- Chicxulub crater
The Chicxulub crater ( // cheek-shə-loob; Maya: [tʃʼikʃuluɓ]) is an ancient impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is located near the town of Chicxulub, after which the crater is named. The crater is more than 180 km (110 mi) in diameter, making the feature one of the largest confirmed impact structures on Earth; the impacting bolide that formed the crater was at least 10 km (6 mi) in diameter.
The crater was discovered by Glen Penfield, a geophysicist who had been working in the Yucatán while looking for oil during the late 1970s. Penfield was initially unable to obtain evidence that the unique geological feature was in fact a crater, and gave up his search. Through contact with Alan Hildebrand, Penfield was able to obtain samples that suggested it was an impact feature. Evidence for the impact origin of the crater includes shocked quartz, a gravity anomaly, and tektites in surrounding areas.
The age of the rocks shows that this impact structure dates from the end of the 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, the geological boundary between the Cretaceous and Tertiary periods, although some critics argue that the impact was not the sole reason and others 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 may have been a piece of a much larger asteroid that broke up in a collision in distant space more than 160 million years ago.
In March 2010, following extensive analysis of the available evidence covering 20 years' worth of data spanning the fields of palaeontology, geochemistry, climate modelling, geophysics and sedimentology, 41 international experts from 33 institutions reviewed available evidence and concluded that the impact at Chicxulub triggered the mass extinctions at the K–T boundary including those of dinosaurs.
In 1978, geophysicists Antonio Camargo and Glen Penfield were working for the Mexican state-owned oil company Petróleos Mexicanos, or Pemex, as part of an airborne magnetic survey of the Gulf of Mexico north of the Yucatán peninsula. Penfield's job was to use geophysical data to scout possible locations for oil drilling. Within the data, Penfield found a huge underwater arc with 'extraordinary symmetry' in a ring 70 km (40 mi) across. He then obtained a gravity map of the Yucatán made in the 1960s. A decade earlier, the same map suggested an impact feature to contractor Robert Baltosser, but he was forbidden to publicize his conclusion by Pemex corporate policy of the time. Penfield found another arc on the peninsula itself, the ends of which pointed northward. Comparing the two maps, he found the separate arcs formed a circle, 180 km (111 mi) wide, centered near the Yucatán village Chicxulub; he felt certain the shape had been created by a cataclysmic event in geologic history.
Pemex disallowed release of specific data but let Penfield and company official Antonio Camargo present their results at the 1981 Society of Exploration Geophysicists conference. That year's conference was underattended and their report attracted scant attention. Ironically, many experts in impact craters and the K–T boundary were attending a separate conference on Earth impacts. Although Penfield had plenty of geophysical data sets, he had no rock cores or other physical evidence of an impact.
He knew Pemex had drilled exploratory wells in the region in 1951, one bored into what was described as a thick layer of andesite about 1.3 km (4,200 ft) down. This layer could have resulted from the intense heat and pressure of an Earth impact, but at the time of the borings it was dismissed as a lava dome – a feature uncharacteristic of the region's geology. Penfield tried to secure site samples but was told such samples had been lost or destroyed. When attempts at returning to the drill sites and looking for rocks proved fruitless, Penfield abandoned his search, published his findings and returned to his Pemex work.
At the same time, scientist Luis Walter Alvarez put forth his hypothesis that a large extraterrestrial body had struck Earth and, unaware of Penfield's discovery, in 1981 University of Arizona graduate student Alan R Hildebrand and faculty adviser William V Boynton published a draft Earth-impact theory and sought a candidate crater. Their evidence included greenish-brown clay with surplus iridium containing shocked quartz grains and small weathered glass beads that looked to be tektites. Thick, jumbled deposits of coarse rock fragments were also present, thought to have been scoured from one place and deposited elsewhere by a kilometres-high tsunami likely resulting from an Earth impact. Such deposits occur in many locations but seem concentrated in the Caribbean basin at the K–T boundary. So when Haitian professor Florentine Morás discovered what he thought to be evidence of an ancient volcano on Haiti, Hildebrand suggested it could be a telltale feature of a nearby impact. Tests on samples retrieved from the K–T boundary revealed more tektite glass, formed only in the heat of asteroid impacts and high-yield nuclear detonations.
In 1990, Houston Chronicle reporter Carlos Byars told Hildebrand of Penfield's earlier discovery of a possible impact crater. Hildebrand contacted Penfield in April 1990 and the pair soon secured two drill samples from the Pemex wells, stored in New Orleans. Hildebrand's team tested the samples, which clearly showed shock-metamorphic materials.
A team of California researchers including Kevin Pope, Adriana Ocampo, and Charles Duller, surveying regional satellite images in 1996, found a sinkhole (cenote) ring centered on Chicxulub that matched the one Penfield saw earlier; the sinkholes were thought to be caused by subsidence of the impact crater wall. More recent evidence suggests the actual crater is 300 km (190 mi) wide, and the 180 km ring an inner wall of it.
The impactor had an estimated diameter of 10 km (6.2 mi) and delivered an estimated 96 teratons of TNT (4.0×1023 J). By contrast, the most powerful man-made explosive device ever detonated, the Tsar Bomba, had a yield of only 50 megatons of TNT (2.1×1017 J), making the Chicxulub impact 2 million times more powerful. Even the most energetic known volcanic eruption, which released approximately 240 gigatons of TNT (1.0×1021 J) and created the La Garita Caldera, was substantially less powerful than the Chicxulub impact.
The impact would have caused some of the largest megatsunamis in Earth's history, reaching thousands of meters high. A cloud of super-heated dust, ash and steam would have spread from the crater, as the impactor burrowed underground in less than a second. Excavated material along with pieces of the impactor, ejected out of the atmosphere by the blast, would have been heated to incandescence upon re-entry, broiling the Earth's surface and possibly igniting global wildfires; meanwhile, colossal shock waves would have effected global earthquakes and volcanic eruptions. 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. The shock production of carbon dioxide caused by the destruction of carbonate rocks would have led to a sudden greenhouse effect. Over a longer period, sunlight would have been blocked from reaching the surface of the earth by the dust particles in the atmosphere, cooling the surface dramatically. Photosynthesis by plants would also have been interrupted, affecting the entire food chain. A model of the event developed by Lomax et al. (2001) suggests that net primary productivity (NPP) rates may have increased to higher than pre-impact levels over the long term because of the high carbon dioxide concentrations.
In February 2008, a team of researchers led by Sean Gulick at the University of Texas at Austin’s Jackson School of Geosciences used seismic images of the crater to determine that the impactor landed in deeper water than was previously assumed. They argued that this would have resulted in increased sulfate aerosols in the atmosphere. According to the press release, that "could have made the impact deadlier in two ways: by altering climate (sulfate aerosols in the upper atmosphere can have a cooling effect) and by generating acid rain (water vapor can help to flush the lower atmosphere of sulfate aerosols, causing acid rain)."
Geology and morphology
In their 1991 paper, Hildebrand, Penfield, and company described the geology and composition of the impact feature. The rocks above the impact feature are layers of marl and limestone reaching to almost 1,000 m (3,300 ft) in depth. These rocks date back as far as the Paleocene. 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, as is shocked quartz. The K–T boundary inside the feature is depressed between 600 to 1,100 m (2,000 to 3,600 ft) compared to the normal depth of about 500 m (1,600 ft) depth 5 kilometres (3.1 mi) away from the impact feature. 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. Such a basin's groundwater dissolved the limestone and created the caves and cenotes beneath the surface. The paper also noted that the crater seemed to be a good candidate source for the tektites reported at Haiti.
Astronomical origin of asteroid
On September 5, 2007, a report published in Nature proposed an origin for the asteroid that created Chicxulub Crater. 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. According to Bottke, the Chicxulub impactor was a fragment of a much larger parent body about 170 km (110 mi) across, with the other impacting body being around 60 km (40 mi) in diameter. In 2011, new data from the Wide-field Infrared Survey Explorer revised the date of the collision which created the Baptistina family to about 80 million years ago. This makes an asteroid from this family highly improbable to be the asteroid that created the Chicxulub Crater, as typically the process of resonance and collision of an asteroid takes many tens of millions of years.
Chicxulub and mass extinction
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 Cretaceous-Tertiary extinction event). Luis and Walter Alvarez, at the time both faculty members at the University of California, Berkeley, postulated that this enormous extinction event, which was roughly contemporaneous with the postulated date of formation for the Chicxulub crater, could have been caused by just such a large impact. This theory is now widely 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. 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.
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 that it contained an abnormally high concentration of iridium. Iridium levels in this layer reached 6 parts per billion by weight or more compared to 0.4 for the Earth's crust as a whole; in comparison, meteorites can contain around 470 parts per billion of this element. It was hypothesized 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 layer of iridium-enriched clay.
Multiple impact theory
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 and the Boltysh crater in Ukraine. Both are 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 nearly 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 demonstrated 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 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, though Shiva and the Chicxulub craters might have been formed 300,000 years apart.
In late 2006, Ken MacLeod, a geology professor from the University of Missouri, completed an analysis of sediment below the ocean's surface, bolstering the single-impact theory. MacLeod conducted his analysis approximately 4,500 km (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. 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.
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- Satellite image of the region (from Google Maps)
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Cretaceous–Tertiary extinction event Proposed K–T boundary craters: Boltysh crater · Chicxulub crater · Shiva crater · Silverpit crater
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