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.