It is an active volcano, which last erupted in 1905–1911 with lava flows that destroyed villages on the central north coast. The island is formed by a massive basaltic shield volcano which rises from the seafloor of the western Pacific Ocean. A possible model for the formation of the volcanic Samoa island chain is explained by the Samoa hotspot situated at the east end of the Samoa Islands. In theory, the Samoa hotspot is a result of the Pacific Tectonic Plate moving over a 'fixed' deep and narrow mantle plume spewing up through the Earth's crust. The Samoa islands generally lie in a straight line, east to west, in the same direction the Plate is moving. In the classic hotspot model, primarily based on studies of the Hawaii hotspot, the volcanic islands and seamounts further away from the Samoa hotspot should be progressively older. However, Savaii, the most western of the Samoa island chain, and Tau Island, the most eastern of the Samoa islands, both erupted in the 20th century, data which is an enigma for scientists. Tau last erupted in 1866. Another discrepancy in the data from the Samoa islands is that subaerial rock samples from Savaii were too young by several million years to fit the classic hotspot model of age progression in an island chain, raising arguments among scientists that the Samoa islands does not have a plume origin. The nearness of Savaii and the Samoa island chain to the Tonga Trench at the south became a possible explanation for these discrepancies as well as the possibility that the islands were formed by magma seeping through cracks in stressed fracture zones. However, in 2005, an international team gathered further submarine samples from the deep flanks and rifts of Savaii. Tests on these later samples showed much older ages, about five million years old, that fit the hotspot model. The discovery in 1975 of Vailuluu Seamount 45 km east of Tau in American Samoa has since been studied by an international team of scientists and contribute towards understanding of the Earth's fundamental processes.