Introduction

Islands

A piece of land surrounded by water on all sides. Generally formed of the following types:

1. 1. Continental islands: These islands were formally a part of the mainland and are now detached from the continent by a shallow lagoon or a deep channel. The rise in sea levels then submerged the low-lying areas that connected them. The evidence of this is the presence of identical flora and fauna on both sides of the land.

Types:
Individual islands: These lie just outside the mainland very much associated with the characteristics of the mainland they were once a part of.

1. 1. • Archipelagoes: Groups of islands varying in shape and size.

1. 1. • Island arcs: Islands form an archipelago in the shape of a loop around the mainland, marking the continuation of mountain ranges that can be traced on the continent
      • Oceanic islands: These are isolated islands that lay thousands of miles away from the mainland. They have flora and fauna different from the nearest mainland. They are usually sparsely populated but are useful stops for airplanes and steamers.

Types:

1. 1. • Volcanic islands: Many islands are actually the topmost part of cones of volcanic mountains that rise from the ocean bed.

1. 1. • Coral islands: These are lower and emerge just above the water surface. These are built by coral animals of various species and are found near the shores of the mainland and in midst of oceans.

Coral reefs:

1. Sources of greatest biodiversity amongst marine ecosystems.
2. They are indicative of the health of the marine ecology. Cant survive in muddied waters or polluted water. They need sediment-free water.
3. Primary food chain. Home to Zoantharia which is an aquatic primary producer.

 Characteristics of coral reefs:

1. Corals are tiny fleshy sea anemones. They extract calcium from the sea and from skeletons to protect their bodies.
2. Each generation dies on top of the previous.
3. They have a symbiotic relationship with the microscopic plant Zoantharia which is photosynthetic in origin.
4. The corals provide protection to Zoantharia and Zoantharia provides food to the corals.
5. When corals are under stress they expel Zoantharia and become white. Ultimately they die without food.

Conditions needed for coral reefs:

1. The shallow platform where sunlight reaches.
2. Temperature 25-27°
3. Warm, tropical, and low latitude waters.
4. Salinity should be 33 ppm.
5. Not at the mouth of the river; they need circulating nutrient-rich water.

Reasons for coral bleaching:

1. Global warming and ozone depletion
2. Ocean acidification,
3. Algal bloom
4. Marine pollution and oil spills.

Formation of coral reefs:

1. Fringing reef: the coral reef is formed bordering an island or continent. A distance is created between the island and the reef. Here small water bodies are seen to form called lagoons.
2. Barrier reef: the reef isn’t continuous but broken. The lagoons can connect to the ocean. Here the coral reef is formed parallel to the coast it’s called a barrier reef.
3. Atoll: the hill is submerged and now only a narrow ring of the reef is visible.

Theory of Reef Formation

Formation of reefs: Subsidence Theory by Darwin

All reefs started as fringing reefs of islands or on top of volcanoes that emerge out of sea bed. Then these islands subside with Earth’s movements.

However, the fringing reefs continue to grow upwards. The corals also continue to grow outwards as more favorable conditions are seen there than at the inner edge.

Finally, a barrier reef is formed and this becomes an atoll as the inner island is completely submerged due to the down warping.

Glacial Control theory: RA Daly

He said that a close relationship exists between the Glaciation and Coral reef formation. The theory states that the Ice Ages saw a great marine erosion due to the absence of coral barriers. The erosion lowered the islands.

Then as the glaciers melted the water levels rose. The coral activity started and now the island was covered by corals. However, as the island submerged due to erosion and wave action, the corals continued to move upward to rise above the water level. This created Atolls.