1. UNIT-4 MOUNTAINS

UNIT-4 MOUNTAINS

Mountains are prominent natural elevations of the Earth’s surface with steep slopes and small summit areas. They are formed mainly due to internal forces of the Earth such as folding, faulting, and volcanic activity. In mountain studies, terms like cordillera, range, system, group, and chain are commonly used. A mountain range is a series of mountains arranged in a narrow belt, while a cordillera is a large complex of ranges and systems. Mountains differ in origin and age. Many of the present-day mountains were once under ancient seas where sediments were deposited. The process of mountain building is called orogeny.

OBJECTIVES

After studying this unit, you should be able to:

Define mountains and their characteristics

Explain types of mountains

Understand mountain-building processes (orogeny)

Describe the significance of mountains

1. CLASSIFICATION OF MOUNTAINS

(Source: AI-generated diagram (OpenAI DALL·E)

Mountains are classified based on their origin and mode of formation. Broadly, they are divided into relict (residual), deformation (tectonic), accumulation (volcanic), dome, and sea-floor mountains. The formation of mountains involves crustal movements, volcanic activity, and erosion. Each type has distinct structural and geological characteristics.

(i) Relict (Residual) Mountains

Relict mountains are formed due to long-term erosion and denudation. They are also called residual mountains because they remain after softer rocks are eroded away. These mountains are generally very old. Their peaks are usually rounded due to prolonged weathering. They do not show sharp and steep features like young mountains. The Aravalli Range in India is a good example of relict mountains.

(ii) Deformation or Tectonic Mountains

Deformation mountains are formed due to crustal movements of the Earth. These movements cause folding and faulting of rock layers. They are mainly associated with plate tectonic activity. This category includes fold mountains and block mountains. These mountains are often large and structurally complex. Most of the world’s major mountain systems belong to this type.

(a) Block Mountains (Fault Mountains)

Block mountains are formed due to faulting of the Earth’s crust. When rocks break and move along faults, some blocks are uplifted while others sink. The raised block is called a horst, and the depressed block is called a graben. These mountains have steep slopes and flat tops. They are formed mainly due to tensional forces. The Black Forest of Germany is an example of block mountains.

(b) Fold Mountains

Fold mountains are formed due to compressional forces in the Earth’s crust. These forces cause rock layers to bend and fold. They are usually formed in geosynclinal regions where thick sediments accumulate. Fold mountains are long, high, and complex in structure. They are considered young mountains when recently formed. The Himalayas and the Alps are examples of fold mountains.

(iii) Accumulation (Volcanic) Mountains

Volcanic mountains are formed by the accumulation of lava, ash, and other volcanic materials. When magma erupts onto the surface, it cools and solidifies to form mountains. These mountains are usually cone-shaped. They are commonly found near plate boundaries. Volcanic mountains may be active, dormant, or extinct. Mount Fuji in Japan is an example.

(iv) Dome Mountains

Dome mountains are formed when magma pushes up rock layers from below without erupting. The rock layers bend upward forming a dome-like structure. Over time, erosion removes the upper layers and exposes the inner rocks. These mountains are generally circular or oval in shape. They are formed mainly due to intrusive igneous activity.

(v) Mountains on the Sea Floor

Some mountains are located beneath the ocean. These are called submarine mountains. They are formed mainly due to volcanic activity and plate movements on the ocean floor. The Mid-Atlantic Ridge is the longest underwater mountain chain in the world. Some peaks of submarine mountains may rise above sea level to form islands. These mountains are important in understanding plate tectonics.

2. MOUNTAIN BUILDING ACTIVITY (OROGENY)

Orogeny refers to the process of mountain building. Mountains are often formed in long sedimentary basins called geosynclines. Sediments accumulate in these basins over millions of years. Later, compressional forces cause folding and uplift of these sediments. This results in the formation of mountain ranges. Orogenic belts are zones where intense deformation has taken place. The Himalayas are an example of an active orogenic belt.

Types of Geosynclines

Geosynclines are classified into (i) Monogeosynclines (ii) Polygeosynclines (iii) Mesogeosynclines.

Monogeosynclines are long and narrow basins.

Polygeosynclines are wider and more complex in structure.

Mesogeosynclines are deep mobile basins located between continents. These basins play an important role in mountain formation. Most fold mountains originate from geosynclinal regions.

Causes of Orogeny

Orogeny is mainly caused by plate tectonic movements. Convection currents in the mantle move tectonic plates. When plates collide, compressional forces develop. These forces cause folding, faulting, and uplift of rocks. Magmatic intrusion and metamorphism also occur during mountain building. Thus, mountain formation is a result of internal dynamic processes of the Earth.

3. MOUNTAIN RANGES OF INDIA

(Source: AI-generated (DALL·E / OpenAI)

4. SUMMARY

Internal geological processes act within the Earth. The main processes are diastrophism and igneous activity. These processes cause mountain building, earthquakes, and volcanic eruptions. A mountain is a high land mass rising above the surrounding area. Mountains are classified as Relict, Tectonic, and Accumulation types. Relict mountains are formed due to long-term erosion. Tectonic mountains include fold and fault mountains formed by crustal movements. Accumulation and volcanic mountains are formed by deposition of sediments or volcanic materials.
In India, Aravalli, Satpura, Vindhyan, Eastern and Western Ghats are old mountains.
The Himalayas are young fold mountains formed by crustal deformation.

2. UNIT-5 EARTHQUAKES

UNIT-5 EARTHQUAKES

An earthquake is the sudden shaking or vibration of the earth caused by the rapid release of energy from within the earth. This release of energy produces seismic waves that travel through the interior and along the surface of the earth. Earthquakes are closely related to movements in the earth’s crust and are generally associated with faulting and tectonic disturbances. They are more common in geologically active regions of the world.

 Objectives

After studying this unit, you should be able to:

Define earthquakes and their causes

Understand seismic waves and their types

Explain the effects of earthquakes

Learn basic methods of earthquake measurement

1. Introduction 

Various physical forces continuously operate inside the Earth and on its surface. These forces constantly modify the shape and size of landforms. Movements within the Earth may be slow and gradual or sudden and violent. Sudden movements inside the Earth produce vibrations in rocks. These vibrations are called earthquakes. Earthquakes occur suddenly, affect large areas, and cause severe destruction to life and property. Unlike mountain building, which occurs very slowly, earthquakes take place within a few seconds. Even with modern scientific progress, earthquakes cannot be predicted accurately in advance. Historical records show that earthquakes were mentioned by early historians like Herodotus and Pliny, and records exist from about 2500 years ago.

2. Recording of Earthquakes

The earliest instrument used to detect earthquakes was called a seismoscope, invented in 136 A.D. in China. This instrument could only indicate that an earthquake had occurred but could not measure its strength. Later, the seismograph was developed to record earthquake vibrations. A seismograph consists of a heavy mass suspended like a pendulum and attached to a firm support fixed to the ground. When the ground shakes, the support moves while the suspended mass tends to remain stationary due to inertia. The relative movement between them is recorded on a rotating drum with photographic paper. The recorded trace of earthquake vibrations is called a seismogram. The study of earthquakes is known as seismology.

3. Earthquake Waves (Seismic Waves)

During an earthquake, energy is released in the form of seismic waves. These waves travel through the Earth in different forms. There are four main types of seismic waves:

1.     P waves,

2.     S waves,

3.     Rayleigh waves, and

4.     Love waves.

1. P waves (Primary waves) are longitudinal waves consisting of alternating compressions and expansions similar to sound waves. They travel fastest and can move through both solids and liquids.
2. S waves (Secondary waves) are shear waves that cause particles to move at right angles to the direction of wave travel. These waves travel slower than P waves and can move only through solids.
3. Rayleigh waves move particles in circular motions in vertical planes.
4. Love waves move particles horizontally at right angles to the direction of wave movement.
Rayleigh and Love waves together are called surface waves or L waves. Among all waves, P waves are the fastest, followed by S waves, while L waves travel the slowest.

(Source: AI-generated diagram (OpenAI DALL·E)

4. Intensity of Earthquakes

The strength of an earthquake is measured using different intensity scales. The intensity depends on the effects of the earthquake on people, buildings, and instruments. One of the earliest scales was the Rossi–Forel scale, which has 10 divisions. Later, the Mercalli scale was introduced with 12 divisions, based on the damage caused by seismic waves. The most widely used modern scale is the Richter scale, which measures earthquake magnitude. Places experiencing equal earthquake intensity are connected by lines called isoseismal lines.

5. Classification of Earthquakes

Earthquakes are classified based on the depth of their focus. The focus or hypocenter is the exact point inside the Earth where the earthquake originates. The point directly above the focus on the Earth’s surface is called the epicenter.  Earthquakes with focus deeper than 300 km are called deep-focus earthquakes. Earthquakes with focus between 70 km and 300 km are known as intermediate-focus earthquakes. Earthquakes with focus less than 70 km deep are called shallow-focus earthquakes, and most earthquakes belong to this category.

6. Causes of Earthquakes

Earthquakes occur due to several causes such as faulting, volcanic activity, landslides, and collapse of underground caverns. The most common cause is movement along faults in the Earth’s crust. Stress gradually accumulates in rocks due to crustal movements. When the rocks can no longer withstand this stress, they suddenly break or slip along faults, producing earthquake waves. According to Elastic Rebound Theory proposed by H. F. Reid, earthquakes occur when accumulated elastic strain in rocks is suddenly released during fault movement. Most earthquakes caused by crustal movements are known as tectonic earthquakes. Earthquakes may also occur due to volcanic activity, when magma and gases move within the Earth’s crust.

(Source: AI-generated diagram (OpenAI DALL·E)

7. Submarine Earthquakes and Tsunamis

Earthquakes that occur beneath the sea are called submarine earthquakes. These earthquakes can cause sudden vertical displacement of the sea floor. This disturbance may generate huge sea waves known as tsunamis. Tsunamis travel long distances and cause severe damage to coastal regions.

8. Distribution of Earthquakes

Earthquakes are not evenly distributed over the Earth. Most destructive earthquakes occur around the Pacific Ocean, forming the Circum-Pacific Belt or Ring of Fire. Another earthquake belt extends through Indonesia, Andaman–Nicobar Islands, and Myanmar. A third belt stretches from the Pamir Knot through Afghanistan, Iran, Turkey, and Greece towards Europe. Another belt runs through the Great Rift Valley of East Africa. These regions are tectonically unstable and experience frequent earthquakes.

3. UNIT-6 VOLCANOES

UNIT-6 VOLCANOES

Volcano is an opening or vent in the earth’s crust through which molten rock material, gases, and ash are expelled to the surface. The process by which magma rises and erupts is known as volcanism. Volcanic activity is closely related to internal heat and tectonic movements within the earth.

Objectives

• After studying this unit, you should be able to:
• Define volcanoes and volcanic activity
• Describe types of volcanoes
• Understand volcanic materials and eruptions
• Explain the impact of volcanoes on Earth

1. Introduction

A volcano is a natural opening or vent in the Earth's Crust through which molten rock, gases, and other volcanic materials escape from the interior of the Earth. The molten rock beneath the surface is called magma, and when it reaches the surface it is known as lava. Volcanic eruptions may be quiet and gradual or violent and explosive depending on the composition of magma and the pressure of gases within the magma chamber.

Volcanic activity plays an important role in shaping the Earth’s surface. It contributes to the formation of new landforms, islands, and mountain ranges, and also influences soil fertility and mineral resources.

2. Causes of Volcanism

Volcanism is mainly caused by the high internal temperature of the Earth and the movement of tectonic plates. Heat generated inside the Earth melts rocks in the Earth's Mantle, producing molten material known as magma.

Several factors contribute to volcanic eruptions:

• Internal Heat of the Earth : Heat generated from radioactive elements melts mantle rocks.
• Movement of Tectonic Plates : According to Plate Tectonics, plates move apart, collide, or slide past each other, creating conditions for magma formation.
• Pressure of Dissolved Gases : Magma contains gases such as water vapour, carbon dioxide, and sulphur dioxide. When pressure increases, these gases expand and force magma upward.

Ø Weak Zones in the Crust : Fractures or cracks in the Earth’s crust allow magma to rise toward the surface.

When the pressure inside the magma chamber becomes greater than the strength of surrounding rocks, a volcanic eruption takes place.

3. Types of Volcanoes Based on Activity

Volcanoes are classified according to their activity into three main types.

3.1 Active Volcanoes

Active volcanoes are those that are currently erupting or have erupted in recent geological time. These volcanoes may show signs such as earthquakes, gas emissions, or lava flows.

3.2 Dormant Volcanoes

Dormant volcanoes are also called sleeping volcanoes. They have not erupted for a long period but still have the potential to erupt again in the future.

3.3 Extinct Volcanoes

Extinct volcanoes are those that have not erupted for thousands of years and are not expected to erupt again because the supply of magma has stopped.

(Source; The image was generated by an AI model (DALL·E by OpenAI))

4. Types of Volcanoes Based on Structure

Volcanoes are also classified according to their shape and structure.

4.1 Shield Volcano

Shield volcanoes are broad volcanoes with gentle slopes formed by repeated eruptions of fluid lava. The lava spreads over large areas before cooling.

4.2 Composite Volcano (Stratovolcano)

Composite volcanoes have steep slopes and are formed by alternating layers of lava, volcanic ash, and other materials. These volcanoes often produce explosive eruptions.

4.3 Cinder Cone Volcano

Cinder cone volcanoes are small and steep-sided volcanoes formed by accumulation of volcanic fragments such as ash and cinders around a central vent.

(Source; The image was generated by an AI model (DALL·E by OpenAI))

5. Volcanic Products

Volcanic eruptions release different types of materials, which are generally classified into gaseous, liquid, and solid products.

5.1 Gaseous Products

Volcanic gases include:

• Water vapour
• Carbon dioxide
• Sulphur dioxide
• Hydrogen sulphide

These gases play a major role in volcanic eruptions and atmospheric changes.

5.2 Liquid Product

The main liquid product of volcanoes is lava. Lava flows across the Earth’s surface and later cools to form igneous rocks.

5.3 Solid Products

Solid materials ejected during eruptions are known as pyroclastic materials. These include:

• Volcanic ash (fine particles)
• Lapilli (small fragments)
• Volcanic bombs (large molten fragments)
• Rock blocks (large solid fragments)

6. Volcanic Landforms

Volcanic activity creates many types of landforms both on the surface and beneath it.

6.1 Extrusive Landforms

Extrusive landforms are formed when lava erupts onto the Earth’s surface. Examples include:

• Lava plateaus
• Volcanic cones
• Craters and calderas

6.2 Intrusive Landforms

Intrusive landforms are formed when magma cools and solidifies beneath the surface. Examples include:

1. Batholiths

2. Laccoliths

3. Dykes

4. Sills

1. Batholiths: Batholiths are very large bodies of intrusive igneous rock formed when huge masses of magma cool slowly deep inside the Earth's Crust. They usually have irregular shapes and may extend over more than 100 km² at the surface.

Key points

• Formed deep underground from slowly cooled magma
• Mostly composed of granite or granodiorite
• Often form the core of mountain ranges
• Exposed on the surface after erosion removes the overlying rocks

2. Laccoliths: A laccolith is a dome-shaped igneous intrusion formed when magma enters between layers of rock and pushes the upper layers upward.

Key points

• Has a flat base and dome-shaped top
• Formed when magma accumulates between rock layers
• Causes the overlying strata to bulge upward
• Usually smaller than batholiths

3. Dykes: A dyke is a vertical or steeply inclined sheet of igneous rock formed when magma fills cracks or fractures in surrounding rocks.

Key points

• Magma cuts across existing rock layers
• Usually narrow but may extend for many kilometres
• Acts as a pathway for magma to reach the surface

4. Sills: A sill is a horizontal sheet of igneous rock formed when magma intrudes between existing layers of rock and solidifies.

Key points

• Lies parallel to surrounding rock layers
• Formed by magma spreading sideways between strata
• Often seen in layered sedimentary rocks

 

7. Effects of Volcanoes

Volcanoes have both constructive and destructive effects.

7.1 Constructive Effects

• Formation of new landforms and islands
• Development of fertile soils suitable for agriculture
• Availability of mineral deposits
• Production of geothermal energy

7.2 Destructive Effects

• Lava flows destroy vegetation, buildings, and infrastructure
• Ash fall damages crops and affects air travel
• Release of toxic gases may harm humans and animals
• Loss of life and property during major eruptions

8. Distribution of Volcanoes

Volcanoes are mainly distributed along tectonic plate boundaries. The most important volcanic region is the Pacific Ring of Fire, which surrounds the Pacific Ocean and contains the majority of the world’s active volcanoes.

Other important volcanic regions include:

• Mid-Atlantic Ridge
• Mediterranean volcanic belt
• Oceanic island chains formed by mantle hotspots

These regions correspond to areas of intense tectonic activity where magma can easily reach the Earth’s surface.