Geology
Christmas Island's south coast
Egeria Point
The Island is believed to be on a tectonic plate moving northwards a few centimetres a year that puts its present location some 700 km, or about 15 degrees of latitude, north of where it first emerged from the sea. One theory is that about 60 million years ago an undersea volcano rose to the surface and a coral atoll formed. Some 20 million years ago, the atoll began to subside and limestone accumulated as the corals sank. About 10 million years ago the subsidence reversed and an Island emerged in a series of uplifts that give it a stepped appearance. Each terrace was formed by the combined effects of fringing reef development and erosion of the sea cliff before the next uplift occurred. Examples of more recent faulting with lava flows (which probably occurred underwater) can be seen at the Dales, Dolly Beach, Waterfall, Ross Hill Gardens, Winifred Beach and Egeria Point.
Caves and sinkholes typical of limestone formations occur at many points on the Island.
The phosphorites commonly found on coral islands are now believed to result from lagoonal marine sediments on Christmas Island, although the chemistry of their formation is unclear. The soils of Christmas Island are derived from two sources - limestone (terra rossa soils) or basaltic extrusive rocks (krasnozem soils).