Why are many Pacific islands made of coral formations?

Many Pacific islands are composed of coral formations due to the unique geological and biological processes of reef development in tropical waters. These islands typically originate in one of two ways:

• Accretion: Rubbly reef rock, broken off by storms and waves, accumulates alongside finer reef detritus. Over time, this material is cemented together by calcium carbonate dissolved in rainwater, eventually forming land that can be colonized by plants and animals.1
• Subsidence: As proposed by Charles Darwin, many islands represent the final stage of a reef growing upward around a sinking volcanic island. As the volcano subsides, the reef continues to grow toward the surface, eventually leaving behind a ring-shaped atoll that encloses a lagoon.23

These processes are supported by the tropical environment, where high light intensity and radiant energy facilitate the growth of corals and their symbiotic zooxanthellae, which accelerate the formation of skeletal structures. Additionally, some islands may be raised above sea level through crustal uplift.451