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[[Image:Yucatan chix crater.jpg|thumb|300px|Radar topography reveals the 180 [[kilometer]] (112 mi) ring of the crater; clustered around the crater's trough are numerous [[sinkhole]]s, suggesting a prehistoric [[oceanic basin]] in the depression left by the impact (Image courtesy NASA/JPL-Caltech)]]
[[Image:Yucatan chix crater.jpg|thumb|300px|Radar topography reveals the 180 [[kilometer]] (112 mi) ring of the crater; clustered around the crater's trough are numerous [[sinkhole]]s, suggesting a prehistoric [[oceanic basin]] in the depression left by the impact (Image courtesy NASA/JPL-Caltech)]]


The Crater''' <!-- ({{Audio-IPA|Mgm pronunciation Knut.ogg|tʃikʃuˈlub}}) -->({{IPA2|tʃikʃuˈlub}}) is an ancient [[impact crater]] buried underneath the [[Yucatán Peninsula]], with its center located near the town of [[Chicxulub, Yucatán|Chicxulub]], [[Yucatán]], [[Mexico]]. The crater is over 180&nbsp;kilometers (110&nbsp;mi) in diameter, making the feature one of the largest confirmed impact structures in the world; the [[asteroid]] or [[comet]] whose impact formed the crater was at least 10&nbsp;km (6&nbsp;mi) in diameter. The crater was named after the nearby town of [[Chicxulub, Yucatán|Chicxulub]], whose name means "tail of the [[Satan|devil]]" in [[Yucatec Maya]].<ref name=penfield />
'''The Chicxulub Crater''' <!-- ({{Audio-IPA|Mgm pronunciation Knut.ogg|tʃikʃuˈlub}}) -->({{IPA2|tʃikʃuˈlub}}) is an ancient [[impact crater]] buried underneath the [[Yucatán Peninsula]], with its center located near the town of [[Chicxulub, Yucatán|Chicxulub]], [[Yucatán]], [[Mexico]]. The crater is over 180&nbsp;kilometers (110&nbsp;mi) in diameter, making the feature one of the largest confirmed impact structures in the world; the [[asteroid]] or [[comet]] whose impact formed the crater was at least 10&nbsp;km (6&nbsp;mi) in diameter. The crater was named after the nearby town of [[Chicxulub, Yucatán|Chicxulub]], whose name means "tail of the [[Satan|devil]]" in [[Yucatec Maya]].<ref name=penfield />


The crater was discovered by [[Glen Penfield]], a geophysicist who had been working in the Yucatán while looking for [[petroleum|oil]] during the late 1970s. The presence of [[tektite]]s, [[shocked quartz]] and [[gravity anomaly|gravity anomalies]], as well as the age of the rocks and [[isotope analysis]], show that this impact structure dates from the late [[Cretaceous]] [[Period (geology)|Period]], roughly 65 [[million years ago]]. The impact associated with the crater is [[Cretaceous–Tertiary extinction event|implicated in causing the extinction]] of the [[dinosaur]]s as suggested by the [[K–T boundary]], although some critics disagree that the impact was the sole reason<ref name=bakker>Bakker interview. "Does the [impact theory] explain the extinction of the dinosaurs? There ''are'' problems..."</ref> and also debate whether there was a single impact or whether the Chicxulub impactor was one of several that may have struck the Earth at around the same time. Recent evidence suggests that the impactor was a piece of a much larger asteroid which broke up in a collision more than 160 million years ago.<ref name=Bottke/>
The crater was discovered by [[Glen Penfield]], a geophysicist who had been working in the Yucatán while looking for [[petroleum|oil]] during the late 1970s. The presence of [[tektite]]s, [[shocked quartz]] and [[gravity anomaly|gravity anomalies]], as well as the age of the rocks and [[isotope analysis]], show that this impact structure dates from the late [[Cretaceous]] [[Period (geology)|Period]], roughly 65 [[million years ago]]. The impact associated with the crater is [[Cretaceous–Tertiary extinction event|implicated in causing the extinction]] of the [[dinosaur]]s as suggested by the [[K–T boundary]], although some critics disagree that the impact was the sole reason<ref name=bakker>Bakker interview. "Does the [impact theory] explain the extinction of the dinosaurs? There ''are'' problems..."</ref> and also debate whether there was a single impact or whether the Chicxulub impactor was one of several that may have struck the Earth at around the same time. Recent evidence suggests that the impactor was a piece of a much larger asteroid which broke up in a collision more than 160 million years ago.<ref name=Bottke/>

Revision as of 14:07, 8 January 2008

For the town the crater is named after, see Chicxulub, Yucatán.
Radar topography reveals the 180 kilometer (112 mi) ring of the crater; clustered around the crater's trough are numerous sinkholes, suggesting a prehistoric oceanic basin in the depression left by the impact (Image courtesy NASA/JPL-Caltech)

The Chicxulub Crater (IPA: [tʃikʃuˈlub]) is an ancient impact crater buried underneath the Yucatán Peninsula, with its center located near the town of Chicxulub, Yucatán, Mexico. The crater is over 180 kilometers (110 mi) in diameter, making the feature one of the largest confirmed impact structures in the world; the asteroid or comet whose impact formed the crater was at least 10 km (6 mi) in diameter. The crater was named after the nearby town of Chicxulub, whose name means "tail of the devil" in Yucatec Maya.[1]

The crater was discovered by Glen Penfield, a geophysicist who had been working in the Yucatán while looking for oil during the late 1970s. The presence of tektites, shocked quartz and gravity anomalies, as well as the age of the rocks and isotope analysis, show that this impact structure dates from the late Cretaceous Period, roughly 65 million years ago. The impact associated with the crater is implicated in causing the extinction of the dinosaurs as suggested by the K–T boundary, although some critics disagree that the impact was the sole reason[2] and also debate whether there was a single impact or whether the Chicxulub impactor was one of several that may have struck the Earth at around the same time. Recent evidence suggests that the impactor was a piece of a much larger asteroid which broke up in a collision more than 160 million years ago.[3]

Discovery

In 1978, geophysicist Glen Penfield had been working for the Mexican state-owned oil company, Petróleos Mexicanos (PEMEX), as a staff member for an airborne magnetic survey of the Yucatán Peninsula. Penfield's job was to use geophysical data to scout possible locations for oil drilling.[4] With aerial surveys, he attempted to use magnetic data to find underground oil deposits. When Penfield examined the survey data, he found a huge underground arc with "extraordinary symmetry" in an enormous ring, 70 km (40 mi) across.[1] Penfield then obtained a gravity map of the Yucatán that had been made in the 1960s. He found another arc, but this one was on the Yucatán itself, and its ends pointed north. He compared the two maps and found that the two arcs joined to form a circle, 180 km (111 mi) wide, with its center near the village of Puerto Chicxulub. Penfield was certain that the arrangement was created by a single event in geologic history.

Although PEMEX would not allow him to release specific data, the company did allow him and PEMEX official Antonio Camargo to present their results at a geological conference in 1981.[5] The conference was under-attended that year, ironically because most geologists were attending a workshop on Earth impacts, and their report attracted very little attention. While Penfield had plenty of geophysical data sets, he had no physical evidence of an impact in the form of rock cores.[4]

Penfield knew that PEMEX had drilled exploratory wells in the region in 1951. One of the wells had bored into a thick layer of andesite about 1.3 kilometers (4,200 ft) down. Such a structure could have resulted from the intense heat and pressures of an Earth impact, but at the time of the borings it had been written off as a "volcanic dome", even though such a feature was out of place in the geology of the region. Penfield attempted to secure samples from these sites, but was told that all the samples had been lost or destroyed.[4] Attempts at returning to the drill sites and looking for rocks proved fruitless. Penfield abandoned his search, published his findings, and returned to working for PEMEX at the same time scientist Luis Walter Alvarez proposed his hypothesis that a giant extraterrestrial body had struck the earth.

File:Chicxulub shockedquartz.png
Penfield with the sample of shocked quartz found at Well #2, Chicxulub.

Meanwhile, Alan R. Hildebrand, a graduate student at the University of Arizona, was searching for a crater, unaware of Penfield's discovery. In 1981 Hildebrand and his faculty advisor, William V. Boynton, published their preliminary theory about an earth impact.[6] Their evidence included a greenish-brown coloured clay with an excess of iridium which contained shocked quartz grains and small beads of weathered glass that appeared to be tektites.[7] There was also the presence of thick, jumbled deposits of coarse rock fragments, thought to have been scoured up from one location and deposited elsewhere by a kilometers-high tsunami that most likely resulted from an Earth impact.[8] Such deposits occur in many locations but seem to be concentrated in the Caribbean Basin at the K–T boundary.[8] At the same time, Florentine Moras, a Haitian professor, discovered what he believed was evidence of an ancient volcano on Haiti; Hildebrand suggested it might be another distinctive feature of an impact nearby.[9] Tests on samples retrieved from the K–T boundary revealed more tektite glass, associated only with asteroid impacts and the site of high yield nuclear detonations.[9]

In 1990, Carlos Byars, a reporter for the Houston Chronicle, contacted Hildebrand and told him that Penfield had discovered what might be the impact crater years earlier.[10] After Hildebrand got in touch with Penfield in April 1990, the two men were able to locate two separate samples from the wells drilled by PEMEX, which were stored in New Orleans, Louisiana.[11] Sending the samples back to Hildebrand, Hildebrand and his team performed tests, which clearly showed shock-metamorphic materials in the samples.

In 1996, a team of California researchers, including Kevin O. Pope, Adriana Ocampo, and Charles Duller, conducted a survey of satellite images of the region. They found that there was a ring of sinkholes centered on Puerto Chicxulub that matched the ring Penfield had found in his data. The sinkholes were likely caused by subsidence of the crater's wall.[12] Recent evidence suggests that the actual crater is 300 kilometers (190 mi) wide, and the 180 kilometer ring is just an inner wall.[13]

Impact specifics

An animation showing the impact, and subsequent crater formation. (University of Arizona, Space Imagery Center)
Artist's impression of a major impact event.

The impactor's estimated size was about 10 km (6 mi) in diameter and may have released an estimated 500 zettajoules (5×1023 joules) of energy, equivalent to 100 teratons of TNT (1014 tons),[14] on impact. By contrast, the most powerful man-made explosive device ever detonated, the Tsar Bomba or Emperor Bomb, had a yield of only 50 megatons,[15] making the Chicxulub impact 2 million times more powerful.[16] Even the largest known explosive volcanic eruption, which released approximately 10 zettajoules and created the La Garita Caldera,[17] was substantially less powerful than the Chicxulub impact.

The impact caused some of the largest megatsunamis in Earth's history. A cloud of dust, ash and steam would have spread from the crater, as the impactor burrowed underground in less than a second.[18] Pieces of the impactor, ejected out of the atmosphere by the blast, would have been heated to incandescence upon reentry, broiling the Earth's surface and igniting global wildfires; meanwhile, shock waves spawned global earthquakes and volcanic eruptions.[19] The emission of dust and particles could have covered the entire surface of the Earth for several years, possibly a decade, creating a harsh environment for living things to survive in. The shock production of carbon dioxide caused by the destruction of carbonate rocks would have led to a dramatic greenhouse effect,[20] and sunlight would have been filtered out by dust particles in the atmosphere. Photosynthesis by plants would also have been interrupted, subsequently affecting the entire food chain.[21][22]

Geology and morphology

In their 1991 paper, Hildebrand, Penfield, and company described the geology and composition of the impact feature.[23] The rocks above the impact feature are layers of marl and limestone reaching to almost 1,000 meters (3,300 ft) in depth. These rocks date back as far as the Paleocene.[24] Below these layers lie more than 500 m (1,600 ft) of andesite glass and breccia. These andesitic igneous rocks were found only within the supposed impact feature; similarly, quantities of feldspar and augite, normally only found in impact-melt rocks, are present,[25] as is shocked quartz.[24] The K–T boundary inside the feature is depressed between 600 and 1,100 m (2,000–3,600 ft) compared to the normal depth of about 500 m (1,600 ft) depth 5 km (3 mi) away from the impact feature.[26] Along the edge of the crater are clusters of cenotes or sinkholes, which suggest that there was a water basin inside the feature during the Tertiary period, after the impact.[26] Such a basin's groundwater dissolved the limestone and created the caves and cenotes beneath the surface.[27] The paper also noted that the crater seemed to be a good candidate source for the tektites reported at Haiti.[28]

Origin

On September 5, 2007 a report published in Nature proposed an origin for the asteroid that created Chicxulub Crater.[21] The authors, William F. Bottke, David Vokrouhlický, and David Nesvorný, argued that a collision in the asteroid belt 160 million years ago resulted in the creation of the Baptistina family of asteroids, the largest surviving member of which is 298 Baptistina. They proposed that the "Chicxulub asteroid" was also a member of this group. The connection between Chicxulub and Baptistina is supported by the large amount of carbonaceous material present in microscopic fragments of the impactor, suggesting the impactor was a member of a rare class of asteroids called carbonaceous chondrites, like Baptistina.[3] According to Bottke, the Chicxulub impactor was a fragment of a much larger parent body about 170 km (105 mi) across, with the other impacting body being around 60 km (40 mi) in diameter.[29][3]

Chicxulub and mass extinction

Main article: Cretaceous–Tertiary extinction event
File:Iridium clay layer.png
Walter Alvarez holding the piece of clay which sparked research into the impact theory. The second green band from the bottom is extremely rich in iridium.

The Chicxulub Crater lends support to the theory postulated by the late physicist Luis Alvarez and his son, geologist Walter Alvarez, that the extinction of numerous animal and plant groups, including dinosaurs, may have resulted from a bolide impact. The Alvarezes, at the time both faculty members at the University of California, Berkeley, postulated that the extinction event roughly contemporaneous with the K–T boundary, could have been caused by just such a large impact.[30] This theory is now widely, though not universally, accepted by the scientific community. Some critics, including paleontologist Robert Bakker, argue that such an impact would have killed frogs as well as dinosaurs, yet the frogs survived the extinction event.[31] Gerta Keller of Princeton University argues that recent core samples from Chicxulub prove the impact occurred about 300,000 years before the mass extinction, and thus could not have been the causal factor.[32]

The main evidence of such an impact, besides the crater itself, is contained in a thin layer of clay present in the K–T boundary across the world. In the late 1970s, the Alvarezes and colleagues reported[33] that it contained an abnormally high concentration of iridium. In this layer, iridium levels reached 6 parts per billion by weight or more compared to 0.4[34] for the Earth's crust as a whole; in comparison, meteorites can contain around 470 parts per billion[35] of this element. It was hypothesised that the iridium was spread into the atmosphere when the impactor was vaporized and settled across the Earth's surface amongst other material thrown up by the impact, producing the relatively iridium-rich layer of clay.[36]

Multiple impact theory

See also: Roche limit

In recent years, several other craters of around the same age as Chicxulub have been discovered, all between latitudes 20°N and 70°N. Examples include the Silverpit crater in the North Sea[37] and the Boltysh crater in Ukraine.[38] Both are much smaller than Chicxulub, but likely to have been caused by objects many tens of metres across striking the Earth.[39] This has led to the hypothesis that the Chicxulub impact may have been only one of several impacts that happened nearly at the same time.[40] Another possible crater thought to have been formed at the same time is the Shiva crater, though the structure's status as a crater is contested.[41]

The collision of Comet Shoemaker-Levy 9 with Jupiter in 1994 proved that gravitational interactions can fragment a comet, giving rise to many impacts over a period of a few days if the comet should collide with a planet. Comets frequently undergo gravitational interactions with the gas giants, and similar disruptions and collisions are very likely to have occurred in the past.[42] This scenario may have occurred on Earth 65 million years ago.[40]

In late 2006, Ken MacLeod, a geology professor from the University of Missouri–Columbia, completed an analysis of sediment below the ocean's surface bolstering the single-impact theory. MacLeod conducted his analysis approximately 4,500 kilometers (2,800 mi) from the Chicxulub Crater to control for possible changes in soil composition at the impact site while still close enough to be affected by the impact. The analysis revealed there was only one layer of impact debris in the sediment, which indicated there was only one impact.[43] Multiple-impact proponents such as Gerta Keller regard the results as "rather hyper-inflated" and do not agree with the conclusion of MacLeod's analysis.[44]

See also

Notes

  1. ^ a b Penfield.
  2. ^ Bakker interview. "Does the [impact theory] explain the extinction of the dinosaurs? There are problems..."
  3. ^ a b c Bottke, Vokrouhlicky, Nesvorny.
  4. ^ a b c Bates.
  5. ^ Weinreb.
  6. ^ Mason.
  7. ^ Hildebrand, Penfield, et al.
  8. ^ a b Hildebrand interview: "Similar deposits of rubble occur all across the southern coast of North America [...] indicate that something extraordinary happened here."
  9. ^ a b Morás.
  10. ^ Frankel, 50.
  11. ^ Hildebrand interview.
  12. ^ Pope, Baines, et al.
  13. ^ Sharpton & Marin.
  14. ^ Bralower, et al.
  15. ^ Adamsky and Smirnov, 19.
  16. ^ Adamsky and Smirnov, 20.
  17. ^ Mason, et al.
  18. ^ Milosh, interview.
  19. ^ Milosh. "On the ground, you would feel an effect similar to an oven on broil, lasting for about an hour [...] causing global forest fires."
  20. ^ Hildebrand, Penfield, et al; 5.
  21. ^ a b Perlman.
  22. ^ Pope, Ocampo, et al.
  23. ^ Hildebrand, Penfield, et al; 1.
  24. ^ a b Hildebrand, Penfield, et al; 3.
  25. ^ Grieve.
  26. ^ a b Hildebrand, Penfield, et al; 4.
  27. ^ Kring, "Discovering the Crater".
  28. ^ Sigurdsson.
  29. ^ Ingham.
  30. ^ Alvarez, W. interview.
  31. ^ Kring, "Environment Consequences".
  32. ^ Keller, et al.
  33. ^ Alvarez.
  34. ^ Web Elements.
  35. ^ Quivx.
  36. ^ Mayell.
  37. ^ Stewart, Allen.
  38. ^ Kelley, Gurov.
  39. ^ Stewart.
  40. ^ a b Mullen, "Multiple Impacts".
  41. ^ Mullen, "Shiva".
  42. ^ Weisstein.
  43. ^ Than.
  44. ^ Dunham.

References

  • Adamsky, Viktor (1994). "Moscow's Biggest Bomb: the 50-Megaton Test of October 1961" (PDF). Cold War International History Project Bulletin (4): 19–21. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  • Alvarez, W. (1979). "Anomalous iridium levels at the Cretaceous/Tertiary boundary at Gubbio, Italy: Negative results of tests for a supernova origin". In Christensen, W.K., and Birkelund, T. (ed.). Cretaceous/Tertiary Boundary Events Symposium. Vol. 2. University of Copenhagen. p. 69. {{cite conference}}: Unknown parameter |booktitle= ignored (|book-title= suggested) (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)CS1 maint: multiple names: editors list (link)
  • Bates, Robin (series producer), Chesmar, Terri and Baniewicz, Rich (associate producers) (1992). The Dinosaurs! Episode 4: "Death of the Dinosaur" (TV-series). PBS Video, WHYY-TV. {{cite AV media}}: External link in |title= (help)
Bakker, Robert T. Interview: The Dinosaurs: Death of the Dinosaur. 1990, WHYY.
Hildebrand, Alan. Interview: The Dinosaurs: Death of the Dinosaur. 1992, WHYY.
Milosh, Gene. Interview: The Dinosaurs: Death of the Dinosaur. 1992, (1990): WHYY.
Moras, Florentine. Interview: The Dinosaurs: Death of the Dinosaur. 1992, (filmed 1990): WHYY.
Penfield, Glen. Interview: The Dinosaurs: Death of the Dinosaur. 1992, WHYY.


21°24′N 89°31′W / 21.400°N 89.517°W / 21.400; -89.517

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