Geology of Chile

The Geology of Chile is mainly a product of the the Andean orogeny which in turn is caused by the subduction zone at at its coast. This subduction have shaped two other features parallel to the Andes, the Intemidiate Depression which is a foreland basin and the Coast Range which is a forearc wedge. Easter Island, and Juan Fernández Archipelago are hotspots moving eastward in Nazca plate. The Antarctic Peninsula in the Chilean Antarctic Territory shares a lot of characteristics with the 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 [U.S. Geological Survey (March 7, 2006). [http://earthquake.usgs.gov/regional/world/events/1960_05_22.php Historic Earthquakes - Chile - 1960 May 22 19:11:14 UTC - Magnitude 9.5: The Largest Earthquake in the World.] Retrieved on 2007-01-09] on the Moment magnitude scale. Four mayor 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 Plate. At the site of Taitao the triple junction of the antarctic, South american and Nazca plates 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 do the snow line, in the Llanquihue it is at 1200 m and at Magallanes it down at 900 m.

Intermidiate Depression

The Intermidiate Depression separes 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 hight with increasing latitude.

In Norte Grande the intermidiate depression is partly coved by a series of salt flats and has the world's largest potassium nitrate deposits. In Norte Chico the depression is absent but it apears again as narrow valley at Santiago. At 34° S the depression goes throug two narrownesses where the two ranges came close again. From the narrowneses southward the valley widdens but is interrupted near Loncoche by the Bahía 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 occasionaly in islands such as Chiloé. Its southern extreme is marked by the Isthmus of Ofqui.

Geologic history

[
Pangaea separation animation]
[
San Rafael Glacier, 1990-2000]

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 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 the adjacent land masses formed Gondwana. Floral affinities among these now-distant landmasses date from the Gondwanaland period (see also: Antarctic Floristic Kingdom). 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 a major transform fault separates Tierra del Fuego from the small Scotia Plate. Across the convert|1000|km|mi|-1|abbr=on 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 it's 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 María glaciation glaciers penetrated into the Pacific Ocean at 42° S dividing the Chilean Coast Range and created what is now Chacao Channel. Chiloé that 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 morraines at the edges of 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 have had a generalized cuaternary rise despite of the holocene transgressions. [http://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0716-02082005000100004&lng=es&nrm=iso]

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 desciptions 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, He entered San Rafael Lagoon throug Río Témpanos (Spanish for "Ice Floe River") without mentioning the any 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. [http://hol.sagepub.com/cgi/reprint/17/7/987]

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 Gómez 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 convert|2700|km|mi|abbr=on east to the Nazca Seamount.ref|seamounts

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 Gómez Ridge, which has been formed by the Nazca Plate floating over the Easter hotspot.ref|hotspot [ [http://www.oxfordjournals.org/our_journals/petroj/online/Volume_38/Issue_06/html/ega038_gml.html#hd15 The Petrogenetic Evolution of Lavas from Easter Island and Neighbouring Seamounts, Near-ridge Hotspot Volcanoes in theSE Pacific ] ] Only at Easter Island, its surrounding islets and Sala y Gómez does the Sala y Gómez Ridge form dry land.

The Juan Fernández 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 km² and the highest peak, El Yunque, is 916 meters. Alexander Selkirk is 50 km² ; its highest peak is Los Innocentes at 1319 meters. Santa Clara is 2.2 km², and reaches 350 meters.

Economic geology

Chile has the world's largest copper reserves and is also the largest producer and exporter of the metal.U.S. Geological Survey (2005). [http://minerals.usgs.gov/minerals/pubs/country/2005/cimyb05.pdf Minerals Yearbook 2005.] ] Some well-known copper mines are Chuquicamata and Escondida. Chile stands for 5% of the western hemispheres gold production of which 41% is biproduct of copper extraction. Apart from copper Chile contain 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 is some minor gas and oil reserves in the southern Magallanes Region, but Chile is otherwise highly dependent of fuel imports.

Guarello Island in Magallanes Region has currently the world's southernmost limestone mine.

See also

*Climate of Chile
*Geography of Chile
*List of earthquakes in Chile
*List of volcanoes in Chile

ources

*Brüggen, Juan. "Fundamentos de la geología de Chile", Instituto Geográfico Militar 1950.
*Duhart, Paul et al. [http://www.scielo.cl/scielo.php?pid=S0716-02082001000200003&script=sci_arttext El Complejo Metamórfico Bahía Mansa en la cordillera de la Costa del centro-sur de Chile (39°30'-42°00'S): geocronología K-Ar, 40Ar/39Ar y U-Pb e implicancias en la evolución del margen sur-occidental de Gondwana]

References

External links

* [http://www.ipgp.jussieu.fr/~dechabal/Geol-millon.pdf Geologic map of Chile]
* [http://www.geo.arizona.edu/geo5xx/geo527/Andes/tectonicandes.html Tectonical development of Chile and the southern Andes]