Land surrounded by water on all sides is called an island. Although one might consider the continents to be islands, they are not regarded as islands due to their massive size. Australia is the smallest continent and all other land surrounded by water that is smaller than Australia, are labeled as islands.
The six major types of islands have been discussed below:
1. Continental Islands
Continental islands represent land masses that were connected to a continent at some point in time but have separated from the continents by the shifting of tectonic plates to form islands surrounded on all sides by water.
How Are Continental Islands Formed?
A number of continental islands were formed when the shifting continents of the world broke apart millions of years ago. According to scientists, initially, there was only one massive mass of land called Pangaea. Over a period of time, movements of the Earth’s crust led to the crust breaking into several pieces and drifting apart. Such tectonic activities gave birth to the different continents of the world and led to the formation of the continental islands. Greenland and Madagascar, both very large islands, were formed in this manner.
Another process of formation of continental islands took place due to sea level changes thousands of years ago. During the most recent glacial period, about 18,000 years back, much of the continental land was ensheathed by massive glaciers. Since much of the water was locked in glaciers, sea level was lower than what it is today. As the world grew warmer, the glaciers retreated and the melting of the ice increased the sea level significantly. This change in sea level triggered flooding of vast tracts of coastal areas, submerging tracts of land permanently and leaving behind several islands. Islands like the British Isles were born in this manner.
The processes of weathering and erosion might also lead to the separation of a piece of land from the mainland, resulting in an island.
2. Oceanic Islands
Oceanic islands are islands that do not sit on the continental shelves and are mostly of volcanic origin. These islands are often referred to as “high islands.” Oceanic islands of volcanic origin are formed when lava spewed out by underwater volcanic activity builds up in layers to reach the surface of the ocean and appear above the water level. The oceanic islands are thus the summits of underground volcanoes.
How Are Oceanic Islands Formed?
Oceanic islands are formed in different ways. They are as follows:
When tectonic plates shift at a subduction zone, volcanic eruptions might take place and lead to the formation of an island. The Japanese islands have been formed in this manner. The country of Japan lies at the site of 4 tectonic plates. Two of these plates, the Eurasian and the North American plates, are linked to continental shelves. These plates are lighter than the two ocean plates, namely the Pacific plate and the Philippine plate. Thus, during tectonic activities, the heavy oceanic plates are subducted beneath the lighter continental plates leading to volcanic activities and island formation.
Oceanic islands are also formed when tectonic plates rift apart from each other. For example, in 1963, the splitting away of the North American plate from the Eurasian plate led to intense volcanic activity that gave birth to the island of Surtsey. It is one of the most recent islands of the world.
Oceanic islands are also formed over a “hot spot” when a continent shifts over the “hot spot”. A hot spot in geology is a crack or break in the Earth’s crust that allows material from the mantle to rise up. Even if the crust over the hot spot shifts, the hot spot stays quite stable beneath the crust. The oceanic islands of Hawaii were formed by a single hot spot. The hot spot is even active today and hosts two volcanoes over it, the Kilauea and the Mauna Loa. The hot spot has also given birth to the newest island of Hawaii, the Loihi which also sits on the hot spot.
3. Artificial Islands
How Are Artificial Islands Created?
Although most islands that are existing today were created by natural processes, some islands are also man-made. Such islands were created to serve various purposes like the extension of habitable land, the creation of new land for agriculture or to encourage tourism.
Often, natural islands have been extended artificially by draining water around these islands or adding material to extend the islands. For example, the Vasilyevsky Island was extended westward by a distance of about 0.5 km during the construction of the St. Petersburg Passenger Port.
Examples of new artificial islands created using material sourced from elsewhere are the islands of Dubai like The World, the Palm Jumeirah, and the Palm Jebel Ali. These massive islands were created by depositing sand dredged from the Persian Gulf on the sea near Dubai’s shores.
Artificial islands are created using either natural materials like sand, earth, and rock or by using synthetic materials like concrete or recycled waste.
4. Barrier Islands
Narrow islands that lie parallel to the coastline, and separate the mainland from the ocean, are known as barrier islands. Barrier islands serve to protect the coastline from storms and waves. The islands are separated from the mainland by sounds or lagoons.
How Are Barrier Islands Formed?
The formation of barrier islands can take place by various processes. For example, barrier islands might be created by the deposition of sediments like gravel, sand or silt by longshore currents. Sandbars are an example of barrier islands formed by the accumulation of sand along the coast by ocean currents. In the long run, these sand bars might also be eroded away by the same currents that formed them.
Barrier islands might also be made from billions of coral exoskeletons.
Barrier islands were also formed during the retreat of glaciers after the last Ice Age. As the ice melted and sea levels rose, sandy coastal areas were submerged and stretches of sandy islands were left behind. An example of this type of island is the Outer Banks island off the southeastern coast of the US.
Barrier islands have also been formed by the glacial moraines. When glaciers melted during the last Ice Age, they left behind piles of gravel, soil, and rock called moraines. As a rise in sea level triggered flooding along the coasts, the moraines near the coast were surrounded by seawater, leaving behind islands.
5. Coral Islands
Coral islands that are built of coral are located in the warm waters of the tropical oceans of the world. Corals are tiny sea creatures that secrete a hard exoskeleton around them that is made of calcium carbonate. Corals grow in colonies and gradually the colony size grows and expands and often surfaces above the water to result in the formation of coral islands. Sand and cement help to bind the coral-based structure of such islands. Coral islands serve as important habitats for a diversity of aquatic creatures. Examples of coral islands include the islands of the Bahamas.
6. Tidal Islands
Tidal islands are continental islands that appear as islands during the high tide. Here the mainland is not completely cut-off from the island but the connecting land between the two is submerged under the water during high tides. The Mont Saint-Michel island is France is an example of a tidal island.