Mountains, Deserts, Islands and Earthquakes
The surface of the Earth is never still. Mountains rise where the crust is pushed upwards, deserts form where rainfall is scanty, islands appear where the ocean floor is built up or the sea level changes, and earthquakes shake the ground when accumulated stress in the rocks is suddenly released. Together these four themes make up a large part of world geography, and they appear in almost every Assam recruitment examination paper. This chapter brings together the classification of mountains, the major ranges and peaks, the types and locations of deserts, the origin of islands, and the causes, measurement and effects of earthquakes, along with the belts of the world that are most prone to them.
Types of mountains
Mountains are classified by the way they are formed. Fold mountains are the most common and the most important for exams; they are formed when two plates collide and the sedimentary rocks between them are squeezed into folds. The Himalayas, the Andes, the Rockies and the Alps are fold mountains. Block mountains are formed when a block of land is uplifted between two faults (a horst), while a sunken block is called a graben or rift valley; examples include the Vosges and the Black Forest. Volcanic mountains are built by the accumulation of lava and ash, as in Mount Fuji and Mount Kilimanjaro. Residual or erosional mountains are the remains of old, worn-down ranges, such as the Aravalli and the Vindhya in India and the Appalachians in North America.
Major ranges and peaks
The Himalayas are the highest mountain range in the world and include the highest peak, Mount Everest, at 8,848.86 metres. The Andes in South America is the longest continental mountain range, and Aconcagua in the Andes is the highest peak of the Western Hemisphere. The Rockies are the backbone of North America, the Alps are the highest range of Europe, and the Urals form the traditional boundary between Europe and Asia. Other famous peaks include K2 (Godwin-Austen), the second highest peak in the world, and Kangchenjunga, the third highest, which lies partly in India.
Deserts
A desert is a region that receives very little rainfall, usually less than 25 centimetres a year. Hot deserts lie near the tropics and have high daytime temperatures, while cold deserts have low temperatures and often occur in the interior of continents or near the poles. The Sahara in Africa is the largest hot desert in the world, and the Thar (Indian) desert lies partly in India and Pakistan. The Arabian and Kalahari deserts are also hot deserts, while the Atacama in Chile is considered the driest place on Earth. The Gobi in Asia and the polar desert of Antarctica are cold deserts; Antarctica is the largest desert of all if cold deserts are counted. Typical desert features include sand dunes, oases, wadis (dry river beds) and yardangs.
Islands
An island is a piece of land completely surrounded by water. Continental islands lie on the continental shelf and were once joined to the mainland, such as Great Britain and Sri Lanka, while oceanic islands rise from the deep ocean floor and are usually volcanic or coral in origin. The largest island in the world is Greenland, and the largest archipelago (group of islands) is Indonesia. Volcanic islands include the Hawaiian islands, and coral islands include the Maldives and Lakshadweep. The Andaman and Nicobar Islands belong to India.
Earthquakes
An earthquake is the sudden shaking of the Earth's surface caused by the release of energy stored in rocks along faults, most often at plate boundaries. The point inside the Earth where the earthquake originates is the focus, and the point directly above it on the surface is the epicentre, where the shaking is strongest. The energy travels as seismic waves - P-waves (primary, longitudinal and the fastest), S-waves (secondary and transverse, which cannot pass through liquids) and surface or L-waves (the slowest but the most destructive). Earthquakes are recorded by a seismograph, and their magnitude is measured on the Richter scale, while intensity is measured on the Modified Mercalli scale. About 90 per cent of earthquakes occur in the Circum-Pacific belt, also called the Ring of Fire. A tsunami is a giant sea wave usually triggered by an undersea earthquake, and India is divided into seismic zones, with the Himalayan belt being the most active.
Quick revision
Remember the four mountain types - fold, block, volcanic and residual - with the Himalayas and Andes as fold mountains, the Andes being the longest continental range and Everest the highest peak. The Sahara is the largest hot desert and the Atacama the driest, while the Gobi and Antarctica are cold deserts. Greenland is the largest island and Indonesia the largest archipelago. The focus is inside the Earth and the epicentre is on the surface above it; P-waves are longitudinal and fastest, S-waves are transverse and cannot pass through liquids, and surface waves are the most destructive. The Richter scale measures magnitude and the Ring of Fire is the most earthquake-prone belt.
How to prepare this topic
- Learn the four types of mountains with one example each.
- Fix the record facts: Everest highest, Andes longest, Sahara largest hot desert, Atacama driest, Greenland largest island, Indonesia largest archipelago.
- Separate hot and cold deserts clearly, since examiners often mix them.
- Understand focus and epicentre with a simple diagram, and learn the three seismic waves in order of speed and damage.
- Remember that the Richter scale is for magnitude and the Mercalli scale is for intensity.
Common exam traps
- The Andes is the longest continental range, but the mid-oceanic ridge system is longer overall.
- Antarctica is the largest desert when cold deserts are included, though the Sahara is the largest hot desert.
- P-waves travel through solids and liquids, but S-waves travel only through solids.
- The epicentre is on the surface and the focus is below it - do not interchange them.
- Greenland is the largest island, but Australia is a continent, not an island.
Frequently Asked Questions
What are the main types of mountains with examples?
Mountains are of four main types. Fold mountains form when plates collide and squeeze rock into folds, as in the Himalayas, Andes, Rockies and Alps. Block mountains form when a block of land is uplifted between faults, as in the Vosges and Black Forest. Volcanic mountains are built of lava and ash, such as Mount Fuji and Kilimanjaro. Residual or erosional mountains are worn-down remains such as the Aravalli, Vindhya and Appalachian ranges.
Which are the largest and driest deserts in the world?
The Sahara in Africa is the largest hot desert in the world. The Atacama in Chile is considered the driest place on Earth. Cold deserts include the Gobi in Asia and the polar desert of Antarctica, and Antarctica is the largest desert of all if cold deserts are counted. Hot deserts have high daytime temperatures while cold deserts have low temperatures and lie in continental interiors or near the poles.
What is the difference between focus, epicentre and seismic waves?
The focus is the point inside the Earth where an earthquake originates, and the epicentre is the point directly above it on the surface, where the shaking is strongest. Energy travels as seismic waves - P-waves are primary, longitudinal and the fastest; S-waves are secondary and transverse and cannot pass through liquids; and surface or L-waves are the slowest but the most destructive. Earthquakes are recorded by a seismograph.
What is the Richter scale and which belt has the most earthquakes?
The Richter scale measures the magnitude, or energy, of an earthquake, while the Modified Mercalli scale measures its intensity, or the damage it causes. About 90 per cent of the world's earthquakes occur in the Circum-Pacific belt, also called the Ring of Fire, which encircles the Pacific Ocean. A tsunami is a giant sea wave usually triggered by an undersea earthquake.
Which is the largest island and the largest archipelago in the world?
Greenland is the largest island in the world, while Indonesia is the largest archipelago, that is, the largest group of islands. Continental islands such as Great Britain and Sri Lanka lie on the continental shelf, while oceanic islands such as the Hawaiian islands and the Maldives are volcanic or coral in origin. The Andaman and Nicobar Islands and Lakshadweep belong to India.