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Geology of Chile
The Geology of Chile is mainly a product of the Andean orogeny which in turn is caused by the subduction zone at the coast. This subduction have shaped two other features parallel to the Andes, the Intermediate Depression which is a graben and foreland basin and the Coast Range which is an accretionary wedge and horst. Easter Island, and Juan Fernandez Archipelago are hotspot islands in the eastward moving Nazca plate. The Antarctic Peninsula in the Chilean Antarctic Territory shares a lot of characteristics with the Andes and is sometimes considered to be an extension of that mountains. This fact is used as an argument for the Chilean claims on Antarctica. As Chile borders the ring of fire, it host a large number of volcanoes, some of them very active like Villarrica and Mount Hudson. The Great Chilean Earthquake or Valdivian Earthquake (Terremoto de Valdivia in Spanish) of 22 May 1960 is the most powerful earthquake ever recorded, rating 9.5 on the Moment magnitude scale. Four major tectonic plates are present within or close to Chile's borders, the Antarctic Plate, the Nazca Plate, the Scotia Plate and the South American Plate.
Morphology
See also: Geography of Chile
The Andes derive in three mayor morphological features are present in most of the country; the proper Andes Mountains, the Chilean Coast Range and the Chilean Central Valley (also called Intermediate Depression and Longitudinal Valley) between them. These features run parallelly in a north-south direction from Morro de Arica to Taitao Peninsula, covering up most of Chile's land surface. Further south from Taitao only the Andes Mountains is present.
North of Taitao Peninsula this subduction zone is made up of the Peru-Chile Trench which is the boundary between the South American and the Nazca Plates. At the site of Taitao the triple junction of the Antarctic, the Nazca plate subducts under the continent.
The Andes
In general the Andes tend to lose height to the south of Chile. In Norte Grande the mountains forms a series a plateaus such as Puna de Atacama and the Altiplano. At a latitude of 27 S, Chile's highest mountain Ojos del Salado reaches 6,893 metres. South of latitude of 42 S the Andes are split into a fjord landscape and the highest mountain is Monte San Lorenzo with 3,706 m. As the mountains lowers so does the snow line, in the Llanquihue it is at 1,200 m and at Magallanes it is down at 900 m.
Intermediate Depression
The Intermediate Depression separates the Andes from the Coast Range. It is delimited a series of faults running in north-south direction. Just like the Andes and the Coast Range it tends to lose height with increasing latitude. In Norte Grande the intermediate depression is partly covered by a series of salt flats and has the world's largest potassium nitrate deposits. In Norte Chico the depression is absent but it appears again as a narrow valley at Santiago. At 34 S the depression goes through two narrows where the two ranges came close again. From the narrows southward the valley widens but is interrupted near Loncoche by the Bahia Mansa Metamorphic Complex that is part of the Coast Range. The valley opens again as Los Llanos near Paillaco. In central and southern Chile (33-42 S) its is partly covered with glacifluvial sediments from the Andes. In Zona Austral, south of 42 S, the depressions is beneath sea level but appears again occasionally in islands such as Chiloe. Its southern extreme is marked by the Isthmus of Ofqui.
Chilean Coast Range
The Chilean Coast Range is a mountain range that runs southward along the coast parallel with the Andean Mountains, from Morro de Arica to Taitao Peninsula where it ends at the Chile Triple Junction. The range is a combined horst, forearc high and accretionary wedge structure. The range was separated from the Andes during the Tertiary rise of the Andes due to the subsidence of the Intermediate Depression.
Geologic history
Paleozoic Era
The oldest rocks in Chile are micaceous schists, phyllites, gneisses and quartzites which are mostly found in the Coast Range of south-central Chile. The schist of southern Chile were initially formed by sedimentation into the proto-Pacific Ocean and underwent later a stage of metamorphism in the forearc wedge of the Peru-Chile Trench.
Mesozoic Era
Some 250 million years ago during the Triassic period, Chile was part of the supercontinent Pangea which concentrated all major land masses in the world. In Pangea Africa, Antarctica, Australia and India were closest to Chile. When Pangea began to split apart in the Jurassic, South America and the adjacent land masses formed Gondwana. Floral affinities among these now-distant landmasses date from the Gondwanaland period . Then India split apart followed by the creation of the Mid-Atlantic Ridge that separates Africa from South America.
The formation of the Andes began in the Jurassic Period. It was during the Cretaceous Period that the Andes began to take their present form, by the uplifting, faulting and folding of sedimentary and metamorphic rocks of the ancient cratons to the east. 27 million years ago South America separated from Antarctica and Australia with the genesis of the Drake Passage. Tectonic forces along the subduction zone along the entire west coast of South America where the Nazca Plate and a part of the Antarctic Plate are sliding beneath the South American Plate continue to produce an ongoing orogenic event resulting in minor to major earthquakes and volcanic eruptions to this day. In the extreme south Magallanes-Fagnano Fault separates Tierra del Fuego from the small Scotia Plate. Across the wide Drake Passage lie the mountains of the Antarctic Peninsula south of the Scotia Plate which appear to be a continuation of the Andes chain.
Cenozoic Era
The Altiplano plateau was formed during the Tertiary and several mechanisms have been suggested as responsible for its formation, aiming to explain why the topography in the Andes incorporates this large area of low relief at high altitude (high plateau) within the orogen:
Existence of weaknesses in the Earth's crust prior to tectonic shortening. Such weaknesses would cause the partition of tectonic deformation and uplift into eastern and western cordillera, leaving the necessary space for the formation of the altiplano basin.
Magmatic processes rooted in the asthenosphere might have contributed to uplift the plateau.
Climate has controlled the spatial distribution of erosion and sediment deposition, controlling the lubrication along the Nazca Plate subduction and hence influencing the transmission of tectonic forces into South America.
Climate also determined the formation of internal drainage (endorheism) and sediment trapping within the Andes, potentially blocking tectonic deformation in the area between the two cordilleras.
Quaternary
The Quaternary glaciations have left visible marks in most of Chile but particularly in Zona Sur and Zona Austral. These include ice fields, fjords, glacial lakes and u-shaped valleys. During the Santa Maria glaciation glaciers penetrated into the Pacific Ocean at 42 S dividing the Chilean Coast Range and created what is now Chacao Channel. Chiloe, which used to be a continuous part of the Chilean Coast Range, became an island after the creation of Chacao Channel. South of Chacao Channel Chile's coast is split by fjords, islands and channels. These glaciers created moraines at the edges of the Patagonian lakes changing their outlets to the Pacific, and then shifting the continental divide.
The last remains of the Patagonian Ice Sheet that once covered up large parts of Chile and Argentina are the Northern Patagonian Ice Field and the Southern Patagonian Ice Field.
The coast in south-central Chile has had a generalized Quaternary rise despite the Holocene transgressions. [*]
It has been suggested that between 1675 and 1850 the San Rafael Glacier advanced considerably as an effect of the Little Ice Age. This is based on the descriptions made by three expeditions that visited the area. The first documented visit to the area was made in 1675 by the Spanish explorer Antonio de Vea, who entered San Rafael Lagoon through Rio Tempanos (Spanish for Ice Floe River) without mentioning the many ice floes for which the river is currently named. De Vea stated also that the San Rafael Glacier did not reach far into the lagoon. In 1766 another expedition noticed that the glacier did reach the lagoon and calved into large icebergs. Hans Steffen visited the area in 1898, noticing that the glacier penetrated far into the lagoon. As of 2001, the border of the glacier has retreated beyond the borders of 1675. [*]
Pacific islands
Easter Island is a volcanic high island, consisting of three extinct volcanoes: Terevaka (altitude 507 metres) forms the bulk of the island. Two other volcanoes, Poike and Rano Kau, form the eastern and southern headlands and give the island its approximately triangular shape. There are numerous lesser cones and other volcanic features, including the crater Rano Raraku, the cinder cone Puna Pau and many volcanic caves including lava tubes.
Easter Island and surrounding islets such as Motu Nui, Motu Iti are the summit of a large volcanic mountain which rises over two thousand metres from the sea bed. It is part of the Sala y Gomez Ridge, a (mostly submarine) mountain range with dozens of seamounts starting with Pukao and then Moai, two seamounts to the west of Easter Island, and extending east to the Nazca Seamount.
Pukao, Moai and Easter Island were formed in the last 750,000 years, with the most recent eruption a little over a hundred thousand years ago. They are the youngest mountains of the Sala y Gomez Ridge, which has been formed by the Nazca Plate floating over the Easter hotspot.
Only at Easter Island, its surrounding islets and Sala y Gomez does the Sala y Gomez Ridge form dry land.
The Juan Fernandez Islands are of volcanic in origin, and were created by a hotspot in the Earth's mantle that broke through the Nazca Plate to form the islands, which were then carried eastward off the hot spot as the Nazca Plate subducts under the South American continent. Radiometric dating indicates that Santa Clara is the oldest of the islands, 5.8 million years old, followed by Robinson Crusoe, 3.8-4.2 million years old, and Alexander Selkirk, 1.0-2.4 million years old. Robinson Crusoe is the largest of the islands, at 93 km and the highest peak, El Yunque, is 916 meters. Alexander Selkirk is 50 km ; its highest peak is Los Innocentes at 1319 meters. Santa Clara is 2.2 km, and reaches 350 meters.
Economic geology
Mining
Chile has the world's largest copper reserves and is also the largest producer and exporter of the metal. Some well-known copper mines are Chuquicamata and Escondida. Chile stands for 5% of the western hemisphere's gold production of which 41% is a by-product of copper extraction. Apart from copper, Chile contains the largest share of the world reserves of rhenium and potassium nitrate. Chile's reserves of molybdenum are estimated to be the third largest in the world. While most of Chile's mineral resources are in the north, there are some minor gas and oil reserves in the southern Magallanes Basin, but Chile is otherwise highly dependent on fuel imports.
Guarello Island in Magallanes Region currently has the world's southernmost limestone mine.
Geothermal energy
Geothermal exploration and concessions are regulated by the Law of Geothermal Concessions since 2000. Currently the Chilean company company Geotermia del Pacifico is exploring a concesion right around the locality of Curacautin for building a geothermal power plant with support of CORFO. Geotermia del Paicifco's studies show that two geothermal fields near Curacautin could be used for energy production with combined capacity of supplying 36,000 homes in 2010. One of the geothemal areas to be develpped are located close to the Tolhuaca hotsprings and the other in Rio Blanco Springs.
Another area being currently explored for geothermal energy production is Cordon Caulle. In northern Chile El Tatio has investigated as early as in the 1920s and was the site of a failed experiment in the 1990s.
Tourism
Tourism focusing on geology is scarce if considering that the natural scenery that attracts many tourist to different locations in Chile is product of geological processes. There are however some sites where geology is made a mayor atraction, such as in the case of the tourism office of the copper mine Chuquicamata. National parks all over Chile have considerable amount of information about flora and fauna but notably often lacks geological information. Santuario de la Naturaleza Granito Orbicular is one of the few protected areas in Chile where tourism centres around the particular geology of the place.
Geological hazards
Due to the tectonic setting of Chile earthquakes, volcanic eruptions and mass movements are frequent. The subduction zone along Chile's coast has produced the most powerful earthquake ever recorded, the 1960 Valdivia earthquake. Earthquakes are known to have triggered eruption at volcanoes as it happened in 1960 with Cordon Caulle. Earthquakes have also produced tsunamis and even earthquakes on the other side of the Pacific Ocean have the potential of sending a tsuname wave to Chile.
Landslides may occur with some frequency in Andes of central and southern Chile, most events happens following earthquakes. The 2007 Aysen Fjord earthquake produced several landslides along the fjords mountains producing thus big tsunami-like waves inside the fjord.
Mud flows are also common in some places and occur after large rainfall events. Lahars have been are among the most lethal volcanic hazards in Chile, destroying towns such as Conaripe in 1964. Although many lahars are triggered by volcanic eruptions they are often mistaken as unequivocal sign of eruption, which is not the case.
See also
Climate of Chile
Geography of Chile
Glaciers of Chile
List of earthquakes in Chile
List of volcanoes in Chile
Sources
Bruggen, Juan. Fundamentos de la geologia de Chile, Instituto Geografico Militar 1950.
Duhart, Paul et al. El Complejo Metamorfico Bahia Mansa en la cordillera de la Costa del centro-sur de Chile (3930'-4200'S): geocronologia K-Ar, 40Ar/39Ar y U-Pb e implicancias en la evolucion del margen sur-occidental de Gondwana
External links
Geologic map of Chile
Tectonical development of Chile and the southern Andes
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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article Geology of Chile
