Hudson Volcano
Hudson Volcano is the most active volcano in the southern part of the Southern Volcanic Zone of the Andes Mountains in Chile, having erupted most recently in 2011. It was formed by the subduction of the oceanic Nazca Plate under the continental South American Plate. South of Hudson is a smaller volcano, followed by a long gap without active volcanoes, then the Austral Volcanic Zone. Hudson has the form of a caldera filled with ice; the Huemules Glacier emerges from the northwestern side of the caldera. The volcano has erupted rocks ranging from basalt to rhyolite, but large parts of the caldera are formed by non-volcanic rocks.
The volcano erupted numerous times in the late Pleistocene and Holocene, forming widespread tephra deposits both in the proximity of Hudson and in the wider region.
Four large eruptions took place in 17,300–17,440 BP, 7,750 BP, 4,200 BP and in 1991 AD ; the second is among the most intense volcanic eruptions in South America during the Holocene. A smaller eruption occurred in 1971. The 7,750 BP and 1991 eruptions had a substantial impact on the human population of Patagonia and Tierra del Fuego: The 7,750 BP eruption devastated the local ecosystem and may have caused substantial shifts in human settlement and lifestyle. During the 1991 eruption, volcanic ash covered a large area in Chile and neighbouring Argentina, causing high mortality in farm animals, aggravating an existing economic crisis, and reaching as far as Antarctica.
Geography and geomorphology
Hudson Volcano lies in the Andes of southern Chile, northwest of Lago Buenos Aires. The name "Hudson" refers to Francisco Hudson, a Chilean Navy captain and hydrographer. Another name of the volcano is Cerro de los Ventisqueros. Politically, Hudson Volcano is in the Aysen Province of Chile's Aysen Region. Most of the volcano is in the municipality of Aysen; the eastern and southern parts are in the municipalities of Coihaique and Rio Ibáñez, respectively. Owing to its remoteness and the dense vegetation at its foot, the volcano is poorly studied; it was recognized as a volcano only in 1970. The closest cities are Puerto Aysen north-northeast and Coihaique northeast; the Carretera Austral highway passes from the volcano. The volcano can be accessed either from the sea along the Huemules River valley or by land via the valley of the Blanco River from Lago Elizalde-Lago Claro. Small populations, mostly farmers, live in the surrounding valleys.The Andean Volcanic Belt includes four volcanic zones separated by gaps without recent volcanoes. From north to south they are the Northern Volcanic Zone, the Central Volcanic Zone, the Southern Volcanic Zone and the Austral Volcanic Zone. Hudson is the second-southernmost volcano of the SVZ, after Rio Murta; erroneously, it is often referred to as the southernmost. Farther south there is the Patagonian Volcanic Gap in the Andean Volcanic Belt, which separates Hudson from the Austral Volcanic Zone and its first volcano, Lautaro. The next volcanoes to the north are Mate Grande and Macá and Cay from Hudson, then Mentolat and the Puyuhuapi volcanic field.
The volcano is a ice-filled caldera that rises above the surrounding terrain. Only the western and southern margins are well-defined. The highest point reaches elevation. The edifice consists partly of volcanic rocks and partly of uplifted basement, and has an eroded appearance, with steep valleys cutting as much as into the outer reaches of the volcano. The total volume of the volcano is about, larger than other SVZ volcanoes, and it covers an area of about. Cinder and spatter cones reach heights of and are sources of lava flows outside of the caldera, especially in the Sorpresa Sur valley. There are two cones northeast of the caldera and one in the far southwest. The landscape of the Andes around Hudson is formed by numerous mountains with deep, glacially carved valleys. Thick volcanic soils occur in the area.
The caldera is filled with about of ice, forming an ice surface at about elevation. Ice flows out of the northwestern margin of the caldera and forms the Ventisquero de los Huemules Glacier. The Huemules Glacier is the largest glacier of Hudson Volcano, being long, and the headwater of the Huemules River. The glacier is covered by tephra and its surface is at too low an altitude for the tephra to be buried under snow; thus from the air the glacier looks like a lava flow. A small crater lake is at its beginning and occupies a crater of the 1991 eruption. Most of the ice in the caldera was destroyed by the 1971 eruption, but by 1979 it had built up again. During the 1991 eruption, cones surrounded by crevasses and small lakes formed in the ice. The recovery of the ice after the 1991 eruption was slower, and by 2002 Huemules was retreating. During eruptions, pyroclastic material and lava can melt the ice. Other glaciers emanating from the ice cap are the Desplayado, Bayo, Ibáñez, El Frio, Sorpresa Sur and Sorpresa Norte glaciers. They were up to long in 1974 but have retreated since then. Together with the Queulat Ice Cap, the Hudson glaciers make up a large part of the regional glacier inventory, and have left well-preserved moraines. The path of some of the glaciers may be influenced by local tectonic lineaments. Numerous rivers originate on Hudson; clockwise from north to south they include the Rio Desplayado to the north, the Rio Bayo to the east, the Rio Ibáñez, the Rio Sorpresa Sur, Rio Sorpresa Norte all to the southeast, and the Huemules River to the northwest. Numerous hot springs occur in the valleys, and produce creeks whose waters have peculiar smells and tastes. Volcanic activity might be responsible for fluctuations in the discharge of the Huemules River.
Geology
Off the western coast of South America, the Nazca Plate subducts beneath the South America Plate at a rate—at Hudson's latitude—of about. This subduction is responsible for volcanism in the SVZ and the rest of the Andean Volcanic Belt except for the AVZ, where the Antarctic Plate subducts.West of Hudson and the Taitao Peninsula, the Chile Ridge enters the Peru–Chile Trench, forming the Chile Triple Junction. The subduction of the ridge has produced a slab window in the downgoing slab, causing volcanism to cease in the Miocene and a gap to open up between the SVZ and the AVZ. The collision began 14 million years ago; since then, the triple junction and the volcanic gap are migrating north. Several fracture zones cut through the downgoing plate, one of which may project under Hudson. South of the volcano, the Tres Montes Fracture Zone forms the northern boundary of the slab window. The subducted plate is still young and hot. The position of Hudson just east of the triple junction may be responsible for the unusually high activity of the volcano. Older volcanism in the region includes back-arc volcanoes in Patagonia and adakitic rocks in the Taitao Peninsula that were emplaced during the last 4 million years.
Hudson rises from the Patagonian Batholith, a formation made up of intrusive rocks that were variously emplaced during the Cretaceous-Miocene. The crust under the volcano is about thick. The volcanism in the SVZ is heavily influenced by faults, including the Liquine-Ofqui Fault Zone which runs parallel to the volcanic belt. In the Hudson area, the LOFZ is formed by two branches connected through perpendicular faults and lies west of the volcano. The LOFZ moves at a rate of about in the area. Recently active faults around the volcano can be recognized in the vegetation.
Composition and magma plumbing system
Hudson has erupted a wide range of volcanic rocks. The cones outside the caldera have produced basaltic andesite and andesite. The Hudson rocks are a potassium-rich calc-alkaline rock suite straddling the alkaline-subalkaline line. Rocks contain only a few phenocrysts, including andesine, apatite, clinopyroxene, ilmenite, oligoclase, olivine, orthopyroxene, plagioclase and titanomagnetite. The composition of Hudson rocks diverges from that of other SVZ volcanoes, with higher concentrations of iron oxide, sodium oxide, titanium oxide and incompatible elements.The cone lavas include mid-ocean ridge basalt and ocean island basalt components as well as crust- or sediment-derived components, while the caldera magmas formed through fractional crystallization, possibly along with the assimilation of crustal material. The three major Holocene eruptions produced uniform magmas with temperatures of, a few percent water by weight and a trachyandesitic to trachydacitic composition. The H2 eruption led to a change of magma chemistry to more mafic compositions, followed by a reversal during the last 1,000 years.
Magma genesis processes can be complex in slab window areas, as melts derived from the asthenosphere that ascended through the window can take part. Magmas ascending into Hudson halt about underground and undergo a first phase of differentiation. Later the magma ascends into shallower reservoirs and is then stored at a few kilometres depth before the large Holocene eruptions. During historical eruptions, the vents opened up in the southwestern sector of the caldera. Some magmas can bypass the magma chamber and directly ascend to the surface through faults, forming the volcanic cones surrounding Hudson.
Climate and vegetation
The climate at Hudson is oceanic, with mean annual temperatures of. Precipitation at the coast reaches per year, increasing to in the Andes and declining to in the eastern valleys. Precipitation is brought by westerly winds and enhanced on the western slopes of the Andes by orographic precipitation, while the eastern slopes are within the rainshadow. Winds usually blow from the north or northwest and are strong; easterly winds are rare.The region is covered by temperate rainforests formed by conifers, broadleaf trees and beeches. Magellanic moorlands with cushion plants occur in the coastal areas. To the east there is a transition to the Patagonian steppe with grasses, herbs and scrubs. Since the 19th century, the vegetation has been altered by human intervention. South of Hudson is the Northern Patagonian Ice Field. Ice coverage has declined at a rate of since 2000.