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We have covered the complete Class 9 Geography Chapter 2 – Shaping of the Earth’s Surface (English Medium) below.
Shaping of the Earth’s Surface || Chapter-2 || class-9(geogrophy)
Introduction
- Earth’s surface is constantly being transformed by powerful forces acting from within and on the surface of the planet.
- Most important ideas that explains these changes is the theory of plate tectonics.
- Which describes how large pieces of the Earth’s crust move slowly over the molten mantle.
What are Landforms?
A landform is a natural feature on the Earth’s surface formed by processes, such as weathering, erosion, deposition, and the movement of the Earth’s crust.
Origin of Landforms
The movement of tectonic plates gives rise to various landforms, such as mountains, volcanoes, plains, and valleys.
Understanding plate tectonics and landforms
Explain to us natural phenomena like earthquakes, volcanic eruptions, and the formation of continents and oceans.
Plate Tectonics
Plate tectonics is an important theory, given by W.J. Morgan.It explains the movement of the Earth’s crust.
What is Plate Tectonics?
- According to this theory, the outermost layer of the Earth is not one single piece but is broken into several large and small pieces called tectonic plates.
- These plates move slowly over the semi-molten layer beneath them.
- Responsible for major physical features and natural phenomena, such as mountains, earthquakes, and volcanoes.
The Earth’s Interior
The Earth is made up of three main layers: crust, mantle, and core.
- Crust
- The outermost layer on which we live.
- Thickness varies from 30-40 Km under continents (Continental Crust 30Km) to 5-7 Km under the ocean (Oceanic Crust 5Km).
2. Mantle
- Below the crust lies the mantle, which is very thick and hot.
- The mostly solid layer between the crust and the Earth’s outer core.
- Approximate thickness: 2900 Km.
3. Core
Core is the innermost layer and is extremely hot and heavy.
(I) Outer Core
- A fluid layer mainly consisting of iron and nickel.
- Approximate thickness: 2200 Km
(II) Inner Core
- A solid, hot spinning metal ball that is the densest part of the Earth.
- Approximate thickness: 1250 Km.
- Total depth from surface to center is approximately 6375 Km.
Lithosphere and Asthenosphere
Lithosphere
- The rigid outer layer of the Earth that includes the crust and upper mantle.
- It is broken into different tectonic plates.
- Approximate thickness: 100 Km
Asthenosphere (Approximate thickness: 200 Km)
- Lies beneath the lithosphere.
- A hot, mobile layer of partially molten rock.
- It is semi-molten and allows the tectonic plates to move.
Properties of Tectonic Plates
They are massive slabs of solid rock that move very slowly, usually a few centimeters per year.
Three Main Types of Tectonic Plates
➔ Continental plates: Which carry continents.
➔ Oceanic plates: Which carry ocean floors.
➔ Mixed plates: Which carry both continents and oceans.
Major Plates of the World
Pacific Plate, Eurasian Plate, African Plate, North American Plate, South American Plate, Indo-Australian Plate, and Antarctic Plate.
The movement of tectonic plates
- The movement of tectonic plates is caused by convection currents in the mantle.
- Heat from the Earth’s core causes molten material in the mantle to rise, while cooler material sinks.
Convection Currents
This continuous movement creates convection currents that push and pull the tectonic plates, causing them to move in different directions.
Plate Boundaries
The edges where tectonic plates meet are called plate boundaries. There are three main types:
1. Convergent Boundary
- Where two plates move towards each other.
- Continental-Continental Collision: When continental plates collide, they form fold mountains, such as the Himalaya.
- Oceanic-Continental Collision: When an oceanic plate collides with a continental plate, the oceanic plate sinks beneath the continental plate, leading to volcanic activity and earthquakes.
2. Divergent Boundary
- Where plates move away from each other.
- Formations: In this case, magma rises from below and forms new crust, creating features such as mid-ocean ridges.
- Example: The Mid-Atlantic Ridge is a good example.
3. Transform Boundary
- Where plates slide past each other without creating or destroying crust.
- Effects: This type of movement mainly causes earthquakes.
- Example: Along the San Andreas Fault in the United States.
Major role of Plate tectonics
- Plays a major role in shaping the Earth’s surface.
- It leads to the formation of mountains, valleys, ocean basins, volcanoes, and earthquakes.
- It explains the distribution of continents and oceans across the Earth.
- Ring of Fire: Most earthquakes and volcanoes occur along plate boundaries, especially around the Pacific Ocean, an area known as the Ring of Fire.
Process of Weathering and Erosion
Weathering and erosion play a vital role in the development of landforms by continuously breaking down and reshaping the Earth’s surface.
Weathering
Weathering is the process through which rocks on the Earth’s surface break down into smaller pieces due to various processes.
Three Main Types of Weathering
1. Physical Weathering
Rocks break into smaller pieces due to temperature changes, frost, or wind.
- Chemical Weathering
Minerals in rocks change because of reactions with water, air, or acids, leading to new substances.
3. Biological Weathering
Caused by plants, animals, or micro-organisms. For example, when plant roots grow into cracks of rocks and split them apart.
Erosion
Erosion is the process by which soil, rocks, and other surface materials are worn away and carried from one place to another by natural agents like water, wind, ice, or waves.
Types of Erosion
- Water erosion: Caused by rivers, rain, or ocean waves.
- Wind erosion: Common in dry and sandy areas.
- Glacial erosion: Where moving ice scrapes and carries rocks.
- Coastal erosion: Where sea waves wear away the land along the shore.
Human and Economic Impacts of Erosion
- On Occupations: Erosion affects many human occupations by changing the land and soil on which people depend.
- For Farmers: Erosion removes the fertile topsoil needed for crop growth, leading to lower yields.
- For rivers and coasts: For those living near rivers and coasts, erosion can wash away land, houses, and roads, causing damage and loss of property.
- On Industries: Industries like tourism and fishing suffer since beaches, rivers, and fertile lands may be destroyed.
- In Construction and Mining: Erosion destabilizes land, posing safety risks.
Ancient Soil and Water Management
➔ Sindhu-Sarasvati Civilization: Employed sophisticated techniques, including contouring, bunding, terracing, dams, and canals for water management.
➔ Sanskrit Texts: Multiple Sanskrit texts document these practices, including: The Vedas
Kṛṣiparāśara, Kautilya’s Arthaśāstra.
➔ Zabo system in Nagaland represents an integrated farming approach using earthen bunds on hillslopes for soil and water conservation.
Agents of Gradation
Agents of gradation are natural forces that wear down, transport, and deposit materials on the Earth’s surface, helping to level or smooth it over time.
Five Main Agents
- Running water: Erodes rocks and soils to form valleys and plains.
- Glaciers: Scrape and carry huge amounts of material, carving U-shaped valleys.
- Wind: Shapes deserts by eroding and depositing sand.
- Waves (Sea waves): Erode coastlines to form cliffs, beaches, and bays.
- Groundwater: Dissolves rocks, such as limestone, creating caves and sinkholes.
➔ Together, these agents are continuously modifying landforms, lowering high areas, and filling up low areas.
Running Water
Rivers shape the land through the processes of erosion, transportation, and deposition, creating a variety of landforms along their course.
Three courses
1. Upper Course
Rivers often form V-shaped valleys, waterfalls, and rapids due to steep gradients and strong erosive forces.
2. Middle Course
As the river starts to lose energy and begins depositing sediments, it starts to meander, forming oxbow lakes and floodplains.
3. Lower Course
The river slows further and deposits large amounts of sediment, forming deltas, levees, and alluvial fans.
Waterfall
A waterfall is a landform where a river flows over a steep cliff or vertical drop, creating a dramatic fall.
Formation: Waterfalls form in the upper course of rivers, where hard rocks resist erosion, while softer rocks below are worn away, creating a sudden drop.
Economic and Social Importance
- Attract tourists, making them important for local tourism and economy.
- Waterfalls are sometimes used for hydroelectric power generation.
- They provide opportunities for recreation, such as trekking and photography and hold cultural or religious significance.
Meander
A meander is a winding curve or bend in the middle or lower course of a river formed due to the lateral erosion and deposition of sediments.
Formation: As the river flows, it erodes the outer banks of bends and deposits sediment on the inner banks, gradually creating large loops.
Importance of Meanders
- Important for humans because the fertile soil deposited along their banks supports agriculture.
- They influence settlement patterns.
- Meandering rivers can be used for navigation and irrigation. Ex- Grand Anicut, also known as Kallanai in Tamil Nadu.
Deltas
A delta is a landform formed at the mouth of a river, where it flows into a sea, ocean, or lake, and deposits the sediments it has carried from upstream.
Formation: Over time, these deposits accumulate to form a fan-shaped or triangular area of land.
Importance of Deltas
- Highly fertile due to their rich alluvial soil, making them ideal for agriculture and supporting crops, such as rice and jute.
- Important for fishing, as the mix of fresh and saltwater creates diverse aquatic life.
- Support dense human settlements and are centres of trade and transportation.
Impact of Deltas
They can also be prone to flooding, which affects human life and activities.