and formation of the Central Scotia Sea. Opening of Drake Passage Between the Late Cretaceous and the Early Oligocene (90–30 Ma) little changed in the region, except for the subduction of the Phoenix Plate along the Shackleton Fracture Zone. The Late Paleocene and Early Eocene (60–50 Ma) saw the formation of South Scotia Sea and South Scotia Ridge — the first sign of separation of the southern Andes and the Antarctic Peninsula — which resulted in seafloor spreading in the West Scotia Sea and hence the initial opening of a deep Drake Passage. The ongoing lengthening of the North Scotia Ridge beyond the Burdwood Bank caused South Georgia to move further east. The banks of the North Scotia Ridge contain volcanic rocks similar to those found in Tierra del Fuego, including on Isla de los Estados on the easternmost tip. During the early Eocene (50 Ma), the Drake Passage between the southern tip of South America at Cape Horn and the South Shetland Islands of Antarctica was a small opening with limited circulation. A change in relative motion between the South American Plate and the Antarctic Plate would have severe effects, causing seafloor spreading and the formation of the Scotia Plate. Marine geophysical data indicates that motion between the South American Plate and the Antarctic Plate shifted from N-S to WNW-ESE accompanied by an eightfold increase in the separation rate. This shift in spreading initiated crustal thinning and by 30–34 Ma, the West Scotia Ridge formed. One of the most prominent features in the Scotia Plate itself is the median valley known as the West Scotia Ridge. It was produced by two plates that are no longer independently active: the Magallanes and Central Scotia Plates. The ridge consists of seven segments separated by right-lateral transform offsets of various lengths. The western region of the Scotia Plate can be dated to 26–5.5 Ma. A complex pattern of spreading prior to 26 Ma is probably present in the oldest parts of this spreading regime; i.e. west of Terror Rise (north of Elephant Island) and on the shelf slope of Tierra del Fuego. Until 17 Ma the Central Scotia Plate moved quickly eastwards, fuelled by the eastern trench migration, but both plates have moved very slowly since. References Notes Sources Tectonic plates Geology of the Atlantic Ocean Geology of Argentina Geology of Magallanes Region