A coral island or group of coral islands forming a ring that is surrounded by deep ocean water and that encloses a shallow lagoon. Atolls range in diameter from about 1 km to over 100 km and are especially common in the western and central Pacific Ocean. They are believed to form along the fringes of underwater volcanoes. → atoll source.
From atollon, atolon, from Divehi (Indo-Aryan language of the Maldive Islands) atolu "reef."
Fr.: source atoll
→ atoll; the name derives from the fact that on X-ray → color-color diagrams these sources often resemble a band of points at constant hard X-ray color, with "islands" of points appearing on time-scales of weeks and months.