Chapter 5 Major Domains Of The Earth

The Earth is the only known planet where life exists. This is possible because the essential elements for life – land, water, and air – are present here.

The Earth's surface is a complex area where the three primary components of the environment interact and overlap:

• The Lithosphere (from Greek 'Lithos' meaning stone) is the solid part of the Earth.
• The Atmosphere (from Greek 'Atmos' meaning vapour) is the layer of gases surrounding the Earth, containing gases like oxygen, nitrogen, and carbon dioxide.
• The Hydrosphere (from Greek 'Hudor' meaning water) encompasses the vast areas of water on the Earth's surface, including water in all its forms (ice, liquid water, water vapour).

The Biosphere (from Greek 'Bios' meaning life) is a narrow zone where the Lithosphere, Hydrosphere, and Atmosphere meet and overlap, supporting all forms of life.

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Lithosphere

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The Lithosphere is the Earth's solid outer layer. It consists of the rocks forming the Earth's crust and the thin layers of soil containing essential nutrients that support living organisms.

The Earth's surface is broadly divided into two main types of features:

• Continents: Large landmasses.
• Ocean Basins: Enormous bodies of water.

All the oceans on Earth are connected to each other. The elevation of land is measured from the level of the sea, which is universally considered as zero elevation.

The highest point on Earth is Mount Everest, reaching 8,848 metres above sea level. The deepest known point is the Mariana Trench in the Pacific Ocean, with a depth of 11,022 metres. This highlights that the deepest parts of the oceans are significantly deeper than the highest mountain peaks are high.

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Continents

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There are seven major continents on Earth, separated by large bodies of water. These are:

1. Asia
2. Europe
3. Africa
4. North America
5. South America
6. Australia
7. Antarctica

A larger portion of the Earth's landmass is located in the Northern Hemisphere.

• Asia: The largest continent, covering about one-third of the Earth's total land area. It lies mainly in the Eastern Hemisphere, and the Tropic of Cancer passes through it. Asia is separated from Europe by the Ural Mountains in the west. The combined landmass of Europe and Asia is known as Eurasia.
• Europe: Much smaller than Asia, located to its west. The Arctic Circle passes through Europe, and it is surrounded by water bodies on three sides.
• Africa: The second largest continent. The Equator ($0^\circ$ latitude) runs almost through its middle, with a large part in the Northern Hemisphere. Africa is unique as it is the only continent through which the Tropic of Cancer, the Equator, and the Tropic of Capricorn all pass. The Sahara Desert, the world's largest hot desert, is located in Africa, and the Nile River, the world's longest river, flows through it. Africa is surrounded by oceans and seas on all sides.
• North America: The third largest continent. It is connected to South America by a very narrow strip of land called the Isthmus of Panama. The continent lies entirely in the Northern and Western Hemispheres and is surrounded by three oceans.
• South America: Located mostly in the Southern Hemisphere and surrounded by two oceans (Pacific on the west, Atlantic on the east). The Andes, the world's longest mountain range, runs along its western length. South America also contains the world's largest river, the Amazon.
• Australia: The smallest continent, located entirely in the Southern Hemisphere. It is surrounded by oceans and seas on all sides and is often called an island continent.
• Antarctica: A huge continent located entirely in the Southern Hemisphere, with the South Pole near its center. It is permanently covered by thick ice sheets and has no permanent human settlements. Many countries, including India (with stations named Maitri and Dakshin Gangotri), have research stations there.

Do you know? Edmund Hillary (New Zealand) and Tenzing Norgay Sherpa (India) were the first men to climb Mount Everest (May 29, 1953). Junko Tabei (Japan) was the first woman (May 16, 1975), and Bachendri Pal was the first Indian woman (May 23, 1984) to reach the summit.

An Isthmus is a narrow strip of land connecting two larger landmasses.

A Strait is a narrow passage of water connecting two larger bodies of water.

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Hydrosphere

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The Earth is often called the 'blue planet' because more than 71 percent of its surface is covered by water, while only 29 percent is land.

The Hydrosphere includes all the water on Earth in its various forms: flowing water in oceans and rivers, lakes, ice in glaciers, underground water, and water vapour in the atmosphere.

Over 97% of the Earth's water is found in the oceans and is salty, making it unsuitable for direct human use. The remaining small percentage is freshwater, mainly in the form of ice sheets and glaciers, or underground. Only a very small amount of freshwater is easily accessible for human use. This explains why, despite the Earth being a 'blue planet' with abundant water, many regions face a shortage of usable freshwater.

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Oceans

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Oceans constitute the major part of the Hydrosphere and are all interconnected. The waters of the oceans are constantly in motion, with three chief types of movements: waves, tides, and ocean currents.

The five major oceans, listed in order of size, are:

1. Pacific Ocean
2. Atlantic Ocean
3. Indian Ocean
4. Southern Ocean
5. Arctic Ocean

• Pacific Ocean: The largest ocean, covering about one-third of the Earth's surface. The Mariana Trench, the deepest known part of the Earth, is located here. The Pacific Ocean is nearly circular and is surrounded by Asia, Australia, North America, and South America.
• Atlantic Ocean: The second largest ocean, with an 'S' shape. It is bordered by North and South America on its western side, and Europe and Africa on its eastern side. The Atlantic's coastline is highly indented, providing excellent natural harbors and ports, making it the busiest ocean for commerce globally.
• Indian Ocean: Unique as it is the only ocean named after a country (India). Its shape is roughly triangular, bordered by Asia to the north, Africa to the west, and Australia to the east.
• Southern Ocean: Encircles the continent of Antarctica and extends northward to about 60 degrees South latitude.
• Arctic Ocean: Located within the Arctic Circle and surrounds the North Pole. It is connected to the Pacific Ocean by a narrow, shallow passage called the Bering Strait and is bounded by the northern coasts of North America and Eurasia.

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Atmosphere

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The Atmosphere is the layer of gases that envelops the Earth. This thin blanket of air is a vital part of the planet, providing the air we breathe and protecting us from the sun's harmful rays.

The atmosphere extends up to a height of approximately 1,600 kilometers from the Earth's surface. It is structured into five distinct layers based on their composition, temperature, and other characteristics, starting from the Earth's surface and moving upwards (Figure 5.4):

1. Troposphere
2. Stratosphere
3. Mesosphere
4. Thermosphere
5. Exosphere

The atmosphere is primarily composed of Nitrogen (78%) and Oxygen (21%), which together make up about 99% of clean, dry air. Other gases, including carbon dioxide and argon, constitute the remaining 1%. Oxygen is essential for respiration in most life forms, while Nitrogen supports the growth of living organisms. Carbon dioxide, even in small amounts, is crucial because it absorbs heat radiated by the Earth, helping to keep the planet warm (contributing to the greenhouse effect), and is necessary for plant growth.

The density of the atmosphere decreases significantly with height, being highest at sea level. This decrease in air density makes breathing difficult at high altitudes, which is why mountain climbers often need to carry oxygen cylinders (Figure 5.5).

The atmosphere also exerts pressure on the Earth's surface, which varies from place to place, creating areas of high and low pressure. Air moves from areas of high pressure to areas of low pressure; this moving air is known as wind.

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Biosphere – The Domain Of Life

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The Biosphere is the unique zone on Earth where the Lithosphere (land), Hydrosphere (water), and Atmosphere (air) come into contact and interact, supporting all forms of life (Figure 5.6).

Within the Biosphere, a wide variety of organisms exist, ranging from microscopic microbes and bacteria to massive mammals. All living organisms, including humans, are interdependent and linked to the Biosphere for their survival.

The organisms in the Biosphere can be broadly classified into the plant kingdom and the animal kingdom.

The interactions between the Earth's three domains (Lithosphere, Hydrosphere, Atmosphere) can have significant impacts. For example, human activities like cutting down forests or clearing land for agriculture can lead to rapid soil erosion on slopes. Natural events such as earthquakes can alter the Earth's surface, causing land to sink, as tragically occurred in parts of the Andaman and Nicobar Islands during the recent Tsunami.

Pollution, caused by discharging waste into lakes and rivers, makes water unfit for human consumption and harms aquatic life. Emissions from industries, power plants, and vehicles pollute the air. An increase in the amount of carbon dioxide in the atmosphere, primarily from burning fossil fuels, leads to a rise in global temperatures, a phenomenon known as global warming.

Maintaining the balance of nature across the Lithosphere, Atmosphere, and Hydrosphere is crucial. This necessitates limiting the unsustainable use of Earth's resources to ensure the continued health and viability of the Biosphere and the life it supports.