Alvarez hypothesis
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[[Image:Trinlake2a.jpg|thumb|300px|K–T boundary exposure (marked by the white line) in [[Trinidad Lake State Park]]]]
[[Image:KT boundary 054.jpg|thumb|300px|Badlands near [[Drumheller, Alberta]] where erosion has exposed the KT boundary.]]
The '''Alvarez hypothesis''' is the theory that the [[Extinction event|mass extinction]] of the [[dinosaur]]s and many other living things was caused by the [[Impact event|impact]] of a large [[asteroid]] on the [[Earth]] sixty-five million years ago, called the [[Cretaceous-Tertiary extinction event]]. Evidence indicates that the asteroid fell in the [[Yucatán Peninsula]], [[Mexico]]. The hypothesis is named after the father-and-son team of scientists [[Luis Alvarez|Luis]] and [[Walter Alvarez]], who first suggested it in [[1980]].
In [[1980]], a team of researchers led by Nobel prize-winning physicist [[Luis Walter Alvarez|Luis Alvarez]], his son geologist [[Walter Alvarez]] and chemists Frank Asaro and Helen Michels discovered that sedimentary layers found all over the world at the Cretaceous–Tertiary boundary contain a [[concentration]] of [[iridium]] hundreds of times greater than normal. Iridium is extremely rare in the earth's crust because it is very dense, and therefore most of it sank into the earth's core while the earth was still molten. The Alvarez team suggested that an asteroid struck the earth at the time of the K–T boundary.<ref name="Alvarez">{{cite journal|title=Extraterrestrial cause for the Cretaceous–Tertiary extinction|author=Alvarez, LW, Alvarez, W, Asaro, F, and Michel, HV|date=1980|journal=Science|volume=208|issue=4448|pages=1095–1108|doi=10.1126/science.208.4448.1095}}</ref> There were other earlier speculations on the possibility of an impact event, but no evidence had been uncovered at that time.<ref>{{cite journal|author=De Laubenfels, MW|title=Dinosaur Extinctions: One More Hypothesis|journal=Journal of Paleontology|volume= 30|issue=1|pages=207–218|date=1956|url=http://www.norwebster.com/astrohit/|accessdate=2007-05-22|format=subscription required}}</ref>
The evidence for the Alvarez impact theory is supported by [[Chondrite|chondritic]] [[meteorite]]s and [[asteroid]]s which contain a much higher iridium concentration than the earth's crust. The [[isotope|isotopic]] ratio of iridium in asteroids is similar to that of the K–T boundary layer but significantly different from the ratio in the earth's crust. [[Chromium]] isotopic anomalies found in Cretaceous–Tertiary boundary sediments are similar to that of an asteroid or a comet composed of [[carbonaceous chondrite]]s. [[Shocked quartz]] granules, glass spherules and [[tektite]]s, indicative of an impact event, are common in the K–T boundary, especially in deposits from around the Caribbean. All of these constituents are embedded in a layer of clay, which the Alvarez team interpreted as the debris spread all over the world by the impact.<ref name="Alvarez"/> The location of the impact was unknown when the Alvarez team developed their theory, but later scientists discovered the [[Chicxulub crater|Chicxulub Crater]] in the [[Yucatán Peninsula]], now considered the likely impact site.
Using estimates of the total amount of iridium in the K–T layer, and assuming that the asteroid contained the normal percentage of iridium found in [[chondrites]], the Alvarez team went on to calculate the size of the asteroid. The answer was about 10 kilometers (6 mi) in diameter, about the size of Manhattan.<ref name="Alvarez"/> Such a large impact would have had approximately the force of 100 trillion tons of [[Trinitrotoluene|TNT]], i.e. about 2 million times as great as the most powerful [[thermonuclear bomb]] ever [[Tsar Bomba|tested]].
The most obvious consequence of such an impact would be a vast dust cloud which would block sunlight and prevent photosynthesis for a few years. This would account for the extinction of plants and [[phytoplankton]] and of all organisms dependent on them (including predatory animals as well as herbivores). But small creatures whose food chains were based on [[detritus]] would have a reasonable chance of survival. It is estimated that sulfuric acid aerosols were injected into the stratosphere, leading to a 10–20% reduction of solar transmission normal for that period. It would have taken at least ten years for those aerosols to dissipate.<ref>{{cite book|author=Ocampo, A, Vajda, V & Buffetaut, E|title=Unravelling the Cretaceous–Paleogene (KT) Turnover, Evidence from Flora, Fauna and Geology in Biological Processes Associated with Impact Events (Cockell, C, Gilmour, I & Koeberl, C, editors)|publisher=SpringerLink|date=2006|pages=197–219|isbn=978-3-540-25735-6|url=http://www.springerlink.com/content/vw75014157p2p278/|accessdate=2007-06-17}}</ref>
Global firestorms may have resulted as incendiary fragments from the blast fell back to Earth. Analyses of [[fluid inclusions]] in ancient [[amber]] suggest that the [[oxygen]] content of the atmosphere was very high (30–35%) during the late Cretaceous. This high O<sub>2</sub> level would have supported intense combustion. The level of atmospheric O<sub>2</sub> plummeted in the early Tertiary Period. If widespread fires occurred, they would have increased the CO<sub>2</sub> content of the atmosphere and caused a temporary [[greenhouse effect]] once the dust cloud settled, and this would have exterminated the most vulnerable survivors of the "long winter".
The impact may also have produced [[acid rain]], depending on what type of rock the asteroid struck. However, recent research suggests this effect was relatively minor. Chemical buffers would have limited the changes, and the survival of animals vulnerable to acid rain effects (such as [[frog]]s) indicate this was not a major contributor to extinction.<ref>{{cite journal|author=Kring, DA|date=2003|title=Environmental consequences of impact cratering events as a function of ambient conditions on Earth|journal=Astrobiology|volume=3|issue=1|pages=133–152|pmid=12809133|doi=10.1089/153110703321632471}}</ref>
Impact theories can only explain very rapid extinctions, since the dust clouds and possible sulphuric aerosols would wash out of the atmosphere in a fairly short time — possibly under ten years.
Although further studies of the K–T layer consistently show the excess of iridium, the idea that the dinosaurs were exterminated by an asteroid remained a matter of controversy among [[geologist]]s and [[paleontologist]]s for more than a decade.
==References==
{{Reflist}}
[[Category: Extinction]]