The correct option is 2 and 3 only.
Explanation
Atolls are ring-shaped coral reefs, islands, or series of islets. An atoll surrounds a body of water called a lagoon. These structures are primarily associated with the subsidence of volcanic islands in tropical and subtropical oceans, as explained by Charles Darwin's Subsidence Theory.
Statement-wise Analysis
- Statement 1 is Incorrect. Atolls are not high mountainous islands; they are typically low-lying islands composed of coral debris and sand. Furthermore, they are found in tropical and subtropical oceans, not polar oceans. Coral polyps, which build the reefs, require warm water (generally between 20°C and 30°C) to survive and calcify, making their existence in polar regions impossible.
- Statement 2 is Correct. Structurally, an atoll consists of a narrow ring of coral reef or closely spaced coral islands that surround a central depression. This ring represents the final stage of reef development after the central volcanic island has subsided completely below the sea surface.
- Statement 3 is Correct. The central depression within the ring of coral is filled with seawater, forming a lagoon. This lagoon is usually connected to the open sea through channels in the reef, maintaining the water level and salinity.
Key Takeaway
Atolls are low-lying, ring-shaped coral formations found in warm tropical waters that encircle a central lagoon, typically resulting from the complete submergence of a volcanic island.