Impact event

Impact events are caused by the collision of large meteoroids, asteroids or comets (generically: bolides) with Earth and may sometimes be followed by mass extinctions of life. For discussion of impacts in general, not just on Earth, see impact crater.

Mass extinctions and impacts

In the past 600 million years there have been five major mass extinctions that on average extinguished half of all species. The largest mass extinction to have affected life on Earth was the Permian-Triassic one that ended the Permian period 250 million years ago and killed off 90% of all species. The last such mass extinction led to the demise of the dinosaurs and has been found to have coincided with a large asteroid impact; this is the Cretaceous-Tertiary (K-T) extinction event. There is no solid evidence of impacts leading to the four other major mass extinctions, though many scientists assume that they are at least related to impacts.

Related Topics:
Mass extinction - Species - Permian-Triassic - Permian - Dinosaur - Asteroid - Cretaceous-Tertiary (K-T) extinction event

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In 1980 Luis Alvarez and his son Walter led a team from the University of California, Berkeley that discovered unusually high concentrations of iridium, an element that is rare in the Earth's crust but relatively abundant in many meteorites. From the amount and distribution of iridium present in the 65 million year old "iridium layer", the Alvarez team later estimated that an asteroid of 10–14 kilometers must have collided with the earth. This iridium layer at the K-T boundary has been found worldwide at 100 different sites. Multidirectionally shocked quartz (coesite), which is only known to form as the result of large impacts or atomic bomb explosions, has also been found in the same layer at more than 30 sites. Soot and ash at levels tens of thousands times normal levels were found with the above.

Related Topics:
Luis Alvarez - Walter - University of California, Berkeley - Iridium - Meteorite - Shocked quartz - Atomic bomb - Soot - Ash

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Anomalies in chromium isotopic ratios found within the K-T boundary layer strongly support the impact theory. Chromium isotopic ratios are homogeneous within the earth, therefore this isotopic anomalies exclude a volcanic origin which was also proposed as a cause for the iridium enrichment. Furthermore the chromium isotopic ratios determined in the K-T boundary are similar to the chromium isotopic ratios found in carbonaceous chondrites. Thus a probable candidate for the impactor is a carbonaceous asteroid but also a comet is possible because comets are assumed to consist of material similar to carbonaceous chondrites.

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Probably the most convincing evidence for a worldwide catastrophe was the discovery of the crater which has since been named Chicxulub Crater. This so-called smoking gun is centered on the Yucatan Peninsula of Mexico and was discovered by Tony Camargo and Glen Pentfield while working as geophysicists for the Mexican oil company PEMEX. What they reported as a circular feature later turned out to be a crater estimated to be 180 kilometers in diameter. Other researchers would later find that the end-Cretaceous extinction event that wiped out the dinosaurs had lasted for thousands of years instead of millions of years as had previously been thought. This would be the final piece of evidence that convinced the vast majority of scientists that this extinction resulted from a point event that is most probably an extra-terrestrial impact and not from increased volcanism and climate change (which would spread its main effect over a much longer time period).

Related Topics:
Chicxulub Crater - Yucatan Peninsula - Mexico - Geophysicist - Oil - PEMEX

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Recently, several craters around the world have been dated to approximately the same age as Chicxulub—for example, the Silverpit crater in the United Kingdom and the Boltysh crater in Ukraine. This has led to the suggestion that the Chicxulub impact was one of several that occurred almost simultaneously, perhaps due to a disrupted comet impacting the Earth in a similar manner to the collision of Comet Shoemaker-Levy 9 with Jupiter in 1994.

Related Topics:
Silverpit crater - United Kingdom - Boltysh crater - Ukraine - Comet - Comet Shoemaker-Levy 9 - Jupiter - 1994

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It was the lack of high concentrations iridium and shocked quartz which has prevented the acceptance of the idea that the Permian extinction (so-called mother of mass extinctions) was also caused by an impact. However, during the late Permian all the continents were combined into one supercontinent named Pangaea and all the oceans formed one superocean, Panthalassa. If an impact occurred in the ocean and not on land at all, then there would be little shocked quartz released (since oceanic crust has relatively little silica) and much less material.

Related Topics:
Continent - Pangaea - Panthalassa - Silica

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Although there is now general agreement that there was a huge impact at the end of the Cretaceous that led to the iridium enrichment of the K-T boundary layer, remnants have been found of other impacts of the same order of magnitude that did not result in any mass extinctions, and in fact there is no clear linkage between an impact and any other incident of mass extinction.

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Nonetheless it is now widely believed, if a little on faith, that mass extinctions due to impacts are an occasional event in the history of the Earth. One such controversial hypothesis is Tollmann's hypothetical bolide, which claims that the Holocene was initiated by an impact.

Related Topics:
Tollmann's hypothetical bolide - Holocene

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Paleontologists Michael Raup and Jack Sepkoski have proposed that an extinction occurs roughly every 26 million years (though many are relatively minor). This led physicist Richard A. Muller to suggest that these extinctions could be due to a hypothetical companion star to the sun called Nemesis periodically disrupting the orbits of comets in the Oort cloud, and leading to a large increase in the number of comets reaching the inner solar system where they might hit Earth.

Related Topics:
Michael Raup - Jack Sepkoski - Richard A. Muller - Nemesis - Oort cloud

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Indeed, in the early history of the Earth, about four billion years ago, bolide impacts were almost certainly common since the skies were far more full of "junk" than at present. Such impacts could have included strikes by asteroids hundreds of kilometers in diameter, with explosions so powerful that they vaporized all the Earth's oceans. It was not until this "hard rain" began to slacken, so it seems, that life could have begun to evolve on Earth.

Related Topics:
Earth - Asteroid - Ocean - Life

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The leading theory of Moon's origin is the giant impact theory, which states that Earth was once hit by a planetoid the size of Mars; possibly the largest impact Earth has ever suffered.

Related Topics:
Moon - Giant impact theory - Planetoid - Mars

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