5. Landforms of the Earth

1. Introduction to Landforms

The surface of the Earth is highly uneven. The major relief features present on the lithosphere are divided into three broad categories: Mountains, Plateaus, and Plains. These are shaped by various internal (tectonic/volcanic) and external (erosion/deposition) processes.

2. Mountains

Mountains are the most prominent features of the Earth's relief. They are natural elevations rising abruptly above the general level of the surrounding country. A landform must be more than 300 metres in height and possess steep slopes to be classified as a mountain.

A. Characteristics of Mountains

They are mostly made of sedimentary rocks, indicating they were formed on sea beds. Igneous and metamorphic rocks can also occur.
They are narrow at the top (peaks) and broader at the base.
Temperature decreases with altitude; hence, high mountains are often covered with snow.
When mountains are arranged in lines, they are called mountain ranges (e.g., The Himalayas, The Alps, The Rockies, and The Andes).

B. Classification of Mountains

Mountains are classified based on their mode of formation:

Concept

1. Folded Mountains: Formed by forces of compression and contortion resulting in a series of arches (anticlines) and troughs (synclines). Most mountains of the world fall into this category. They have sedimentary rocks on top and crystalline rocks in the core.

Simple folded: Alternating anticlines and synclines (e.g., Jura mountains).
Complexly folded: Intricate drainage, overlapping folds.
Nappe/Decken: Extreme recumbent folds that are broken and thrust forward.

2. Block Mountains: Formed as a result of faulting. The earth's crust breaks into blocks separated by fissures. When a block is raised between two faults, it forms a block mountain. The depressed area between two faults forms a rift valley. One slope is typically steep while the other is gentle.
3. Dome Mountains: Produced by the intrusion of molten rocks (magma) from underneath, pushing the surface upwards like a blister. They dip outwards uniformly from the center.
4. Mountains of Accumulation: Formed by the accumulation of volcanic material (e.g., Cinder cones, Lava cones, Composite cones) or sand dunes.
5. Relict (Residual) Mountains: Formed by the continuous removal of surrounding material. When a mountain is subjected to heavy denudation/erosion over millions of years, its height is lowered, leaving behind scattered mounds or stumps of hard, resistant rocks.

Important

Man and Mountains:
Advantages: They act as natural barriers against enemies and extreme winds. They influence precipitation (rainfall), provide rich pastures, timber, and are sources of major rivers. They offer sites for hydro-electricity, mining, and act as tourist/health resorts.
Drawbacks: They create immense difficulties for transport (roads/railways), offer very little leveled land for agriculture, and impose harsh climates, making them thinly populated.

3. Plateaus

A plateau is an elevated tract of relatively flat land (tableland), usually limited on at least one side by a steep slope or cliff falling abruptly to lower land. Unlike mountains, plateaus have a large flat or undulating surface on top.

A. Classification of Plateaus

Based on Rock Structure:

Volcanic Plateaus: Formed by volcanic eruptions where basic lava flows and accumulates over vast areas (e.g., North Western Deccan Plateau, Columbia Plateau).
Tectonic Plateaus: Formed by tectonic movements that uplift the land (e.g., Meseta Central in the Iberian Peninsula).
Erosional Plateaus: Formed by the continuous erosion of mountain tops by glaciers and rivers, creating elevated flatlands.

Based on Surrounding Topography:

Intermontane Plateaus: Plateaus enclosed by mountain ranges. These are the highest and most extensive plateaus in the world (e.g., Plateau of Tibet, Plateaus of Mexico, Bolivia, and Mongolia).
Piedmont Plateaus: Situated at the foot of mountains. They are flanked by mountains on one side and plains or seas on the other (e.g., Colorado Plateau in NA, Patagonia Plateau in SA).
Continental Plateaus: Massive tablelands that rise abruptly from bordering lowlands or the sea. They rarely have mountainous rims (e.g., Plateaus in East Antarctica).

Fact

Effect of Plateaus on Man: Due to rugged topography and lower temperatures, extensive farming is limited. Instead, stock raising is common. However, volcanic plateaus with basaltic/black soil (like the Deccan Trap) are highly retentive of moisture and excellent for crop farming (cotton). Many plateaus are incredibly rich in minerals, making mining the chief occupation (e.g., tin and copper in Bolivia and Peru).

4. Plains

Plains are relatively flat, low-lying regions generally about 500 feet above sea level with a gradual slope. Because they are the most suitable landforms for human habitation, the largest concentration of the world's population is found in plains.

A. Classification of Plains

1. Coastal Plains:

Situated near the sea coast, formed by the emergence of sea bottoms. They consist of loosely compacted layers of mud and sand left by sea waves.

2. Erosional Plains:

Formed by the wearing down of elevated landmasses by agents of denudation (running water, wind, ice).

Glaciated Plains: Marked by rounded hillocks, broad open valleys, lakes, and swamps. Mostly found in NW Europe and North America.
Desert Plains: Characterized by wind erosion marks and salt lakes (Playas).
Peneplain: A low-relief plain formed by protracted (long-term) erosion, often covered with a deep layer of rock waste.
Cuestiform Plains: Marked by concentric ridges and lowlands formed by river action on alternating hard and soft rocks (e.g., Paris and London basins).
Karst Plains: Landscapes created by the dissolving of soluble bedrock like limestone or dolomite, creating sinkholes and caves.

3. Depositional Plains:

Formed by the deposition of debris or silting up of depressions. They are highly fertile.

Alluvial Plains: Formed by river deposits. Includes Piedmont Alluvial Fans (gravel deposited at the foot of hills where the river slows) and Flood Plains.
Inter-riverine Plains (Doabs): Plains formed between two rivers, usually made of older, elevated alluvium (e.g., Pampas of Argentina).
Delta Plains: Flat, triangular plains formed at the mouth of a river where it meets the sea and deposits its heavy load (e.g., Ganga, Nile, Mississippi deltas).
Drift Plains: Formed by glacial deposits (Moraines, Till plains, Outwash plains).
Aeolian Loess Plains: Formed by wind depositing fine, chemically undecomposed dust. Very fertile (e.g., NW China, Pampas).
Lacustrine Plains: Formed by the filling up of ancient lake beds.

4. Structural Plains:

Formed due to the large-scale uplift or subsidence of landmasses (e.g., The Great Plains of the USA, which were once submerged under the ocean).

Important

Man and Plains: Depositional and alluvial plains have historically been the cradles of civilization. Their level surface makes laying transport networks (railways/roads) extremely easy and cheap. The soft, highly fertile arable soil makes them the "granaries of the world." Furthermore, easy access to water allows for excellent irrigation and the growth of dense populations and major industries.