The Tuvalu seamount chain is a Mid-Pacific linear volcanic chain associated with a crustal hotspot that is believed to have formed the underlying sea mounts some 50–70 million years ago. However, the age and volcanic origin of these underlying structures are not important for soil formation as all atolls in the group are the surface expressions of thick deposits of calcareous biominerals (coral reef materials) overlying these ancient volcanic mounds.
Tuvalu consists of three reef islands and six true atolls. The true atolls are more or less continuously emerged or slightly emerged calcareous reefs surrounding a central lagoon, whereas the reef islands have no true lagoon. The atolls of Tuvalu are all low atolls, where the maximum height of the emerged portion (usually less than 5 m) is made up of accumulations of broken reef material deposited by storms. All islands in Tuvalu are free of any major deposits of volcanic materials, but there are phosphatic materials concentrated in small areas.
