The land stirs: Volcanoes and the eastern highlands

Australia is currently the only continent in the world without active volcanoes, but this was not so in the past. There has been regular volcanism throughout Australia's geological history, especially of the silica-rich, felsic type along the eastern margin during the Paleozoic and Mesozoic, and the most recent mafic phase has barely finished. Basalt eruptions may have started in the late Cretaceous, as early as 90–85 Ma, were most active through much of the Cenozoic (from 55 Ma) and persisted until the most recent eruptions, in South Australia, only 4.6 ka ago. There is a close relationship between these basalts and the Great Divide of eastern Australia. Rich soil derived from the weathered basalt on elevated country combined with cooler and wetter climates along the eastern margin is the basis for many of Australia's finest agricultural and horticultural areas. THE VOLCANIC PROVINCES The mainland has no active volcanoes, though there are two active volcanoes in the offshore Australian territory, which includes Heard Island and the McDonald Islands. These islands are part of the Kerguelen volcanic system, related to a large igneous province of mantle plume association, and lie about 4000 km southwest of Australia and 1500 km north of Antarctica. Big Ben volcano on Heard Island has a high point at Mawson Peak which is 2745 m in elevation. It has a thick mantle of snow and glacial ice and last erupted in 1992 and 2016. The McDonald Islands volcano is only 230 m high and erupted between 1997 and 2005, following a long hiatus in activity thought to have extended back tens of thousands of years. Most of the volcanism in Australia produced rhyolite and andesite, rocks which formed mainly during a subduction-related active margin setting, and involved considerable melting of continental crust. In addition, three episodes of within-plate, mantle-derived basaltic igneous activity are evident: in the early to mid Cambrian (at about 515 Ma) in northern Australia, during the final assembly of Gondwana; in the Jurassic and early Cretaceous (185–132 Ma) in southern and western Australia, before and during the initial Gondwanan breakup; and in the late Cretaceous (about 85 Ma) to Holocene (5.6 ka) interval, along eastern Australia.