Great Chagos Bank

The Great Chagos Bank (satellite picture)

The Great Chagos Bank is a major coral structure within the Chagos Archipelago (the British Indian Ocean Territory). It is the largest atoll structure in the world, with a total area of 4,881 square miles. Most of the structure is underwater, but with parts emerging as islands.

Islands

Despite its enormous size, the Great Chagos Bank is largely a submarine structure. There are only four emerging reefs, mostly located on the western rim of the atoll, except for lonely Nelson Island which lies wholly isolated in the middle of the northern fringe. These reefs have seven or eight individual low and sandy islands, with a total land area of about 2.2 square miles. All islands and their surrounding waters are a Strict Nature Reserve since 1998. The total length of the eastern and southern expanses of the bank, as well as the reefs in its central area are wholly submerged.

The islands of the Great Chagos Bank, starting clockwise from the south, are:

• Danger Island (a mile and a quarter long from north to south, by half a mile wide, land area 163 acres, vegetated with palm trees up to 40 feet high.
• Eagle Islands
  • Eagle Island, vegetated with high coconut trees, land area 605 acres
  • Sea Cow Island, vegetated with trees, land area 44 acres.
• Three Brothers and Resurgent Islands, vegetated with high coconut trees, land area 99 acres.
  • South Island (largest of the group), 57 acres
  • Middle Island, 20 acres
  • unnamed rocky islet
  • North Island, 15 acres
• Nelson Island (1.2 miles long from east to west, up to 0.3 mile wide, land area 99 acres, with bushy vegetation 10 feet high.

Cartography of the submerged reefs

The Great Chagos Bank was surveyed for the first time by Commander Robert Moresby of the Indian Navy in 1837; all other maps that would be drawn for over a century and a half were based on his chart. Although the charts of atolls made up of mostly emerged reefs, like Peros Banhos and Diego Garcia, were relatively accurate, the cartography of the vast sunken reefs forming the Great Chagos Bank proved quite a challenge. The real shape of these sunken reefs was known only when satellite imagery became available in the latter part of the 20th century.

Moresby's original hydrographic drawings were somewhat at variance with the true shape of the submerged reef, especially in areas where there were no emerging islands close by, like in the South east of the bank. The outlines of the first hydrographic surveys were marked in the 1980s navigational maps of the Chagos with a dotted line and the legend "existence doubtful" until the 1998 edition.

Outside links

• Geochronology of Basement Rocks from the Mascarene Plateau, the Chagos Bank and the Maldives Ridge