spreading ridges can also diminish volcanism, as observed at the Chile Rise further south. The buoyancy of these crustal structures may hinder subduction, reducing water supply to the mantle and inducing the formation of melts. Not all volcanism in Chile is relative to subduction of the Nazca Plate; the Chile Ridge exhibits submarine volcanism, and volcanism in the Austral Volcanic Zone is triggered by the Antarctic Plate subducting beneath the South American Plate. , one hundred ninety-eight South American volcanoes were on the Global Volcanism Program's list of volcanoes with Holocene activity. Local setting Licancabur is part of the Central Volcanic Zone and is situated at the western edge of the Altiplano. Among the active volcanoes of the region are Putana (erupted at the end of the 19th century), Llullaillaco (1868) and Lascar (1993). Other stratovolcanoes are Tacora, Nevados de Payachata, Isluga, Tata Sabaya, Ollague, Tocorpuri, Sairecabur and Socompa. The Central Volcanic Zone has more than 1,100 volcanic centres—many older than the Pleistocene, since the area's prevailing arid climate prevented substantial erosion. During the Miocene, the area around Licancabur was the site of major ignimbrite-forming eruptions. Llano de Chajnantor (southeast of Licancabur) is the site of several observatories, including the Llano de Chajnantor Observatory. Licancabur is a -high symmetrical cone. The cone rises above the surrounding terrain, covering a base diameter of . The volcano has a total volume of , with mean slopes of the cone being about 30°. The volcano has erupted blocky lava flows, with thicknesses ranging from ; younger lava flows are thinner. Young lava flows on the western sides of the volcano are long; older flows extend . Some old lava flows reach Laguna Verde, and -long pyroclastic flow deposits have been found. There are no glacial cirques on the volcano. The volcano's eastern and western flanks have distinct appearances, since it formed on a basement which dips towards the Salar de Atacama. West of Licancabur are deposits stemming from debris avalanche activity, including blocks wide. Crater and lake The top of the mountain features a -wide crater which contains an elliptical lake. The lake is below the crater rim, it has a length of and a depth of . Fed by snowfall, it is one of the highest lakes in the world. Based on its relative lack of salt deposits, in 1955 researchers postulated that the lake spilled through a notch in the crater's