GC Leong Test: Island And Coral Reefs - UPSC MCQ

Oceanic islands: 

• These islands are normally small and are located in the midst of oceans. They have no connection with the mainland, which may be hundreds or thousands of miles away. 

• They have flora and fauna unrelated to those of the continents. The Galapagos Islands have many unique species of animals.

• Due to their remoteness from the world's major trading centres, most of the oceanic islands are very sparsely populated.

Coral islands. Unlike the volcanic islands, the coral islands are much lower and emerge just above the water surface. These islands, built up by coral animals of various species, are found both near the mainland and in the midst of oceans.

Coral Reefs: 

• Many kinds of coral animals and marine organisms such as coral polyps, calcareous algae, shell-forming creatures, and lime-secreting plants live in large colonies in tropical seas. 

• Though they are very tiny creatures, their ability to secrete calcium carbonate within their tiny cells has given rise to a particular marine landform type. They exist in numerous species of many forms, colours and shapes.

• Under favourable conditions, they grow in great profusion just below the water level. Taking coral animals as a whole, the polyps are the most abundant and also the most important.

There are also non-reef- building species such as the "precious corals' of the Pacific Ocean and the 'red coral of the Mediterranean which may survive in the colder and even the deeper waters. As a rule, they thrive well only in the warmer tropical seas.

• The water temperature must not fall 68°F. (20°C.). 

• This virtually limits the areal distribution of corals to the tropical, and subtropical zones. 

• Again they will not flourish where there are cold currents because of the cold water's upwelling from the depths that cools the warm surface water. 

• This explains why coral reefs are generally absent on the western coasts of continents. 

• On the other hand, the warming effect of the warm currents, e.g. the Gulf Stream, means that corals are found far to the north of the West Indies in the Atlantic Ocean. 

• The Pacific and the Indian Oceans, however, have the most numerous coral reefs

The depth of water exceeding 180 feet is not suitable for coral reefs because sunlight is too faint for photosynthesis to take place beyond this depth. Coral reefs require sunlight to survive, as it provides the energy for photosynthesis. The energy from photosynthesis is used by the coral polyps to create a calcium carbonate skeleton, which forms the basis of the reef.

Beyond a certain depth, sunlight cannot penetrate the water and becomes too faint for photosynthesis to take place. This depth is generally around 180 feet, although it can vary depending on factors such as water clarity and quality.

Therefore, both assertion (A) and reason (R) are true, and R is the correct explanation of A.

Coral animals and marine organisms like coral polyps which live in large colonies in tropical seas have the ability to secrete calcium carbonate within their tiny cells. Polyps reside in tiny cups of coral and help to form coral reefs. Coral reefs begin to form when free swimming coral larvae attach to submerged rocks or other surfaces.
The conditions necessary for the formation of coral reefs are as follows:
1. Water temperature must not fall below 20 degree Celsius.
2. Depth of water should not exceed 180 feet because beyond this sunlight is too faint for photosynthesis to take place.
3. The water should be saltish and free from sediment. They survive best in moving ocean water away from silty coasts or muddy mouths of streams.

• A fringing reef is a coralline platform lying close to the shore extending outwards from the mainland. 

• It is sometimes separated from the shore by a shallow lagoon. 

• It is widest when fringing a protruding headland but completely absent when facing the mouth of a stream. 

• The outer edge grows rapidly because of the splashing waves that continuously renew the supply of fresh food.

• Atolls are ring-shaped coral reefs, islands, or series of islets that encircle a lagoon. They form when a coral reef grows around a volcanic island that later subsides into the ocean.
• The first statement is correct as atolls resemble barrier reefs but are circular and lack land in the center.
• The second statement is also correct because atolls typically have breaks in the encircling reef, allowing water to flow in and out of the lagoon.

 Therefore, Correct Answer - Option C