Araguainha crater
Encyclopedia
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
on the border of Mato Grosso
Mato Grosso
Mato Grosso is one of the states of Brazil, the third largest in area, located in the western part of the country.Neighboring states are Rondônia, Amazonas, Pará, Tocantins, Goiás and Mato Grosso do Sul. It also borders Bolivia to the southwest...
and Goiás
Goiás
Goiás is a state of Brazil, located in the central part of the country. The name Goiás comes from the name of an indigenous community...
states, Brazil, between the villages of Araguainha
Araguainha
Araguainha is a town and municipality in the state of Mato Grosso in the Central-West Region of Brazil.-References:...
and Ponte Branca
Ponte Branca
Ponte Branca is a town and municipality in the state of Mato Grosso in the Central-West Region of Brazil.-References:...
. With a diameter
Diameter
In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...
of 40 kilometres (24.9 mi), it is the second largest known impact crater in South America, and possibly the oldest one.
The crater was formed 244.40 ± 3.25 million years ago in the Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
period, when the region was probably a shallow sea. The date is actually quite close to the Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
/Triassic boundary, one of the largest mass extinction events in Earth's history. The impact punched through Paleozoic
Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
sedimentary units belonging to the Paraná Basin
Parana Basin
The Paraná Basin is a large sedimentary basin situated in the central-eastern part of South America. About 75% of its areal distribution occurs in Brazil, from Mato Grosso to Rio Grande do Sul states. The remainder area is distributed in eastern Paraguay, northeastern Argentina and northern...
formations, and exposed the underlying Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
basement rocks. It is estimated that the crater was initially 24 kilometres (14.9 mi) wide and 2.4 kilometres (1.5 mi) deep, which then widened to 40 kilometres (24.9 mi) as its walls subsided inwards.
Description
Araguainha is a complex crater with annular and radial faults, exposed to the surface and eroded, crossed by the Araguaia RiverAraguaia River
The Araguaia River or, in Portuguese, Rio Araguaia is one of the major rivers of Brazil, and the principal tributary of the Tocantins. It has a total length of approximately 2,627 km. Araguaia means "river of macaws" in the Tupi language....
. The crater has an uplifted central core, shaped like an elliptical basin, consisting of exposed basement granite. Surrounding this core is a ring of shocked granite and overlying breccias; then another ring of ridges and mountains, 6.5 kilometres (4 mi) in diameter and up to 150 metres (492.1 ft) high, consisting of folded and steeply tilted Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
sandstones. This central region is surrounded by an annular depression floored by rocks from Devonian and Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
sandstone formations. The outer rim of the crater consists of remnants of semi-circular grabens in highly deformed Permo-Carboniferous sediments. Evidences of impact origin include shatter cone
Shatter cone
Shatter cones are rare geological features that are only known to form in the bedrock beneath meteorite impact craters or underground nuclear explosions...
s, impact breccias, and shocked quartz
Shocked quartz
Shocked quartz is a form of quartz that has a microscopic structure that is different from normal quartz. Under intense pressure , the crystalline structure of quartz will be deformed along planes inside the crystal...
.
History and studies
The earliest report on the Araguainha structure was published in 1969 by Northfleet et al., who interpreted it as an uplift of the Phanerozoic sediments caused by a CretaceousCretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
syenite
Syenite
Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or present in relatively small amounts Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or...
intrusion
Intrusion
An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...
. A reconnaissance geological survey by Silveira Filho and Ribeiro (1971) noted the occurrence of lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
s, breccia
Breccia
Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....
s and tuff
Tuff
Tuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered...
s around the central core, and deduced that Araguainha was as a crypto-volcanic structure. In 1973 R.S. Dietz
Robert S. Dietz
Robert Sinclair Dietz was Professor of Geology at Arizona State University. Dietz was a marine geologist, geophysicist and oceanographer who conducted pioneering research along with Harry Hammond Hess concerning seafloor spreading, published as early as 1960–1961...
and B.M. French reported the occurrence of impact breccias and shocked quartz, and recognized the structure as an impact crater. A detailed study of the crater by Á.P. Crósta
Álvaro Penteado Crósta
Álvaro Penteado Crósta is a Brazilian geologist, specialized in remote sensing and mineral exploration. He is an authority on the meteor craters of Brazil and known for the Crosta method/technique.-Biography:A. P...
in 1981-1982 reported further petrological and mineralogical evidence of the impact. Further geomorphologic evidence was published by Theilen-Willige in 1981. The impact was first dated (at 243 ± 19 million years ago, with Rb-Sr method
Rubidium-strontium dating
The rubidium-strontium dating method is a radiometric dating technique that geologists use to determine the age of rocks.Development of this process was aided by Fritz Strassmann, who later went on to discover nuclear fission with Otto Hahn and Lise Meitner....
) by Deutsch et al. in 1992. In 1992 Engelhardt et al. (1992) published a detailed study of the uplifted core and revised date of about 246 million years ago, later revised to about 244 million years ago. A magnetic survey was conducted in 1994 by Fischer and Masero.
Access and conservation
The Araguainha Dome can be reached by car from GoiâniaGoiânia
-Climate:The city has a tropical wet and dry climate with an average temperature of . There's a wet season, from October to April, and a dry one, from May to September. Annual rainfall is around 1,520 mm....
or from Cuiabá
Cuiabá
Under the Koppen climate classification, Cuiaba features a tropical wet and dry climate. Cuiabá is famous for its searing heat, although temperatures in winter can arrive sporadically at 10 degrees, indeed atypical, caused by cold fronts coming from the south, and that may only last one or two...
. The unpaved state road MT-306, between Ponte Branca and Araguainha, cuts across the central uplift. As of 1999, the local population was not aware of the Dome's nature and scientific importance.
External links
- Araguainha at Earth Impact Database
- Satellite image of the region (from Google Maps)
- DOME