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 islands' foundations are actually two of a series of undersea features known as the Vening Meinesz Seamounts. This range of mountains also takes in Christmas Island and extends in a north-easterly direction from a prominent seafloor feature of the Indian Ocean known as the Ninetyeast Ridge. The Cocos atolls 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).
The Cocos (Keeling) Islands were the only coral atolls that Charles Darwin visited in 1836 when he developed his well known theory of atoll formation (Darwin 1842). He considered that the upgrowth of coral reefs continued long after the seamounts that supported them had subsided. While others have presented alternative theories, Darwin's subsidence theory of coral reef development has gained wide acceptance.
North Keeling Island is 2.0 km long and 1.3 km wide. Its shape is approximately rectangular with the long axis bearing slightly north-east. Like the southern atoll it is a true coral island. In form, the island is low and flat. The shore rises fairly steeply to a height of three to five metres and from this peripheral ridge the ground slopes gently down to a large, shallow, sandy-bottomed lagoon which occupies the greater part of the interior. The island varies from sand to rubble. On the northern shore there is a broad sandy beach. This continues along the western shore but with varying amounts of shingle. In profile the sandy beach rises up to about five metres above mean sea level.
The southern shore of the island is composed of a spectacular steep shingle beach, with a series of berms, or ridges, being identifiable. Much of the eastern shore is composed of a series of shingle berms, these are particularly well-developed just south of the channel into the lagoon, but continue to the north as well. There are also outcrops of coral conglomerate. A broad platform of conglomerate extends out over the reef flat at the eastern part of the island, almost closing the channel into the lagoon completely.
The lagoon shore of the island slopes gradually and deepens slowly. At low tide its maximum depth is less than two metres.