Islands are pieces of land surrounded by water on all sides. They come in all sizes—from small rocky outcrops to large landmasses like Greenland. Depending on how they were formed, islands can be continental or oceanic.
Coral reefs, on the other hand, are underwater or partially submerged structures built over thousands of years by tiny sea organisms called coral polyps. These reefs are not just beautiful; they’re also crucial for marine biodiversity and coastal protection.
Types of Islands
1. Continental Islands
These islands were once part of the mainland or are situated on the continental shelf. They share similar geological characteristics with nearby continents.
(a) Individual Continental Islands
- These were once connected to continents but got separated due to rising sea levels or land subsidence.
- Examples:
- Greenland (largest island, near North America)
- Sri Lanka (once part of the Indian subcontinent)
(b) Archipelagos
- A group or chain of islands formed close to the mainland, often sitting on shallow continental shelves.
- Examples:
- Andaman and Nicobar Islands (India)
- British Isles
(c) Festoons
- Arc-shaped island chains typically found near subduction zones, often volcanic in origin but situated on continental margins.
- Examples:
- Japanese Islands
- Kuril Islands
2. Oceanic Islands
These islands have no direct geological link to continents and often emerge from the ocean floor.
(a) Volcanic Islands
- Formed when magma from beneath the ocean crust erupts and builds up over time, rising above the sea surface.
- Often part of mid-ocean ridges or hotspot activity.
- Examples:
- Hawaii, Iceland, Canary Islands
- Lakshadweep (has volcanic origin traces)
(b) Coral Islands
- Formed from coral reef deposits, often found in warm tropical oceans. These are usually flat and low-lying.
- Examples:
- Maldives, Lakshadweep, Tuvalu
Coral Reefs
Coral reefs are limestone structures formed by coral polyps, which secrete calcium carbonate. These reefs grow best in shallow, warm, and sunlit waters between 30°N and 30°S latitudes.
Ideal conditions for coral growth:
- Temperature: 20–25°C
- Clear, unpolluted saline water
- Plenty of sunlight (shallow depths)
Types of Coral Reefs
1. Fringing Reefs
- Found close to the shoreline, sometimes attached to it.
- The lagoon (if present) is shallow and narrow.
- Examples:
- Reefs around Red Sea
- Reefs near Andaman Islands
2. Barrier Reefs
- Separated from the coast by a wide and deep lagoon.
- Located farther offshore, forming a “barrier” to the mainland.
- Examples:
- Great Barrier Reef (Australia) – world’s largest
- Belize Barrier Reef
3. Atolls
- Ring-shaped reefs enclosing a lagoon, often found on submerged volcanic islands.
- No central landmass remains.
- Examples:
- Lakshadweep Islands, Maldives, Marshall Islands
Probable Origin of Coral Reefs – Darwin’s Theory
The most widely accepted theory of coral reef formation was proposed by Charles Darwin.
Darwin’s Subsidence Theory:
- Fringing reefs form around volcanic islands.
- As the island gradually sinks (subsides), the reef keeps growing upward and outward, forming a barrier reef.
- Eventually, the island may completely submerge, leaving behind a ring-shaped atoll.
This theory explains the transformation from fringing reef → barrier reef → atoll.
