Bosumtwi Impact Crater - Ghana
Saturday, 13. December 2008, 16:36:52
When I visited Bosumtwi Lake in 2003 it was first and foremost to watch birds, I have to admit, although the geological aspects as usually kept lurking somewhere behind. It irritated me enormously that many people (still) thought that it is a volcanic crater - even supported in this belief by tourist guide books! Incredible!
The Bosumtwi impact crater is almost completely filled by Lake Bosumtwi - Ghana's largest natural lake. Maclaren postulated an impact origin of the Bosumtwi Crater already in 1931. Numerous studies have followed, and the impact hypothesis confirmed by all the typical impact indicators. The 1.07 Million years old crater has a rim-to-rim diameter of about 10.5 km.
Before I go on, a few words on tektites:
The term “Tektite” was coined by Suess in 1900 and is derived from a Greek word for “molten”, tektos. Tektites are natural glass objects up to a few centimeters in size, which most scientists argue were formed by the impact of large meteorites on Earth's surface. Tektites are typically black or olive-green, and their shape varies from rounded to irregular. Areas with concentrations of tektites on the Earth's surface are known as strewn fields.
One such strewn field is the so-called Ivory Coast strewn field (with Ivory Coast tektites) to the west of the Bosumtwi Crater. The first suggestions that the Bosumtwi crater is the source crater for the Ivory Coast tektites were made in the early 1960s. Ivory Coast tektites were first reported in 1934 from a geographically rather restricted area in the Ivory Coast (Cote d'Ivoire), West Africa. Microtektites and tektites were later reported from deep-sea sediments and other West African locations. Ivory Coast tektites and the Bosumtwi crater have the same age, and there are close similarities between the isotopic and chemical compositions of the tektites and crater rocks, which strongly support a connection between the crater and the tektites. This makes the Bosumtwi crater one of only three impact structures that have been identified as source craters of a tektite strewn field. (Ries Crater being another - connected to the moldavite strewn field that extends from about 200 to 450 km from the center of the Ries to the east north east).
The principal criteria for determining if a geological feature is an impact structure are listed at the Earth Impact Database. I shall here concentrate on two of them in connection with the Bosumtwi Impact Crater. Impact breccias and multiple planar deformation features (PDFs) in minerals (a.o. found in what is referred to as shocked quartz).
Cores drilled in the Bosumtwi Lake have shown that the lake sediments are underlain by impact breccias, first a layer of polymict and suevitic impact breccia (suevite is also known as “fall back breccia”) and beneath that a thinner layer of monomict impact breccia. In a monomict all the small pieces of rocks in the breccia are of the same type, meaning they came from a single parent body. A polymict has more sources, indicating some sort of collision and melding of material from different areas of the same parent or different parents.
A study of shock metamorphism and shocked rocks of the Bosumtwi Impact Crater was published yesterday (12 December 2008) in the journal Science. I read it with great interest, but as it is rather technical I won’t go into much detail here. So far only three investigations have explored the relative vertical decay of recorded shock pressure in complex impact structures (i.e. craters with diameters greater than 2-4 km on Earth, see drawing below), two for the ~40 km diameter Puchezh-Katunki impact structure and another for the ~65 km diameter Kara crater. So also in this aspect the Bosumtwi Crater is special.
Cr isotope analyses of an Ivory Coast tektite indicates that the Bosumtwi impactor was an ordinary chondrite - a stony meteorite. Most meteorites that are recovered on Earth are chondrites.
Selected References:
Shock Metamorphism of Bosumtwi Impact Crater Rocks, Shock Attenuation, and Uplift Formation by Ferrière et al., Science of 12 December 2008.
An Ordinary Chondrite Impactor Composition for the Bosumtwi Impact Structure, Ghana, West Africa: Discussion of Siderophile Element Content and Os and Cr Isoptope Data by Koeberl and al., in Lunar and Planetary Science XXXV (2004)
• http://www.sciencemag.org/cgi/content/short/322/5908/1678
• http://www.unb.ca/passc/ImpactDatabase/images/bosumtwi.htm
• http://www.univie.ac.at/geochemistry/koeberl/bosumtwi/
• http://originoftektites.com/Chapter_2.php
The Bosumtwi impact crater is almost completely filled by Lake Bosumtwi - Ghana's largest natural lake. Maclaren postulated an impact origin of the Bosumtwi Crater already in 1931. Numerous studies have followed, and the impact hypothesis confirmed by all the typical impact indicators. The 1.07 Million years old crater has a rim-to-rim diameter of about 10.5 km.
Before I go on, a few words on tektites:
The term “Tektite” was coined by Suess in 1900 and is derived from a Greek word for “molten”, tektos. Tektites are natural glass objects up to a few centimeters in size, which most scientists argue were formed by the impact of large meteorites on Earth's surface. Tektites are typically black or olive-green, and their shape varies from rounded to irregular. Areas with concentrations of tektites on the Earth's surface are known as strewn fields. One such strewn field is the so-called Ivory Coast strewn field (with Ivory Coast tektites) to the west of the Bosumtwi Crater. The first suggestions that the Bosumtwi crater is the source crater for the Ivory Coast tektites were made in the early 1960s. Ivory Coast tektites were first reported in 1934 from a geographically rather restricted area in the Ivory Coast (Cote d'Ivoire), West Africa. Microtektites and tektites were later reported from deep-sea sediments and other West African locations. Ivory Coast tektites and the Bosumtwi crater have the same age, and there are close similarities between the isotopic and chemical compositions of the tektites and crater rocks, which strongly support a connection between the crater and the tektites. This makes the Bosumtwi crater one of only three impact structures that have been identified as source craters of a tektite strewn field. (Ries Crater being another - connected to the moldavite strewn field that extends from about 200 to 450 km from the center of the Ries to the east north east). The principal criteria for determining if a geological feature is an impact structure are listed at the Earth Impact Database. I shall here concentrate on two of them in connection with the Bosumtwi Impact Crater. Impact breccias and multiple planar deformation features (PDFs) in minerals (a.o. found in what is referred to as shocked quartz).
Cores drilled in the Bosumtwi Lake have shown that the lake sediments are underlain by impact breccias, first a layer of polymict and suevitic impact breccia (suevite is also known as “fall back breccia”) and beneath that a thinner layer of monomict impact breccia. In a monomict all the small pieces of rocks in the breccia are of the same type, meaning they came from a single parent body. A polymict has more sources, indicating some sort of collision and melding of material from different areas of the same parent or different parents.
A study of shock metamorphism and shocked rocks of the Bosumtwi Impact Crater was published yesterday (12 December 2008) in the journal Science. I read it with great interest, but as it is rather technical I won’t go into much detail here. So far only three investigations have explored the relative vertical decay of recorded shock pressure in complex impact structures (i.e. craters with diameters greater than 2-4 km on Earth, see drawing below), two for the ~40 km diameter Puchezh-Katunki impact structure and another for the ~65 km diameter Kara crater. So also in this aspect the Bosumtwi Crater is special.
Cr isotope analyses of an Ivory Coast tektite indicates that the Bosumtwi impactor was an ordinary chondrite - a stony meteorite. Most meteorites that are recovered on Earth are chondrites.
Selected References:
Shock Metamorphism of Bosumtwi Impact Crater Rocks, Shock Attenuation, and Uplift Formation by Ferrière et al., Science of 12 December 2008.
An Ordinary Chondrite Impactor Composition for the Bosumtwi Impact Structure, Ghana, West Africa: Discussion of Siderophile Element Content and Os and Cr Isoptope Data by Koeberl and al., in Lunar and Planetary Science XXXV (2004)
• http://www.sciencemag.org/cgi/content/short/322/5908/1678
• http://www.unb.ca/passc/ImpactDatabase/images/bosumtwi.htm
• http://www.univie.ac.at/geochemistry/koeberl/bosumtwi/
• http://originoftektites.com/Chapter_2.php
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