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Christmas Island National Park

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Physical Characteristics


Climate

Christmas Island lies at the southern edge of the equatorial low pressure belt that moves north and south of the equator during the course of the year. This confers a typical tropical, equatorial climate with a wet and a dry season.

Adult Brown Boobies

Adult Brown Boobies

The wet season is from December to April when the north-west monsoon blows. For the rest of the year south-east trade winds bring slightly lower temperatures and humidity, and much less rain. The mean annual rainfall is 2154.0 mm. During the monsoon, heavy downpours lasting several days, and periods of humid calm weather are punctuated by gusty north-westerly winds. In years of significant El Nino activity in the Pacific Ocean, rainfall on the Island tends to be relatively low. Cyclones and cyclonic swells from the north-west sometimes affect the Island during the wet season. Humidity and temperatures vary little with ranges from 80-90%, and temperatures of average maximum 28 Celsius in March/April. The average minimum falls to 22 Celsius in August/September.

The following charts provide an indication of rainfall, temperatures and relative humidity for the Christmas Island region. Data for the charts was sourced from the Bureau of Meteorology.

Chart - Average Temperatures for Christmas Island

Chart - Average rainfall for Christmas island

Chart - Average humidity for Christmas Island

Geology

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

Continental Drfit

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).

Topography

North West Point

The North West Point of Christmas Island

The coast consists of mostly sheer, rocky cliffs from 10-20 m high with a few, small sand and coral rubble beaches. The interior is a slightly undulating plateau, from 160-360 m above sea level. A series of steep slopes or cliffs with intervening narrow terraces separate the central plateau from the shore. Generally a wider terrace is located immediately inland above the shoreline.

Hydrology

Most rain percolates through the soil and limestone and surface runoff only occurs after heavy rain. 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 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 from some of the cave systems on the Island are thought to be the outflows from the edges of such a lens.

 

Migrate back to the Christmas Island Home Page

Return to Christmas Island Home Page

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