The geology of the Falkland Islands

Plate tectonics have produced some surprising juxtapositions as the Earth’s continental fragments have drifted and jostled over the eons. Microplates seem to have enjoyed most freedom of movement, and none more so than that supporting the Falkland Islands. Though this archipelago is situated in the south-west corner of the South Atlantic Ocean, about 650 km east from Tierra del Fuego and the Strait of Magellan, its geology tells of an African heritage. Charles Darwin provided the first evidence for that – although he didn’t appreciate it at the time. HMS Beagle visited the Falkland Islands twice, in 1833 and 1834, and during the first visit Darwin discovered fossil shells, mostly brachiopods. His first impression had been unfavourable, but after that discovery he noted in his diary: “The whole aspect of the Falkland Islands were however changed to my eyes … for I found a rock abounding with shells; and these of the most interesting age.” Darwin published his account of Falklands’ geology in 1846. The “interesting age” proved to be Devonian, and as more data were acquired a close and surprising similarity was established with the fauna of equivalent age in South Africa. This similarity was soon extended to other aspects of the Falklands rock succession, whilst the geology of neighbouring Patagonia proved to be quite different. These relationships were not readily explicable without recourse to continental drift, so were largely ignored for many years despite a remarkably prescient interpretation by a South African geologist, Ray Adie, in 1952. Not content with a straightforward African connection, as championed by Alexander Du Toit in his 1937 book ‘Our Wandering Continents’, Adie proposed that what we would now call the Falklands microplate had rifted from the east coast of South Africa, and had then been rotated through 180° as it drifted to its present position. His evidence was drawn from the alignment of sedimentological and structural features from the two areas. Half a century later, and though the jury is still out on some of the details, Adie’s proposal is looking to be essentially correct. The close geological correlation between South Africa and the Falkland Islands is now put down to their original proximity in a reconstructed Gondwana supercontinent (Fig. 1).