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 than 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.
Christmas Island experiences a tropical equatorial climate with wet and dry seasons. The wet season is from December to April when the island comes under the influence of the north-west monsoons. During the rest of the year, the south-east trade winds bring slightly lower temperatures and humidity with much less rain.
Tropical cyclones occasionally pass close to the island during the monsoon season, bringing strong winds, rain and rough seas. Since settlement, no cyclone has been recorded passing directly over the island.
The mean annual rainfall is 1,930 millimetres. Most of this rain falls between November and May. February and March are usually the wettest months.
Because of the oceanic influence, the relative humidity does not vary seasonally as much as rainfall. Humidity usually ranges between 80 – 90%. Temperatures on the island vary little from month to month. The average daily maximum temperature reaches a high of 28° Celsius in April and the average daily minimum temperature falls to 22° Celsius in August.
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