Cerro Azul (Chile volcano)

Regional setting Volcanism in the Chilean Andes is caused by subduction of the Nazca and Antarctic tectonic plates under the South American Plate. Volcanoes in Chile occur in the Central (CVZ), South (SVZ), and Austral Volcanic Zones (AVZ). The gap that separates the Central and South Volcanic Zones is caused by shallow-angle subduction in the Pampean flat-slab segment where the more buoyant Juan Fernández Ridge subducts under the South American continent. This buoyant region prevents the slab (subducting tectonic plate) from diving deep into the mantle, The Patagonian Volcanic Gap, which separates the South and Austral Volcanic Zones, is caused by the subduction of the Chile Ridge, though it is less clear whether this gap also is due to flat-slab subduction; it may instead arise because melting of the subducting slab there produced felsic igneous rocks instead of volcanoes. Offshore volcanism also occurs in Chile. Intraplate volcanism generated from the Easter and Juan Fernández hotspots has formed many Chilean islands, including Isla Salas y Gómez, Easter Island, and the Juan Fernández Islands. Underwater volcanism occurs due to seafloor spreading along the Chile Ridge. Local setting volcano, and close to it is the pyramidal Cerro Azul. Cerro Azul is part of the South Volcanic Zone, which runs through central and western Chile and extends south to Argentina. This range includes at least nine caldera complexes, more than 70 of Chile's stratovolcanoes and volcanic fields that have been active in the Quaternary, and hundreds of minor eruptive centres. The South Volcanic Zone is the most volcanically active region in Chile, and produces around one eruption per year. Its largest historical eruption was at Quizapu Crater, located on the north side of Cerro Azul's summit (see below), and its most active volcanoes are Llaima and Villarrica. Cerro Azul, just south of Descabezado Grande volcano, is part of the Descabezado Grande–Cerro Azul eruptive system, a volcanic field that comprises its two large namesake volcanic edifices and several smaller vents, including 12 Holocene calderas. The cone of Cerro Azul has a total volume of about , and is a young feature, formed in the Holocene. The cone has a few volcanic craters; Because of aerodynamic drag, a Plinian eruption excavates a circular crater. As the earlier eruptions had already formed an approximately circular caldera, the Plinian eruption was able to proceed efficiently, with minimal drag and minimal reshaping of the crater. The Quizapu Crater is almost perfectly circular, and rises to a prominence of above the surrounding portions of the volcano. == Climate and vegetation ==

Cerro Azul is situated in a Mediterranean climate zone, characterized by hot and dry summers but mild and wet winters. The temperatures and precipitation are strongly dependent on topography. In the Andes the annual average maximum temperatures lie in the range , while minimum temperatures are below . Annual precipitation is up to 800 mm (31.5 inches). Vegetation in the Andes varies with elevation. Above 1,600 meters (5,249 ft) the slopes of mountains are covered by Alpine-like steppe, while below there are zones of Nothofagus forest, Hygrophilous forest, Sclerophylous forest, and matorral. The number of plant species is likely to exceed 2,000, although no comprehensive study of the flora of Central Chile has been undertaken. == Eruptive history ==

Cerro Azul has a history of eruptions dating back to at least 1846. The known events include effusive eruptions (lava flows), which created the Quizapu vent, explosive eruptions, and phreatic eruptions. Pyroclastic flows have also been observed as a result of some of these explosive eruptions. The earliest recorded eruption began on November 26, 1846, while the volcano's last eruption began on August 9, 1967. This first recorded eruption of Cerro Azul was effusive, and formed the volcanic vent at Quizapu. Hornblende–dacite lava erupted with small masses of tephra, which had been degassed shortly before the eruption. Early 20th century Cerro Azul was quiet from 1846 to the beginning of the 20th century. After a possible precursor explosive event in 1903, Cerro Azul once again erupted in 1907. Between 1907 and 1914, plumes and clouds of ash frequently rose out of the caldera, and at least a few of these events were explosive. On September 8, 1914, an explosive eruption sent a plume 6 or 7 kilometers (about 4 mi) into the air over 8 minutes. By 1916, these eruptions had produced a caldera nearly identical to the one in existence today. On April 10, Cerro Azul finally erupted, releasing a towering column or plume of white gas. After 10 AM, the plume turned black with ash and began to form an umbrella shape. The ash was carried by wind into Puesto El Tristan in Argentina, about away, where beginning at 1 PM it rained down for hours. At 4 PM, coarser sandy material and some pumice lapilli began to fall. and minuscule amounts of andesitic and basaltic scoria. At least one eruptive period lasted for 18 hours, creating an "exceptionally uniform" deposit. The eruption had a Volcanic Explosivity Index (VEI) of at least 5. == Threats and preparedness ==

Cerro Azul is in the South Volcanic Zone, where many volcanoes pose a threat to human life. Among the other active volcanoes of the South Volcanic Zone are Mount Hudson, Llaima, and Villarrica. Every known type of eruption (Hawaiian, Strombolian, Plinian, subplinian, phreatic, phreatomagmatic, and Vulcanian) has occurred at some point in the range. Past eruptions of Quizapu Crater ejected enormous amounts of ash that traveled as far as Brazil. After the 1932 eruption, the local vegetation was devastated, and the area remained barren until the 1990s, though human life was not impacted. Despite the extent of its eruptions, the threat to humans from Quizapu is relatively small because of the remote location of Cerro Azul. Nevertheless, the size of past eruptions is large enough for scientists to be worried. Evidence of a potentially deadly threat lies in a lahar at Descabezado Grande. There is, however, a possibility that a large reservoir of rhyodacite magma may exist under the Azul–Descabezado complex. If this is the case, all previous eruptions in 1846–1967 were only preliminary, preclimactic leaks from the large magma chamber beneath, and a large caldera-forming eruption may be expected in the future. If Cerro Azul were to erupt, relief efforts could be orchestrated. The Volcanic Disaster Assistance Program (VDAP) formed in response to the famous 1985 eruption of Nevado del Ruiz in Colombia, and responded to the 1991 eruption of Mount Hudson in Chile. == See also ==