Geology of Chile
The Geology of Chile is mainly a product of the Andean and preceding orogenies which are caused by the long-lived convergent boundary at South America's western coast. While in the Paleozoic and Precambrian this boundary was affected by the accretion of terranes and microcontinents it has since then developed into a pure subduction zone. The subduction have shaped four features parallel to its strike; the Andes, the Intermediate Depression which is a graben and foreland basin, the Coast Range which is an accretionary wedge and horst and the Peru-Chile Trench off the coast. As Chile lies in an active continental margin, it host a large number of subduction volcanoes and nearly all of the territory of Chile is subject to earthquakes arising from strains in the subducting Nazca and Antarctic Plates or shallow strike-slip faults.
Mineral resources in the northern regions of Chile have since late 19th century made up the bulk of the country's export and state income. Chile is currently a leading producer of copper, lithium and molybdenum. Much of the countries mineral wealth have accumlated thanks to long lived volcanic and magmatic activity as well as the extreme aridity that have prevailed over Atacama Desert for millions of years.
The Chilean territories of Easter Island, and Juan Fernández Archipelago are extinct volcanic hotspot islands in the eastward moving Nazca plate. The Antarctic Peninsula, claimed as part of the Chilean Antarctic Territory, shares a series of characteristics with the southern Andes.
General characteristics
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 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) it 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 Chiloé. 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 (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 Magallanes-Fagnano Fault separates Tierra del Fuego from the small Scotia Plate. Across the 1,000 km (620 mi) 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 María glaciation glaciers penetrated into the Pacific Ocean at 42° S dividing the Chilean Coast Range and created what is now Chacao Channel. Chiloé, 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.[1]
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 Río Témpanos (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.[2]
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 2,700 km (1,700 mi) east to the Nazca Seamount.[3]
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.[4][1] 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
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 Cordón 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 Coñaripe in 1964. Although many lahars are triggered by volcanic eruptions they are often mistaken as unequivocal sign of eruption, which is not the case.
Earthquakes
 |
Seismicity of the Chilean coast identifies and describes the seismic activity of an area of Chile. Seismicity refers to the frequency, type and size of earthquakes experienced over a period of time. The Chilean coast is southern part of South America which is near a Pacific Ocean subduction zone.
Major earthquakes in Chile occur in a small number of source areas. Those which affect coastal regions are aligned offshore from Concepción to the south. The major epicenters produce a predictable pattern of seismic and tsunami effects.[2]
Area of the city most affected by earthquake
City blocks most damaged by fire
The first systematic seismology of Chile began after an earthquake and fire devastated Valparaiso in 1906.[3]
Significant events which devastated coastal communities in the 20th and 21st centuries include:
- 1906 Valparaiso earthquake. The 8.8 Chilean quake in August was preceded by the Ecuador-Columbia quake (8.8 magnitude) in January and the San Francisco quake (7.9 magnitude) in April.[4]
- 1960 Valdivia earthquake. The 9.5-magnitude quake in Chile (largest in modern history)[5] was comparable in scale to earthquakes in Alaska (2nd largest) and the Kamchatka Peninsula (5th largest).[6]
- 2010 Chile earthquake. The 8.8-magnitude quake in Chile (6th largest)[5] was comparable in scale to undersea seismic events near Indonesia in 2004 (3rd largest) and near Japan in 2011 (4th largest).[6]
The seismicity of the Chilean coast and the top six quakes ever recorded appear to be clustered in two time periods: a 12-year span between 1952 and 1964 and a 7-year span between the 2004 and 2011; however, this is understood as a statistical anomaly[6]
The phenonmenon comparably large quakes that happen on the same or neighbouring faults within months of each other can be explained by a sound geological mechanism; but this does not fully demonstrate a relationship between events separated by longer periods and greater distances[7]
See also
- Climate of Chile
- Geography of Chile
- Glaciers of Chile
- List of earthquakes in Chile
- List of volcanoes in Chile
Sources
- Brüggen, Juan. Fundamentos de la geología de Chile, Instituto Geográfico Militar 1950.
- Duhart, Paul et al. 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
- ^ The Petrogenetic Evolution of Lavas from Easter Island and Neighbouring Seamounts, Near-ridge Hotspot Volcanoes in theSE Pacific
- ^ Lomitz, Cinna. "Major earthquakes and tsunamis in Chile during the period 1535 to 1955," International Journal of Earth Sciences, Vol. 59, No. 3, abstract.
- ^ Moreno, Teresa. (2006). The Geology of Chile, p. 264., p. 264, at Google Books
- ^ "Chile 1906 Valparaiso Earthquake Centennial," CNRS International Magazine (France). 2006.
- ^ a b Phillips, Campbell. "The 10 biggest earthquakes in history," Australian Geographic, 14 March 2011.
- ^ a b c Pappas, Stephanie. "Sumatra, Japan, Chile: Are Earthquakes Getting Worse?" LiveScience, 11 March 2011.
- ^ Brahic, Catherine. "The megaquake connection: Are huge earthquakes linked?" New Scientist (UK). 16 March 2011.
