Christmas Island's south coast
The Island is the summit of a submarine mountain. It rises steeply to a central plateau dominated by stands of rainforest. The plateau reaches heights of up to 361 metres and consists mainly of limestone with layers of volcanic rock.
The Island's 80 km coastline is an almost continuous sea cliff ranging in height up to 20 metres. There are thirteen places where breaks in the cliff give way to shallow bays and small sand and coral beaches. The largest of these bays forms the Island's port at Flying Fish Cove.
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).
Although it has a high rainfall, Christmas Island lacks permanent surface water. Nearly all rainfall goes quickly underground to join a karst drainage system. A few perennial streams flow at the Dales, Ross Hill Gardens, The Ravine, Jones Spring, Freshwater Spring, Dolly Beach, Hosnie's Springs and Waterfall.
Underground water accumulates in the caves and sinkholes at the interface of limestone and the underlying volcanic rock strata. Here it either flows along the interface, as in the case of the flow system from Grant's Well through Jedda Cave and Jane-up beyond, or flows down fractures in the volcanic rock. The flows along the limestone-volcanic rock interface emerge in some places as springs. There is evidence of a freshwater lens floating on top of underlying seawater and flows just above sea level.
Ramsar wetland site
The island has unusual relict populations of mangroves which were isolated from the ocean when the island lifted. Whilst mangroves of this group are distributed widely across the region, the stand represented at Hosnies Spring is most unusual in that it occurs high above sea level (24-37 metres), the mangroves are unusually tall (up to 30-40 metres high) and because it appears that the stand has persisted at the site for approximately 120 000 years. The stand is maintained by the permanent freshwater spring. The site is registered as a wetland of international importance - find out more about Hosnies Spring here