Landforms of the Earth

Both A and R are true and R is not the correct explanation of A.

Explanation — The Himalayas and Alps are indeed very high and have sharp peaks and steep slopes because they are young fold mountains and have not faced much erosion. On the other hand, old fold mountains have been worn down by denudation over millions of years. However, the reason given talks about old fold mountains and does not explain why the Himalayas and Alps are high and steep. Hence, R is not the correct explanation of A.

Both A and R are true and R is the correct explanation of A.

Explanation — Construction of railways and roads is easier on plains because the land is flat and even, which reduces the need for cutting, filling, or tunneling. This makes transportation infrastructure more cost-effective and less technically challenging in plain areas.

Extensive flat areas of low altitude, a gently and undulating terrain without prominent hills or depressions are known as plains. For example, the Indo-Gangetic plains of North India.

The process of mountain building on large scale leading to the formation of the intensely deformed belts which constitute mountain ranges is called orogenesis.

The different types of mountains are as follows:

1. Fold mountains — The fold mountains are formed by horizontal movements of the crust resulting in wrinkle like folds that vary greatly in size. The folds have arched or upraised parts known as anticlines and troughs known as synclines. Based on the period of their formation they are of two types — young fold mountains like the Himalayas and old fold mountains like the Aravalis.
2. Block mountains — Sometimes movement of earth's crust causes cracks or faults, where such movement leaves a block of higher land standing between two areas of lower land, the highland is known as a 'Block Mountain' or Horst. For example — the Vosges and Black forest mountains, Satpura and Vindhya mountains in India.
3. Residual mountains — When an area of highland remains standing above the general level after rivers and other natural agents have lowered the surface of the surrounding area, the term residual mountain is used. Sometimes such highlands are called mountains of denudation. For example — Helvellyn Residual Mountain

Plateau is an elevated tract of relatively flat land, usually limited on at least one side by a steep slope falling abruptly to lower land. They are results of diastrophism (large scale deformation of the Earth's crust). They have been modified by various agents of erosion and in many cases by volcanism and minor Earth movements.

For example, Plateau of Tibet, the Mexican Plateau.

Various types of plateaus with examples are:

1. Intermontane Plateaus — Plateau of Tibet, The Mexican Plateau.
2. Volcanic Plateaus — Deccan Plateau, South African Plateau.

A few advantages of plains are:

1. Construction of settlements, rails, roads, etc. is easy on flat and soft surface.
2. Fertile soil of alluvial plains are granaries of the world. In arid areas, irrigation can be provided easily and economically.
3. Most of the big cities are situated in plains as most of the plains provide fine climate and fertile land. Movement is easy which encourages agricultural and industrial development.

The types of depositional plain are:

1. Alluvial plains — These are formed by gradual accumulation of silt brought down by rivers. These deposits may be fan shaped known as Piedmont alluvial fan. When the river widens its valley or overflows its banks during floods, the clay and sand deposit to make flood plains. Examples are Indo-Gangetic plains, Plains of northern China and Mississippi plain. At the mouth of a river, triangular shaped plain of deposited material is called delta plain.
2. Glacial deposits — When the glaciers melt, the debris are deposited. Such plains are extensive in size and very fertile. They have irregular surface. For example, Plains of Central North America and Western European plain.
3. Aeolion Loess Plains or deposition by winds — These plains are formed by the materials which have been transported or deposited by winds. For example, Loess deposits of North China.
4. Lacustrine plains or river deposits — These plains result from the fillings of lake beds. When rivers enter a lake, they are muddy. The plains of Hungary in Europe and the plains of surrounding great lakes of North America are examples.

The main uses of mountains are:

1. Mountains are effective climatic barriers.
2. The swift streams of mountains are frequently sources of hydro-electric power.
3. They have deposits of minerals. They provide gold, silver, copper, lead etc.
4. They provide scope for lumbering industries.
5. Agriculture has been developed on the slopes of mountains.
6. They provide pastures.
7. They act as natural barriers or borders between two countries.

Plain is an extensive fertile tract of flat land, a gently and undulating terrain without prominent hills or depressions, usually formed by deposition of eroded material brought by rivers, glaciers and wind.

A portion of land surface rising considerably above the surrounding area, either as a single eminence, (generally above 600 mts), or as an extensive chain are called mountains.

For example, Himalayan mountain range.

Two characteristics of young fold mountains are:

1. Ruggedness of relief and rounded contours of mountain areas.
2. Most of the active volcanoes are found in their neighbourhood.

Young fold mountains are useful as they act as climatic barriers. For example, the Himalayas prevent the cold Siberian winds from entering the Indian sub-continent. Also, these mountains provide us swift rivers to harness hydroelectricity, make perfect tourist destination and they are rich in mineral deposits.

Alluvial plains are formed by gradual accumulation of silt brought down by rivers. As a river leaves the mountains, the water spreads out and fast speed is arrested. The sluggish water deposits the material brought by it forming the plains. They mostly deposit the eroded materials along the foot of hills forming Piedmont Alluvial plain, like Bhabhar in India. Plains are also formed by the flooded rivers and are known as flood plains.

Examples of fertile alluvial plains are Indo-Gangetic plains, Mississippi plain, etc.

1. Fold Mountain — The Himalayas
2. Block mountain — Fault Block Mountain (Pakistan)
3. Volcanic plateau — The Columbian Plateau
4. Structural plain — Russian platform

Geomorphology

Block Mountain

Residual Mountain

Block Mountain

Alps

Bolivian Plateau

Great Plains of USA

Piedmont alluvial plain

Fold Mountain

intermontane plateau

Structural plain

Mountain building

The Fold Mountains in the West Indies are relatively young and lie along active plate boundaries where tectonic plates are constantly moving. This movement creates intense geological activity, making these regions prone to frequent volcanic eruptions. Hence, a belt of active volcanoes is commonly found along such mountain ranges.

The key reason for this difference lies in the nature of the land. The Ganga Valley is a flat and low-lying area, which allows water to accumulate—an ideal condition for rice cultivation. In contrast, the Araku Valley has steep slopes where water drains quickly. This well-drained terrain is suitable for growing coffee, which requires less standing water. Therefore, the type of crop grown in each valley depends on the slope and water retention of the land.

The plateaus that are surrounded by high hills and mountains on all sides are called intermontane plateaus. For example — Tibetan plateau.

The mountains that are formed as a result of cracking of earth's crust or faulting, when there is deep submergence of a large landmass leaving behind raised platform, are called Block mountains. For example, the Vosges.

Below diagram shows the formation of Block Mountains:

Below diagram shows the formation of a floodplain:

As they have flat top with steep sided slope.

As these mountains are formed when surrounding area of a highland is lowered down by the long term denudation and erosion by various agents of gradation.

Alluvial plains are formed by the deposition of alluvium by the rivers; therefore they have fertile soil which enhances the cultivation of grains.

As Tibet is enclosed by Kunlun Mountain from the north & Himalayas from the south.

The old fold mountains have been subjected to the forces of denudation (such as weather, rivers, glaciers, etc.) for long geological periods. Due to this, they are much lower and less rugged than young fold mountains.

The north western Deccan is an example of a lava plateau because it was formed by extensive volcanic activity that deposited large amounts of basaltic lava over the area millions of years ago, resulting in a broad, flat plateau with distinctive basaltic rock formations.