Geography Notes Chapter 2 Class 9 Maharashtra Board

Notes For All Chapters – Geography Class 9th

Introduction

What are Endogenetic Movements?

These are movements inside the earth that cause natural events like earthquakes and volcanoes.
They happen because of energy and instability in the earth’s interior, especially in the mantle (layer below the crust).

Why Study Them?

These events can be disasters, causing loss of life and property (e.g., Nepal earthquake in 2015).
Understanding them helps us prepare and learn about earth’s changes.

Example from News (Fig. 2.1):

Nepal Earthquake (25 April 2015): 7.9 on Richter scale, killed nearly 1500 people, epicenter at Lamjung, focus 11 km deep.
Affected India, China, Bhutan, Pakistan, Bangladesh; had 17 aftershocks.

What Causes These Movements?

Inside the Earth:

Radioactive materials in the mantle release huge energy as waves.
These waves move and create instability, leading to movements in the crust (earth’s outer layer).

Activities to Understand (Try This):

Notebook Activity (Fig. 2.2 & 2.3): Stack notebooks, place objects (chalk, duster), pull one out or push them. Objects fall or shake, showing how earth moves and affects things.
This mimics how energy waves shake the ground during earthquakes.

Types of Internal Movements

Classification (Table in Chapter):

1. Velocity (Speed):

Slow Movements: Happen over a long time (e.g., forming mountains, continents).
Sudden Movements: Happen quickly (e.g., earthquakes, volcanoes).

2. Direction:

Horizontal: Sideways movements.
Upward/Downward: Up or down movements.

3. Landforms:

Continent-building: Makes continents and plateaus.
Mountain-building: Makes mountains by folding or faulting.

Slow Movements

What Happens?

Slow movements shape the earth’s surface over millions of years, forming mountains and continents.

Two Forces:

Pressure (Compression): Waves move toward each other.
Tension: Waves move away from each other.

Effects (Fig. 2.4):

On Hard Rocks: Breaks or faults form.
On Soft Rocks: Folds or bends form.

(a) Mountain-Building (Orogenic) Movements

Fold Mountains:

How They Form: Pressure from energy waves pushes soft rock layers into folds (like crumpled paper).
Activity (Fig. 2.5): Push a paper strip from both ends-it folds, showing how mountains rise.
Examples: Himalayas, Alps, Rockies, Andes, Aravalis (Fig. 2.7 shows Himalayas).
Features: Large folds uplift the surface, creating peaks and ridges.

Block Mountains:

How They Form: Tension pulls rocks apart, forming faults; a block between faults lifts up.
Activity (Fig. 2.9): Move notebooks apart-middle one stays up, like a block mountain.
Examples: Black Forest (Europe), Meghalaya Plateau (India).
Features: Flat tops, steep slopes, no peaks at first.

Rift Valleys:

How They Form: Tension creates two faults; land between them sinks.
Activity: Similar to block mountain but focus on sinking part.
Examples: Narmada Rift Valley (India), Great Rift Valley (Africa), Rhine River Valley (Europe).
Features: Deep with steep slopes (Fig. 2.10).

(b) Continent-Building (Epeirogenic) Movements

How They Happen: Slow up or down movements lift or sink large crust areas.

Results:

Uplifting forms continents or plateaus (e.g., Meghalaya Plateau).
Sinking makes sea-beds if land goes below sea level.

Why Important: Creates vast landforms over time.

Sudden Movements

What Are They?
- Quick events like earthquakes and volcanoes caused by sudden energy release.

Earthquakes

What is an Earthquake?

Shaking of the earth’s surface due to energy waves from the crust or mantle.
Measured by Richter scale (e.g., Nepal quake was 7.9).

Causes (Fig. 2.11):

Moving, colliding, or sliding plates.
Fractures in rocks from tension.
Volcanic eruptions.

Key Terms:

Focus (Hypocenter): Point inside earth where energy starts.
Epicenter: Point on surface above focus where tremors hit first (perpendicular to focus).

Seismic Waves (Fig. 2.12):

Primary (P) Waves:

Fastest, reach first, move back and forth.
Travel through solids, liquids, gases; shake buildings side to side.

Secondary (S) Waves:

Slower, reach after P waves, move up and down.
Only through solids; more destructive than P waves.

Surface (L) Waves:

Slowest, travel along crust, most destructive.
Cause major damage to buildings and land.

Seismogram:

Instrument that records seismic waves as a graph (Fig. 2.11).
Modern ones detect tiny tremors too.

Effects:

Cracks, landslides, ground uplifts or sinks, tsunamis, avalanches.
Buildings collapse, transport and communication fail, loss of life.

Volcanoes

What is a Volcano?

Hot materials (magma, ash, gases) burst out from the mantle to the surface.
Magma becomes lava when it reaches the surface.

Types by Eruption:

Central-Type (Conical) Volcano:
- Magma comes out a single vent, forming a cone-shaped mountain.
- Examples: Mt. Fujiyama (Japan), Mt. Kilimanjaro (Tanzania) (Fig. 2.16).
Fissure-Type Volcano:
- Magma flows out from many cracks, forming flat plateaus.
- Example: Deccan Plateau (India) (Fig. 2.17).

Types by Periodicity:

Active: Erupt regularly now (e.g., Mt. Fujiyama, Mt. Stromboli).
Dormant: Not erupted for long but could (e.g., Mt. Vesuvius, Barren Island).
Extinct: No eruptions for ages, unlikely to erupt (e.g., Mt. Kilimanjaro).

Effects:

Loss of life/property, tsunamis, air pollution from ash and gases.
Fertile land from ash, new land or islands, minerals near surface, crater lakes.

Plates and Their Role

What are Plates?

Earth’s crust is made of big pieces called plates (e.g., Indo-Australia, Eurasia).
Oceans and continents sit on these plates.

Plate Boundaries:

Subduction (Consuming): One plate slides under another, causing earthquakes/volcanoes.
Constructive (Creating): New crust forms as plates move apart.
Most volcanoes and earthquakes happen here (see map).

Map Questions:

Plates: Indo-Australia, Africa, Eurasia, etc.
Americas: Earthquake zones on west side (Andes).
Asia: Himalayas zone.
Africa: Volcanoes in east (rift valley).