blocks are called Horsts, while the sunken blocks are termed Graben or rift valleys. The Vosges mountain range in Europe is an excellent example.

Answer: Sand dunes are low, hill-like structures formed by the accumulation of sand deposited by the wind. When the wind blows in a desert, it lifts and transports sand. As soon as the wind speed drops or hits an obstacle like a bush or rock, the sand falls and deposits in a mound, creating a sand dune. They are mostly found in arid regions like the Thar Desert.

Answer: Glaciers are massive "rivers of ice" that move incredibly slowly due to gravity. As they slide down mountains, they scrape, scratch, and carve out the valley floor like giant sandpaper. This process destroys massive boulders and cuts deep, wide troughs known as U-shaped valleys.

Answer: Deltas are highly populated because:

1. The soil deposited by the river is extremely fertile alluvial soil, perfect for agriculture.
2. Abundant availability of water supports farming, transport, and industries.
3. The flat terrain makes it easy to build houses, roads, and railway networks.

Answer: Weathering is categorized into three types:

1. Physical/Mechanical Weathering: Physical forces break rocks into pieces. Example: Thermal expansion (rocks expanding during hot days and contracting during cold nights until they crack).
2. Chemical Weathering: Chemical reactions decompose minerals inside rocks. Example: Acid rain reacting with limestone.
3. Biological Weathering: Living organisms break rocks down. Example: Plant roots growing into rock cracks and wedging them apart.

Human Acceleration: Modern human actions speed up this process massively. Mining and blasting with explosives shatter deep bedrock instantly. Deforestation removes soil cover, exposing bare rocks directly to harsh sunlight and rain. Industrial pollution releases sulfur and nitrogen oxides into the air, turning normal rainfall into devastating acid rain that rapidly dissolves structural stone and natural monuments.

Answer: The earth's surface is a playground of balance between internal (endogenic) and external (exogenic) forces.

• Internal Forces as Builders: Movements originating deep inside the mantle push magma up, smash tectonic plates together, and warp the crust. These actions lift flat lands up into towering fold mountains, carve deep rift valleys, and create expansive volcanic plateaus. Without them, the earth would be a uniform, featureless sphere.
• External Forces as Sculptors: The moment a landform is elevated, external agents like wind, rain, rivers, and ice attack it. They constantly chip away at high mountain peaks, carve deep canyons, smooth out rocky plateaus, and carry all that debris down to fill up low-lying basins.

Essentially, internal forces build the raw rough blocks of land, and external forces endlessly carve, smooth, and polish them into the landscapes we see today.

Answer: Sea waves continuously strike the coastal rocks, creating incredible erosional and depositional features:

1. Sea Cliffs: Steep, near-vertical rocky faces formed right on the edge of the coast due to wave erosion at the base.
2. Sea Caves: Over time, waves hollow out weak spots in a cliff face, forming large, cave-like openings.
3. Sea Arches: As erosion continues inside a cave, it breaks all the way through the rock wall, leaving only the roof standing as an arch.
4. Stacks: Eventually, the roof of the arch collapses under its own weight, leaving an isolated vertical pillar of rock standing out in the sea.
5. Beaches: On the depositional side, sea waves drop sand, gravel, and sediment along the shoreline, forming beautiful sandy beaches.

Q13. What type of valley feature did the students witness, and what force created it? (2 Marks)
Answer: The students witnessed a gorge (or canyon), which is a deep, narrow valley with vertical rock walls. This is created by the erosional down-cutting action of a fast-moving river, which is an exogenic force.

Q14. Why is this hilly station prone to earthquakes according to geological principles? (2 Marks)
Answer: The region is prone to earthquakes because it is located near an active convergent tectonic plate boundary. As plates continuously push against one another, immense friction and stress build up in the rocks. When this stress is suddenly released, it creates powerful seismic shockwaves known as earthquakes.

Answer: A) Both A and R are true and R is the correct explanation of A. Explanation: The sudden drop in water speed forces the river to drop its heavy load of sediment, blocking its own path and creating a delta.