Kiribati consists of 33 atolls and reef islands spread over an area exceeding 5 million km2 of ocean straddling the equator between 04°43' N and 11°25' S latitude and between 169°32' E and 150°14' W longitude. The total land area, however, only slightly exceeds 800 km2. All are formed on underlying seamounts caused by crustal hotspot activity associated with plate tectonics. Ages of these seamounts range from about 135 to 40 million years ago.
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. The atolls of Kiribati are all low atolls, where the maximum height of the emerged portion (usually less than 8 m) is made up of accumulations of broken reef material deposited by storms.
The Gilbert Islands are a group of 16 atolls and slightly raised limestone islands without lagoons with a total land area of 279 km2.
About 1,480 km east of the Gilberts lies the Phoenix Group, a cluster of eight largely uninhabited atolls and reef islands.
Further east are the Line Islands including Kiritimati (Christmas Island) situated some 3,330 km from. Kiritimati is the world's largest atoll in terms of contiguous landmass (388 km2) accounting for almost half of Kiribati's dry surface.
