GEOLOGY

Almost all isolated oceanic islands sit atop the remains of ancient volcanoes. The Cocos (Keeling) Islands are two coral atolls which have developed on top of old volcanic seamounts, rising from the depth of 5000 metres in the north east Indian Ocean.

The islands' foundations are two of a series of undersea features known as the Vening Meinisz Seamounts. This undersea range of mountains also includes Christmas Island and extends in a north north-easterly direction from a prominent Indian Ocean sea floor feature known as the Ninetyeast Ridge. The Cocos atolls are two peaks in a section of the range known as the Cocos Rise and are connected by a narrow underwater bank at a depth of 700-800 metres.

Atolls are more or less circular coral reefs enclosing a lagoon, but without any land inside. On large atolls, parts of the reef have been built up by wave action and wind to form low island chains connected by the reef. The environmental aspects of atoll islands are unique in some respects. For example there is no rock other than coral limestone composed of calcium carbonate. This means that plants requiring other minerals such as silica, can not be cultivated without the aid of fertilisers or some outside source of rock from a larger island composed of volcanic or other igneous rock. The palm tree is native to atoll islands because it thrives on brackish water and the seed, or nut, is distributed widely by floating from one island to another.

A VISIT FROM CHARLES DARWIN

Charles Darwin visited the Cocos (Keeling) Islands in 1836 aboard the HMS Beagle and it was during this visit that he developed his theory of atoll formation. He spent some time exploring the southern atoll and also visited North Keeling. In his publication on coral reefs in 1842, he was the first to propose the theory of reef formation and evolution, building on his discovery of coralline fossils in inland areas and in mountains earlier in the journey and his visit to the islands. That theory, which is still held as valid, explains the dynamics of the three principal categories of coral formation.

When a volcano emerges from tropical seas, it creates conditions favourable for the development of a fringe-like coral reef, or fringe reef, about its sloping coasts. A coral bank develops that tends upward and outward, increasing in thickness and width. After the phases of eruption and emersion, the volcano slowly sinks. This process can bring about the formation of a barrier reef when a channel of deep water forms between the island and the fringe reef. In practice it is often the old fringe reef that is found to be detached from the coast because of the sinking. When, later, the volcano finally submerges below the surface, only the ring of coral around the old island remains, and an atoll is born. The many atolls of the Indian and Pacific Oceans vary in diameter from 1 to 160 kilometres, more frequently varying between 5 and 30 kilometres. The study of sediments in the depths of existing atolls seems to effectively demonstrate that they rest on ancient volcanic soils and in some cases they have been dated back some thousands or even several million years. The thickness of the corals underlying the Cocos (Keeling) Islands is not known, but dredging of basaltic rocks in local waters suggest that it is in the order of 500-1000 metres.

Charles Darwin wrote " April 12th.-- in the morning we stood out of the lagoon on our passage to the Isle of France. I am glad we have visited these island; such formations surely rank high amongst the wonderful objects of this world. Captain Fitzroy found no bottom with a line 7200 feet in length, at the distance of only 2200 yards from the shore: hence this island forms a lofty submarine mountain, with sides steeper even than those of the most abrupt volcanic cone. The saucer shaped summit is nearly 10 miles across; and every single atom form the least particle to the largest fragment of rock, in this great pile, which however is small compared with the very many other lagoon-islands, bears the stamp of having been subject to organic arrangement. We feel surprise when travellers tell us of the vast dimensions of the Pyramids and other great ruins, but how utterly insignificant are the greatest of these, when compared to these mountains of stone accumulated by the agency of various minute and tender animals! This is a wonder which does not first strike the eye of the body, but, after reflection, the eye of reason".