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The Chicxulub crater (; ) is an 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 date of the Chicxulub impactor, which created it, coincides precisely with the Cretaceous–Paleogene boundary (K–Pg boundary).〔Renne et al.〕 The crater is more than in diameter and in depth, making the feature one of the largest confirmed impact structures on Earth; the impacting bolide that formed the crater was at least in diameter. The crater was discovered by Antonio Camargo and Glen Penfield, geophysicists who had been looking for petroleum in the Yucatán during the late 1970s. Penfield was initially unable to obtain evidence that the geological feature was a crater, and gave up his search. Through contact with Alan Hildebrand, Penfield obtained 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 marked by the impact shows that this impact structure dates from roughly 66 million years ago, the end of the Cretaceous period, and the start of the Paleogene period. It coincides with the K-Pg boundary, the geological boundary between the Cretaceous and Paleogene. The impact associated with the crater is thus implicated in the Cretaceous–Paleogene extinction event, including the worldwide extinction of non-avian dinosaurs. This conclusion has been the source of controversy. In March 2010, 41 experts from many countries reviewed the available evidence: 20 years' worth of data spanning a variety of fields. They concluded that the impact at Chicxulub triggered the mass extinctions at the K–Pg boundary.〔Schulte, et al.〕〔Rincon.〕 In 2013 a study compared isotopes in impact glass from the Chicxulub impact with the same isotopes in ash from the boundary where the extinction event occurred in the fossil record; the study concluded that the impact glasses were dated at 66.038 ± 0.049 Ma, and the deposits immediately above the discontinuity in the geological and fossil record was dated to 66.019 ± 0.021 Ma, the two dates being within 32 kyr of each other, or almost exactly the same within experimental error.〔Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary - Renne et al〕 ==Discovery== 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.〔Verschuur, 20–21.〕 Penfield's job was to use geophysical data to scout possible locations for oil drilling.〔Bates.〕 In the data, Penfield found a huge underwater arc with "extraordinary symmetry" in a ring across.〔Penfield.〕 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.〔Verschuur, 20.〕 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, 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.〔Weinreb.〕 That year's conference was underattended and their report attracted scant attention. Coincidentally, many experts in impact craters and the K–Pg 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 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.〔Mason.〕 Their evidence included greenish-brown clay with surplus iridium containing shocked quartz grains and small weathered glass beads that looked to be tektites.〔Hildebrand, Penfield, et al.〕 Thick, jumbled deposits of coarse rock fragments were also present, thought to have been scoured from one place and deposited elsewhere by a kilometers-high tsunami resulting from an Earth impact.〔 Such deposits occur in many locations but seem concentrated in the Caribbean basin at the K–Pg boundary.〔Hildebrand interview: 'Similar deposits of rubble occur all across the southern coast of North America () indicate that something extraordinary happened here.'〕 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.〔Morás.〕 Tests on samples retrieved from the K–Pg 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.〔Frankel, 50.〕 Hildebrand contacted Penfield in April 1990 and the pair soon secured two drill samples from the Pemex wells, stored in New Orleans.〔Hildebrand interview.〕 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 cenote (sinkhole) 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.〔Pope, Baines, et al.〕 More recent evidence suggests the actual crater is wide, and the 180 km ring is in fact an inner wall of it.〔Sharpton & Marin.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Chicxulub crater」の詳細全文を読む スポンサード リンク
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