Low island
A low island is, in geology (and sometimes in archaeology), an island of coral origin. The term applies whether the island was formed as a result of sedimentation upon a coral reef or of the uplifting of such islands. The term is used to distinguish such islands from high islands, whose origins are volcanic.
This distinction is important to understand, as there are some "low islands", such as Makatea, Nauru, Niue and Banaba, which rise several hundred feet above sea level, while a number of "high islands" rise no more than a few feet above sea level, often classified as "rocks".
Low islands are the kind of islands which ring the lagoons of atolls.
The two types of islands are often found in close proximity, especially among the islands of the South Pacific Ocean, where low islands are found on the fringing reefs which surround most high islands.