Islands | Explained

An island can be defined as ‘a body of land surrounded by water from all the sides. However, continents are also the mass of land which is surrounded by water. However, there is a difference between the continents and islands with respect to their size. Hence, the islands have a smaller mass of land than the continents. Greenland is the world's largest island, covering an area of 2.166 million sq km.

Types of Islands

There are six major kinds of islands. These are: 

1. Continental Islands 

2. Tidal Islands,

3. Barrier Islands,

4. Oceanic Islands, 

5. Coral Islands and 

6. Artificial Islands. 

Continental Island

Continental islands are those islands which were once part of the mainland. However, due to the shift of the tectonic plates, these land masses are separated from the continents. These islands are surrounded by water from all sides.

Fig. Continental Islands

Processes of Formation of Continental Islands

• Due to Shifting of Continents: Millions of years ago there was only one landmass in the world which was termed Pangaea.  The supercontinent Pangaea broke apart and drifted away forming the different continents of the world. Some of the smaller portions of these islands drifted apart due to plate tectonics forming separate Islands. Examples of such islands are Greenland and Madagascar.

• Due to Glaciations: Eustatic Changes i.e. the rise and fall of the sea level due to continuous glacial and interglacial periods are responsible for the formation of islands. For Example, about 18,000 years ago, during the most recent glacial period, most of the earth was covered by ice. As the ice melted, the sea level rose and it flooded the low-lying areas of the continents, thus separating the landmasses from the mainland. The land lying at higher altitudes did not submerge leading to the formation of the Islands.

• For Example The British island has been formed due to this process. It used to be a part of mainland Europe

• Due to the Accumulation of Sediments by River: These islands are formed due to the accumulation of sand, silt, and gravel deposits in the deltas or the lower course of the river over a long period. However, these river islands may also disappear when the volume of river water or its speed increases. Examples of such islands are Marajó in the Amazon River, Brazil and Majuli in the Brahmaputra River in India.

Tidal Island

Tidal islands are part of continental islands such that they remain separated from the mainland only during the high tide. Here, the connecting land between them is not completely eroded but submerged because of the rising water levels.

Examples: The island of Mont Saint-Michel in France, and Cramond Island in Scotland

Barrier islands

The lagoons or shallow water separate the mainland from a piece of land termed barrier islands. These are narrow pieces of landmass which occur in chains and lie parallel to the coastline. These islands protect the mainland from ghastly storms and high sea waves by acting as a barrier. Hence, these islands have been termed barrier islands. Such Island chains can be found along approximately 13-15% of the world’s total coastlines.

Formation of Barrier Islands

• Due to Accumulation of Sediments by Ocean Currents: Some of the barrier islands are parts of continental islands which have been formed due to the deposition of sediments like sand, gravel and silt accumulated by ocean currents. Sandbars are an example of such barrier islands formed due to high water levels during storms or during the high tide season by the accumulation of sand along the coast. Such Sandbars are ephemeral as these sandbars might also be eroded away by the same ocean currents. Examples of such Islands are Virginia barrier islands.

• Due to Glaciations: Some of the barrier islands were formed during the end of the last Ice Age. When the glacier melted, the sea levels rose and coastal areas were submerged and stretches of sandy islands were left behind. These barrier islands are formed by the glacial moraines. Examples of such islands are Outer Banks islands off the south-eastern coast of the US.

• Coral Barrier Islands: Some of the barrier islands can also be formed due to the formation of barriers from billions of coral exoskeletons. The Great Barrier Reef in the Coral Sea off the Queensland coastline in Australia is a fine example of a barrier island formed due to the deposition of coral.

Oceanic islands/Volcanic Islands

These islands are formed due to volcanic eruptions in the ocean floor or marine volcanic eruptions. These are also referred to as ‘high islands’ regardless of their height. These islands are formed due to eruptions of lava layers which build up and it protrude above the sea level.

Formation Oceanic Islands

• Due to Subduction of Tectonic Plates: Island arcs are formed due to the convergence of the Oceanic crusts when the two tectonic plates collide with each other. The Japanese islands are situated in such subduction zone of four tectonic plates which includes two continental plates of North America and Eurasia with two oceanic plates of the Pacific and the Philippines. It is an example of such islands. The Aleutian Islands are another example of such formations.

• Due to Rifts in Tectonic Plates: The formation of islands in the middle of a tectonic plate due to rifting due to which lava comes out and islands are formed. Examples of such islands are the island of Surtsey. It was formed when the Eurasian plate split away from the North American plate in 1963. It can be termed as the world’s newest island.

• Due to shifting of hotspots or Plum Volcanoes: When the continental crusts moves over the ‘hot spot’ or a plum volcano then islands can be formed. A ‘hot spot’ is a crack in the earth's crust through which hot lava spills out. The volcano remains stable beneath the crust even after the plates surpass the volcanoes. Examples of such volcanoes are the Hawaiian Islands in the Pacific Ocean which is formed by a single hotspot.

Coral Islands

These islands are formed by the skeletal remains of small sea creatures called Corals and other associated organic materials. Corals are found mainly in shallow depths of tropical and sub-tropical warm water. These organisms produce an external skeleton which is made of limestone (calcium carbonate) that accumulates to form small to large landmasses as islands.

Formation of Coral Islands 

• By Forming Coral Reefs: The microscopic corals grow in colonies which gradually expand from the sea floor and protrude above the ocean surface forming coral islands. Sand, rocks, as well as other inorganic and organic materials help to adhere to the coral detritus to form the reefs. Coral reefs are termed biodiversity hotspots as these are shelters for a large number of flora and fauna. Examples of such Islands are  the islands of the Bahamas in the Atlantic Ocean and the Caribbean Sea. 

• By Forming Atoll: Coral islands can also grow as a coral reef which surrounds a lagoon on the outskirts of a volcanic oceanic island. These coral islands are termed atolls. When a volcanic ocean island is moved from the hot spot due to plate tectonics, this island slowly sinks into the sea and the coral continues to grow. When these corals build up larger colonies it comes above the water in the form of a coral island. Examples of such islands are the Tuamotus Islands in French Polynesia and the Caroline Islands in the north of New Guinea

Artificial islands

The artificial islands are created by humans These islands are built to provide more land for infrastructural development, agriculture, tourism and recreational activities.

Formation of Artificial Islands

• By Extension of Existing Islands: Some artificial islands are constructed as the extension of already-existing islands. It is done by draining water around them or adding land-forming materials to extend them. Examples: The currently existing Hong Kong International Airport was created by the extension upon the existing islands of Chek Lap Kok and Lam Chau

• Land Reclamation: Some of the artificial islands are formed by reclaiming the lands. It is done by using natural materials like sand, rocks, gravel and synthetic materials like concrete. Examples of such islands are Flevopolder in the Netherlands. It is the largest artificial island with an area of 970 sq.km), Palm Jumeirah and Palm Jebel Ali in Dubai are also fine examples of such islands.

Fig. Example of artificial island (Palm Jumeirah, Dubai)

Basic Terminology

• Islets: Many islands are very small, which may be less than half a hectare (one acre). These tiny islands are called islets. 

• Eyots: Islands in rivers are sometimes called aits or eyots. For Example, Majuli Island in Brahmaputra river is the largest riverine island. 

• Archipelagos: An archipelago is a chain, cluster, or collection of islands, or a sea that contains a number of scattered islands. Indonesia is the world’s largest archipelago. It consists of about 17000 islands in the Pacific Ocean. The islands like the Aleutian Islands in the U.S. state of Alaska, are cold and ice-covered throughout the year. Islands like Tahiti are located in warm, tropical waters. Other islands like Greek islands located in the Aegean Sea are found in closely spaced groups.

Note for UPSC Aspirants: For UPSC aspirants interested in exploring further, here are some keywords to guide your research:Continental Islands, Tidal Islands, Barrier Islands, Oceanic Islands, Coral Islands, Artificial Islands, Processes of Formation, Islets, Eyots, Archipelagos