GeologyGeology

Geology

The Cocos (Keeling) Islands consist of two separate coral atolls, 24 km apart, which have formed atop an old volcanic seamount that rises from a depth of 5,000 m in the north-eastern Indian Ocean. Bathymetric research shows that the atolls’ foundations are actually two of a series of undersea features known as the Vening-Meinesz Seamounts. This range of mountains also includes Christmas Island and extends in a north-easterly direction from a prominent seafloor feature of the Indian Ocean known as the Ninetyeast Ridge. The atolls of the Cocos (Keeling) Islands are two peaks in a section of the range known as the Cocos Rise (Jongsma 1976).

The two atolls are connected by a narrow submarine bank at a depth of 700-800 metres (Gibson-Hill 1948). Approximately 100 kilometres south-west of Cocos lies the Umitaka-Mary or Muirfield Seamount which rises to 16 metres below the surface of the ocean. Taken as a series from Umitaka-Mary in the south-west to Christmas Island in the north-east, these seamounts display a sequence in the evolution of atolls. Over the last 100 million years the Indo-Australian plate has moved steadily north towards the Java Trench. As it has done so it has passed over a ‘hot spot,’ or plume rising from deep in the mantle, producing a chain of seamounts with a south-west to north-east trend. Sediments on the Cocos Rise are very thin (100 metres to 200 metres thick) and no mineralisation or accumulation of petroleum has occurred (Jongsma 1976).

The Cocos (Keeling) Islands were the only coral atolls that Charles Darwin visited in 1836 as he developed his well-known theory of coral atoll formation (Darwin 1842), in which he asserted that the upgrowth of coral reefs continued long after the seamounts that supported them had subsided. While others including Guppy (1889) and Wood-Jones (1912) presented alternative theories, Darwin’s subsidence theory of coral reef development gained wide acceptance. The thickness of the corals underlying Cocos is not known, but the dredging of basaltic rocks in local waters suggest that it is in the order of 500-1,000 metres (Bunce 1988).

North Keeling Island is surrounded by a broken, irregular fringing reef, except at the north-west corner. Like the shingle bank around the shore of the island, the reef is narrower on the sheltered sides of the island and broader on the exposed sides. On the east coast it is continuous across the mouth of the lagoon and forms a wide bar, which at times blocks the entrance.