Christmas Island is the summit of a submarine mountain, rising steeply to a central plateau dominated by stands of rainforest. This plateau reaches heights of up to 361 metres and consists mainly of limestone and layers of volcanic rock.
Geologists believe that about 60 million years ago the cone of an undersea volcano surfaced, forming a basin on which a coral atoll then began to form. More then 20 million years ago, the atoll appears to have sunk slowly and limestone was deposited as the coral began to build up. Ten million years later, this process stopped and an island began to emerge from the sea in a series of uplifts. Each of the island terraces was formed by the development of the fringing reef and sea cliff erosion before being uplifted. The lagoon drained and the central plateau was formed. Evidence of these volcanic origins can be seen at The Dales, and Dolly Beach where the basalt rock is exposed, forming the bed of freshwater streams.
The island's 80 kilometre coastline is an almost continuous sea cliff, of up to 20 metres in height. In a few places, the cliff gives way to shallow bays with small sand and coral shingle beaches. The largest of these bays forms the island's only port; Flying Fish Cove.
The island is surrounded by a narrow encircling coral reef. There is virtually no coastal shelf, and the sea plummets to a depth of about 500 metres within 200 metres of the shore.