What is "microatoll"

n. A colony of coral, usually 1-3 metres in diameter that is dead on the top as a result of exposure at low tide.

A microatoll is a discoid colony of coral, dead on the top but living around the perimeter. Growth is mainly lateral, as upward growth is limited by exposure to air. Microatolls may be up to in diameter. They are named for their resemblance to island atolls formed during the subsidence of volcanic islands, as originally suggested by Darwin (1842). They act as natural recorders of sea level, which allows the monitoring of sea level changes in response to global warming. They have also been used to quantify and date changes in relative sea level in seismically active areas, and to provide information on changes in sea surface temperature using oxygen isotope values as a proxy.