Chapter 2: Dynamic Earth – Notes

1. Earth as a Dynamic Planet

The Earth is constantly changing.
Changes happen due to:
- Internal forces (inside Earth)
- External forces (on the surface)
These forces shape:
- Mountains
- Ocean basins
- Valleys

2. Internal Forces (Inside the Earth)

Caused by movements inside the Earth.
Main reason: movement of Plate Tectonics
Effects:
- Earthquakes
- Volcanic eruptions
- Mountain formation
- Ocean basin formation

3. External Forces (On the Surface)

These forces reshape landforms slowly.
Main processes:
- Weathering
- Erosion
- Mass movement
Agents (tools of nature):
- Rivers
- Wind
- Glaciers
- Waves & ocean currents
- Underground water

4. Natural Disasters

Caused by internal + external forces.
Examples:
- Earthquakes
- Landslides
- Avalanches
- Floods
- Dust storms

Studying them helps us reduce damage and stay safe.

Plate Tectonics

5. What is Plate Tectonics?

Earth’s outer layer is broken into large plates.
These plates move over a soft layer called the asthenosphere.
Their movement causes:
- Earthquakes
- Volcanoes
- Mountains
- Ocean basins

6. Theories Related to Plate Tectonics

a) Continental Drift Theory

Proposed by Alfred Wegener (1912)
Idea:
- All continents were once one landmass called Pangaea
- Later, they drifted apart
Evidence:
- Matching coastlines
- Similar fossils
- Rock formations

b) Seafloor Spreading Theory

Given by Harry Hess and Robert S. Dietz
Idea:
- New crust forms at mid-ocean ridges
- Old crust moves away
Explains how continents move

c) Convection Currents

Heat from Earth’s core creates circular movement in mantle
Hot material rises, cold sinks
This movement pushes tectonic plates

d) Types of Plate Boundaries

Divergent → Plates move apart
Convergent → Plates collide
Transform → Plates slide past each other

Interior of the Earth

7. Layers of the Earth

a) Crust

Outer layer (5–40 km thick)
Thicker on continents, thinner under oceans
Supports life

b) Mantle

Below crust (about 2900 km thick)
Made of rocks
Divided into:
- Upper mantle
- Lower mantle
Boundary between crust & mantle:
- Mohorovicic Discontinuity

c) Core

Innermost layer (about 3500 km thick)
Made of metals (iron & nickel)
Divided into:
- Outer core → liquid
- Inner core → solid

Earth behaves like a magnet due to metals.

8. Temperature & Density

Density increases towards the centre
Temperature increases:
- About 1°C for every 32 metres
Inner core remains solid due to high pressure

External Processes

9. What are External Processes?

Natural processes on Earth’s surface
Examples:
- Weathering
- Erosion
- Mass movement

Weathering

10. Definition

Breaking of rocks into smaller pieces at the same place

11. Factors Affecting Weathering

Temperature changes
Water action
Wind (sand abrasion)
Plant roots
Animals and human activities

12. Types of Weathering

a) Physical Weathering

No change in chemical composition
Causes:
- Heating & cooling
- Frost action
Example:
- Exfoliation (peeling of rock layers)

b) Chemical Weathering

Rocks dissolve due to chemical reactions
Forms:
- Cavities
- Sinkholes
- Caverns

c) Biological Weathering

Caused by plants, animals, humans
Roots break rocks
Animals dig and loosen rocks

Erosion

13. Definition

Carrying away of weathered materials

14. Agents of Erosion

Rivers
Rainwater
Wind
Glaciers
Sea waves

15. Factors Affecting Erosion

Rainfall
Temperature
Slope of land
Type of soil
Vegetation
Human activities

Over millions of years, erosion changes landforms
Example: The Himalayas were once part of the Tethys Sea

Mass Movements

16. Definition

Movement of rocks/soil downwards due to gravity

17. Types

Landslide → Dry rocks move down steep slope
Earth flow → Wet soil moves slowly
Mudflow → Water-filled debris flows quickly

18. Effects

Common in hilly areas like:
- Uttarakhand
- Himachal Pradesh
- Jammu & Kashmir
Causes:

Loss of life
Damage to property

Agents of Denudation – Notes

1. Meaning of Denudation

Denudation means wearing away or making land bare.
It includes:
- Weathering
- Erosion
- Transportation
- Deposition

2. Main Agents of Denudation

Rivers
Rainwater
Glaciers (Ice)
Sea waves
Winds

3. Rivers (Most Important Agent)

Carry rocks from mountains to plains.
Do three main works:
- Erosion (cutting land)
- Transportation (carrying materials)
- Deposition (dropping materials)
Form:
- Valleys
- Gorges

4. Rainwater

Water seeps into rocks through cracks.
Forms:
- Underground water
- Springs
- Caves (in limestone areas)

5. Glaciers (Ice)

Moving masses of ice in mountains.
Work:
- Erode land
- Change V-shaped valleys → U-shaped valleys
When they melt → form rivers

6.Sea Waves

Erode coastal areas.
Also deposit materials on shores.
Do:
- Erosion
- Transportation
- Deposition

7. Winds

Important in deserts like Thar Desert.
Remove and carry sand and dust.
Shape landforms like sand dunes.

Role of Humans in Land Changes

Humans also degrade land:
- Cutting trees → causes wasteland
- Construction → destroys land
- Factory waste → pollutes land
- Dumping garbage → degrades soil

Work of River (3 Stages)

1. Upper Course (Mountain Stage)

Steep slope → fast flow
Work:
- Erosion + Transportation
Landforms:
- Gorges
- V-shaped valleys
- Waterfalls
- Rapids

Gorge = narrow valley with steep sides

2. Middle Course (Plain Stage)

Gentle slope → slower flow
Work:
- Deposition begins
Landforms:
- Flood plains (fertile land)
- Meanders (loops in river)

Rivers useful for:

Navigation
Irrigation

3. Lower Course (Delta Stage)

Very slow flow
Work:
- Heavy deposition
Features:
- Distributaries (small channels)
- Deltas (fertile land)

Types of Glaciers

1. Mountain Glaciers

Found in mountains
Move along valleys
Form rivers like Ganga & Yamuna
Change V-shaped valleys to U-shaped

2. Continental Glaciers

Very large ice sheets
Found in Polar Regions
Cover large land areas
Move slowly

1. What is plate tectonics and why is it important?

Plate tectonics is the theory that Earth’s lithosphere is divided into large and small plates that move over the asthenosphere. Their movement causes earthquakes, volcanoes, mountains, and ocean basins. It is important because it explains how Earth’s surface changes continuously and helps us understand the formation of major landforms over millions of years.

2. Explain the structure of the Earth.

The Earth has three main layers: crust, mantle, and core. The crust is the outer layer supporting life. The mantle lies below it and is made of semi-molten rocks. The core is the deepest layer made of iron and nickel, divided into a liquid outer core and a solid inner core, with extremely high temperature and pressure.

3. What is Continental Drift Theory?

The Continental Drift Theory was proposed by Alfred Wegener. It states that all continents were once joined together in a supercontinent called Pangaea. Over time, they slowly drifted apart to form present-day continents. Wegener supported his theory with evidence like matching coastlines, fossils, and rock formations, but could not explain the mechanism of movement.

4. What is Seafloor Spreading Theory?

Seafloor Spreading Theory was given by Harry Hess and Robert Dietz. It explains that new oceanic crust is formed at mid-ocean ridges when magma rises and cools. This pushes older crust away from the ridge, causing the seafloor to spread. It provides a mechanism for continental movement and is supported by magnetic patterns on the ocean floor.

5. What are convection currents in the mantle?

Convection currents are circular movements in the Earth’s mantle caused by heat from the core. Hot material rises while cooler material sinks. This continuous movement creates pressure that pushes tectonic plates in different directions. These currents are responsible for the movement of continents and play a key role in plate tectonics.

6. Explain plate boundaries in plate tectonics.

There are three types of plate boundaries. At divergent boundaries, plates move apart and new crust is formed. At convergent boundaries, plates collide, causing mountains or subduction. At transform boundaries, plates slide past each other, causing earthquakes. These boundaries explain most geological activities on Earth’s surface.

7. What is denudation and what processes does it include?

Denudation is the wearing away of Earth’s surface. It includes weathering, erosion, transportation, and deposition. These processes are carried out by natural agents like rivers, wind, glaciers, rainwater, and sea waves. Denudation gradually lowers mountains, fills basins, and forms plains, continuously reshaping Earth’s surface over time.

8. Describe weathering and its types.

Weathering is the breakdown of rocks at the same place without movement. It occurs due to temperature changes, water, wind, plants, and animals. There are three types: physical weathering (breaking due to heat and cold), chemical weathering (dissolving of minerals), and biological weathering (caused by plants and animals breaking rocks).

9. Explain the role of rivers in shaping landforms.

Rivers are powerful agents of erosion and deposition. In the upper course, they form valleys, gorges, and waterfalls. In the middle course, they create floodplains and meanders. In the lower course, they deposit sediments and form deltas. Rivers also transport materials and continuously reshape the Earth’s surface over time.

10. What is the role of humans in land degradation?

Humans contribute to land degradation through deforestation, urbanization, and industrial waste. Cutting trees leads to soil erosion and reduced rainfall. Construction activities damage natural land surfaces, while pollution from factories and waste dumping reduces soil fertility. These activities disturb natural balance and accelerate land degradation.

11. How do wind and sea waves act as agents of erosion?

Winds erode dry and loose particles in deserts and form sand dunes. Sea waves erode coastal areas by breaking rocks and also deposit materials along the shore. Both processes involve erosion, transportation, and deposition. They continuously change deserts and coastal landscapes and create new landforms over time.

Landforms