Coral island

A coral island is built from broken-down coral and other organic debris. These islands are found in tropical and subtropical regions, usually as part of a much larger underwater coral reef. They are often called "low islands" to set them apart from "high islands," which are created by volcanic activity. Low islands form when sediment builds up on a coral reef or when such a reef is pushed upward.

Coral reefs are among the planet's oldest ecosystems. Over millions of years, they create massive limestone structures. These reefs support more species of plants and animals than any other habitat. The reefs build hard, chalky skeletons that provide shelter for fish in their crevices and surfaces for barnacles to attach. These structures also lift plants toward the sunlit surface waters and create calm areas where fish and plants can thrive. Over time, a reef may rise to the surface and become a coral island, starting a new land-based ecosystem.

The formation of a coral island often begins with a volcanic island over a hot spot. As the volcano emerges, a fringing reef grows around its edge. When plate tectonics move the volcano away from the hot spot, it can no longer resist wave erosion and sinks. Once submerged, the coral must keep growing to stay in the sunlit surface layer. This growth forms an atoll with a shallow central lagoon. Sediment builds up in the lagoon, creating an island made entirely of carbonate materials. Plant remains later add to the island's growth.

The term "low island" refers to geological origin, not strict height. Some low islands, like Banaba, Makatea, Nauru, and Niue, rise hundreds of feet above sea level, while some volcanic high islands are only a few feet high and are often called "rocks." Low islands surround the lagoons of atolls. Both types are often found near each other, especially in the South Pacific, where low islands sit on the fringing reefs around many high islands.

Most coral islands are in the Pacific Ocean, but they also exist in the Atlantic and Indian Oceans. Examples include the American territories of Jarvis, Baker, and Howland Islands in the Pacific; atolls in Colombia's Archipelago of San Andrés, Providencia and Santa Catalina in the Atlantic; India's Lakshadweep Islands (39 coral islands plus smaller islets and banks); several islands of Kiribati; the Maldives; St. Martin's Island in Bangladesh (8 square kilometers); and islands near Pattaya and Ko Samui in Thailand.

Coral is vital for biodiversity and fish populations, so protecting reefs is important. Many coral islands are small and low-lying, making them vulnerable to storms and rising sea levels. Human activities can harm reef systems and slow the creation of coral island chains through chemical and physical changes. Reefs grow in shallow, warm, nutrient-poor waters where they are not overwhelmed by phytoplankton. Adding fertilizers to runoff can cause phytoplankton to explode, choking out coral. Too much sediment blocks sunlight, starving the zooxanthellae living on coral and causing coral bleaching. Ocean acidity also matters: coral is made of calcium carbonate, which dissolves in carbonic acid. Increased carbon dioxide from human combustion forms carbonic acid in rainwater, raising ocean acidity and slowing coral growth. While low islands may have fewer habitats than high islands, studies in Palau show that species diversity in surrounding waters depends more on island size than on origin.

Low islands have poor, sandy soil and little fresh water, making farming difficult. They support human habitation less well than high islands and are threatened by sea level rise from global warming. People living on low islands mostly rely on fishing. Low islands generally have an oceanic climate.