Geographically, the Maldives is categorized as a coral archipelago. Unlike Iceland or Reunion Island, which are composed of basaltic volcanic rock, the islands of the Maldives are formed by the growth of coral reefs on the rim of a sunken volcanic chain. As the original volcanoes subsided or sea levels rose, the coral kept growing upward, eventually forming atolls. Therefore, the visible land surface is purely organic and sedimentary in origin. This makes the Maldives extremely low-lying and geographically distinct from the high, rugged volcanic islands found elsewhere in the Indian and Atlantic Oceans.