Chicxulub Crater

Coordinates: 21°24′N, 89°31′W

Chicxulub Crater (IPA: /tʃikʃu'lub/) (cheek-shoo-LOOB) is an ancient impact crater buried underneath the Yucatán Peninsula, with its center located approximately underneath the town of Chicxulub, Yucatán, Mexico. Investigations suggest that this impact structure is dated from the late Cretaceous Period, about 65 million years ago. The meteorite's estimated size was about 10 km (6 mi) in diameter, releasing an estimated 500 zettajoules (5.0×10²³ joules) of energy, approximately 100 teratons of TNT^[1], on impact. By contrast, the most powerful explosive device ever detonated, the Tsar Bomba or Emperor Bomb, had a yield of only one-twentieth of a gigaton, which would make this impact 2,000,000 times more powerful than the Emperor Bomb.

The impact would have caused some of the largest megatsunamis in Earth's history. These would have spread in all directions, hitting the Caribbean island of Cuba especially hard. The emission of dust and particles caused environmental changes close to a nuclear winter, during which the surface of the Earth was totally covered by a cloud of dust for several years. (Pope, et al., 1997)

This timing is in good agreement with the theory postulated by the late physicist Luis Alvarez and his son Walter, a geologist, for the extinction of the dinosaurs. The Alvarezes, at the time both faculty at the University of California, Berkeley, postulated that the extinction of the dinosaurs, roughly contemporaneous with the K-T boundary, could have been caused by the impact of just such a large meteorite. This theory is now widely, though not universally, accepted by the scientific community. ^[2] The main evidence is a widespread, thin layer of clay present in this geological boundary across the world. In the late 1970s, the Alvarezes and colleagues reported ^[3] that it contained an abnormally high concentration of iridium – 6 parts per billion by weight or more compared to 0.4^[4] for the Earth's crust as a whole. Meteorites can contain around 470 parts per billion^[5] of this element. It was hypothesised that the iridium was spread into the atmosphere when the meteorite was vapourised and settled across the Earth's surface amongst other material thrown up by the impact, producing the relatively iridium-rich layer of clay.^[6]

Discovery

In early 1990, Alan R. Hildebrand, a graduate student at the University of Arizona, visited a small mountain village named Beloc in Haiti. He was investigating certain K-T deposits that include thick, jumbled deposits of coarse rock fragments, which were apparently scoured up from one location and deposited elsewhere by a kilometers-high tsunami that most likely resulted from an Earth impact. Such deposits occur in many locations but seem to be concentrated in the Caribbean Basin.

Hildebrand found a greenish brown coloured clay with an excess of iridium and containing shocked quartz grains and small beads of weathered glass that appeared to be tektites. He and his faculty adviser, William V. Boynton, published the results of the research in the scientific press, suggesting that the deposits were the result of an Earth impact and that the impact could not have been more than 1,000 kilometers (620 mi) away.

However, no crater was known to exist in the Caribbean basin. Hildebrand and Boynton also reported their findings to an international geological conference, sparking substantial interest.

Evidence pointed to possible crater sites off the north coast of Colombia or near the western tip of Cuba. Then Carlos Byars, a reporter for the Houston Chronicle, contacted Hildebrand and told him that a geophysicist named Glen Penfield had discovered what might be the impact crater in 1978, buried under the northern Yucatán Peninsula.

In that year, Penfield had been working for Petroleos Mexicanos (PEMEX, the Mexican state-owned oil company) as a staff member for an airborne magnetic survey of the Yucatán Peninsula. When Penfield examined the survey data, he found a huge underground "arc", with its ends pointing south, in the Caribbean off the Yucatan that was inconsistent with the region's geology.

Penfield then obtained a gravity map of the Yucatan that had been made in the 1960s. He found another arc, but this one was on the Yucatan itself, and its ends pointed north. He matched up the two maps and found that the two arcs joined up in a circle, 180 kilometers (112 miles) wide, with its center at the village of Puerto Chicxulub.

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. The conference was under-attended in that year, ironically because most geologists were attending a workshop on Earth impacts, and their report attracted very little attention, though it did get back to Byars.

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.

After Hildebrand got in touch with Penfield, the two men were able to locate two separate samples from the wells drilled by PEMEX in 1951. Analysis of the samples clearly showed shock-metamorphic materials. Studies by other geologists of the debris found in Haiti at Beloc also showed it to be clearly the result of an impact.

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. (Pope, et al., 1996) Further studies have reinforced the consensus. Indeed, some evidence has accumulated that the actual crater is 300 kilometers (186 miles) wide, and the 180 kilometer ring is just an inner wall. (Sharpton & Marin, 1997)

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 United Kingdom, and the Boltysh crater in Ukraine, both much smaller than Chicxulub but likely to have been caused by objects many tens of metres across striking the Earth. This has led to the hypothesis that the Chicxulub impact may have been only one of several impacts that happened all at the same time. 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.

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. This scenario may have occurred on Earth 65 million years ago.

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, indicating only one impact. Reuters quoted Multiple Impact proponent Gerta Keller as saying, "Unfortunately, these claims are rather hyper-inflated and do not withstand close examination."

See also

• Cretaceous-Tertiary extinction event
• Permian-Triassic extinction event
• Wilkes Land crater

References

• Pope KO, Baines KH, Ocampo AC, Ivanov BA (1997). "Energy, volatile production, and climatic effects of the Chicxulub Cretaceous/Tertiary impact". Journal of Geophysical Research 102 (E9): 21645-64. PMID 11541145. 
• Pope KO, Ocampo AC, Kinsland GL, Smith R (1996). "Surface expression of the Chicxulub crater". Geology 24 (6): 527-30. PMID 11539331. 
• Rojas-Consuegra, R., M. A. Iturralde-Vinent, C. Díaz-Otero y D. García-Delgado (2005). "Significación paleogeográfica de la brecha basal del Límite K/T en Loma Dos Hermanas (Loma Capiro), en Santa Clara, provincia de Villa Clara. I Convención Cubana de Ciencias de la Tierra.". GEOCIENCIAS 8 (6): 1-9. ISBN 959-7117-03-7. 
• Sharpton VL, Marin LE (1997). "The Cretaceous-Tertiary impact crater and the cosmic projectile that produced it". Annals of the New York Academy of Sciences 822: 353-80. PMID 11543120. 
• Single Massive Asteroid Wiped Out Dinosaurs Reuters, December 1, 2006
• The Chixulub Debate

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