Diego Garcia - Geography

Geography

Diego Garcia is the largest land mass in the Chagos Archipelago (which includes Peros Banhos, the Salomon Islands, the Three Brothers, the Egmont Islands and the Great Chagos Bank), being an atoll occupying approximately 174 square kilometres (67 sq mi), of which 27.19 square kilometres (10 sq mi) is dry land. The continuous portion of the atoll rim stretches 40 miles (64 km) from one end to the other, enclosing a lagoon 13 miles (21 km) long and up to 7 miles (11 km) wide, with a 4 miles (6 km) pass opening at the north. There are three small islands located in the pass.

The island consists of the largest continuous atolls in the world. The dryland rim varies in width from a few hundred metres to 2.4 km. Typical of coral atolls, it has a maximum elevation on some dunes on the ocean side of the rim of nine metres (30ft) above mean low water. The rim nearly encloses a lagoon about 19 kilometres (12 mi) long and up to 8 kilometres (5.0 mi) wide. The atoll forms a nearly complete rim of land around a lagoon, enclosing 90 percent of its perimeter, with an opening only in the north. The main island is the largest of about 60 islands which form the Chagos Archipelago. Besides the main island, there are three small islets at the mouth of the lagoon: West Island (3.4 ha/8.4 acres); Middle Island (6 ha/14.8 acres); and East Island (11.75 ha/29 acres). A fourth island shown on some maps, Anniversary Island one km (1,100 yards) southwest of Middle Island, appears as just a sand bar on satellite images. Both Middle Island and Anniversary Island are part of the Spur Reef complex.

The total area of the atoll is approximately 170 square kilometres (65.6 sq mi). The lagoon area is approximately 120 square kilometres (46.3 sq mi) with depths ranging down to about 25 m (80 feet). The total land area (excluding peripheral reefs) is approximately 30 square kilometres (12 sq mi). The coral reef surrounding the seaward side of the atoll is generally broad, flat, and shallow at about 1 m (3 feet) below mean sea level in most locations and varying from 100 to 200 m (300 to 650 feet) in width. This fringing seaward reef shelf comprises an area of approximately 35.2 square kilometres (14 sq mi). At the outer edge of the reef shelf, the bottom slopes very steeply into deep water, at some locations dropping to more than 450 metres (1,500 ft) within 1 km (0.6 miles) of the shore.

In the lagoon, numerous coral heads present hazards to navigation. The shallow reef shelf surrounding the island on the ocean side offers no ocean-side anchorage. The channel and anchorage areas in the northern half of the lagoon are dredged, along with the pre-1971 ship turning basin. Significant salt-water wetlands called barachois exist in the southern half of the lagoon. These are small lagoons off of the main lagoon, filled with seawater at high tide and dry at low tide. Scientific expeditions in 1996 and 2006 described the lagoon and surrounding waters of Diego Garcia, along with the rest of the Chagos Archipelago, as "exceptionally unpolluted" and "pristine".

There are no endemic species of plants, birds, amphibians, reptiles, mollusks, crustaceans, or mammals on Diego Garcia or in the surrounding waters. There are several endemic fish and aquatic invertebrates. All plants, wildlife, and aquatic species are protected to one degree or another. In addition, much of the lagoon waters are protected wetlands as a designated Ramsar site, and large parts of the island are nature preserves. Diego Garcia is frequently subject to earthquakes caused by tectonic plate movement along the Carlsberg Ridge located just to the west of the island. One was recorded in 1812; one measuring 7.6 on the Richter Scale hit on November 30, 1983 at 21:46 local time and lasted 142 seconds, resulting in a small tsunami which raised wave height in the lagoon to 1.5 metres (5 ft), and another on December 2, 2002, an earthquake measuring 4.6 on the Richter Scale struck the island at 12:21 a.m.

In December 2004, a tsunami generated near Indonesia caused minor shoreline erosion on Barton Point (the northeast point of the atoll of Diego Garcia).