Active Andean volcanism: its geologic and tectonic setting

The Andean volcanic arc includes over 200 potentially active Quaternary volcanoes, and at least 12 giant caldera/ignimbrite systems, occurring in four separate segments referred to as the Northern, Central, Southern and Austral Volcanic Zones. Volcanism results from subduction of the Nazca and Antarctic oceanic plates below South America. Active volcanoes occur where the angle of subduction is relatively steep (25°), and active arc segments are separated by regions below which subduction angle decreases and becomes relatively flat (<10°) at depths 100 km. Segments of low angle subduction formed beginning in the Miocene in association with subduction of buoyant oceanic plateaus and ridges, and current segmentation of subduction geometry and active Andean volcanic zones is clearly a transient feature related to Neogene tectonics. A genetic relation between subduction and volcanism is confirmed by geochemical studies indicating that generation of Andean magmas is initiated by dehydration and/or melting of subducting oceanic lithosphere and interaction of these slab-derived fluids/melts with the overlying mantle wedge. Continental crust is incorporated into Andean magmas by a combination of both subduction of crust into the subarc mantle and assimilation of crust into mantle-derived magmas. Variations in the rate of subduction erosion and subduction of continental crust significantly affect not only Andean magma chemistry, but also the along-strike intraplate mechanical coupling in the subduction zone and the dynamics of mountain building in the Andes. Crustal components are most significant in magmas erupted in the Central Volcanic Zone, where the crust is extremely thick (70 km) and estimated rates of subduction erosion of the continental margin, possibly equivalent to as much as 4% of the volume of subducting oceanic crust, are also greatest due to the hyper-arid climate conditions and low sediment supply to the trench. Obvious hazards associated with Andean volcanoes include lava and pyroclastic flows, lahars, debris flows resulting from sector collapse, and tephra falls. More than 25,000 people have been killed by the 600 eruptions of these volcanoes catalogued since the year 1532, most of these by lahars generated during the eruption of Nevado del Ruiz, Colombia, in 1985. Despite the fact that 20 million people live within <100 km of an active Andean volcano, mostly in low-lying areas in the intermontane valleys of Colombia and Ecuador and the Central Valley of south-central Chile, only <25 of these volcanoes are continuously monitored for signs of activity