Strong evidence that the dinosaurs were killed-off 66 million years ago by an asteroid hitting Earth has been found in Chicxulub crater under the Gulf of Mexico. An international team has measured an abundance of the rare element iridium in the crater and similarly high
concentrations of the element are known to occur in sediments laid down at the time of the Cretaceous–Paleogene boundary (K–Pg) extinction event, which saw many species on Earth vanish. Measuring 200 km across, the Chicxulub crater is believed to have been created by an 11 km-wide asteroid crashing into Earth. The impact would have sent vast amounts of vaporized rock into the atmosphere, blocking out the Sun and creating a winter that could have lasted several decades. The result,
scientists believe, was the mass extinction of 75% of species on Earth including the non-flying dinosaurs. The crater was discovered in the 1990s, but the idea that the K–Pg extinction was caused by an asteroid impact was proposed a decade earlier by a team that included the physics Nobel laureate Luis Alvarez. They found an unusually high amount of iridium in sedimentary rocks laid down at the K–Pg boundary. Iridium is rare in the Earth’s crust because it is a siderophile, which means
that it dissolves in iron and therefore tends to sink into the Earth’s core. Iridium is much more abundant in asteroids, leading Alvarez and colleagues to conclude that the vaporization of an asteroid released large amounts of iridium into the atmosphere, which then fell to the ground as dust as the dinosaurs disappeared. As well as the subsequent discovery of the Chicxulub crater, the impact extinction theory is backed up by evidence that huge tsunamis occurred in
the Gulf of Mexico and Caribbean regions at the time. However, the evidence linking the Chicxulub impact to the K–Pg extinction is not conclusive. The iridium could have been put into the atmosphere by another asteroid impact or impacts; and some scientists have suggested that increased volcanic activity, rather than an asteroid, could have caused the extinction. In 2016, Sean Gulick at the University of Texas at Austin and Joanna Morgan of Imperial College London led an international team of scientists on the International Ocean Discovery Program on an expedition to the Chicxulub crater. They took about 900 m of rock core samples and found a similar spike in iridium content in sediment laid down just after the crater was formed. Indeed, the sedimentary rock containing iridium is so thick that they were able to date the dust to about two decades after the impact. Similar abundancesStudies of the cores also revealed high levels of several other elements associated with asteroids and have been found in similar abundances in K–Pg sediments at 52 sites around the world. Read more  Dinosaur extinction linked to colliding asteroids “We are now at the level of coincidence that geologically doesn’t happen without causation,” says Gulick. “It puts to bed any doubts that the iridium anomaly [in the geologic layer] is not related to the Chicxulub crater.” A separate study of the cores done in 2019 reveals that the crater rock has been depleted of sulphur when compared to surrounding limestone. This suggests that the impact blew large amounts of sulphur into the air, where it would have contributed to the cooling and then fallen as acid rain – making the situation on the ground even worse. The research is described in Science Advances. The term iridium anomaly commonly refers to an unusual abundance of the chemical element iridium in a layer of rock strata at the Cretaceous–Paleogene (K–Pg) boundary. The unusually high concentration of a rare metal like iridium is often taken as evidence for an extraterrestrial impact event. Anomaly characteristics[edit]The type locality of this iridium anomaly is near Raton, New Mexico.[citation needed] Iridium is a very rare element in the Earth's crust, but is found in anomalously high concentrations (around 100 times greater than normal) in a thin worldwide layer of clay marking the boundary between the Cretaceous and Paleogene periods, 66 million years ago. This boundary is marked by a major extinction event, including that of the dinosaurs along with about 70% of all other species. The clay layer also contains small grains of shocked quartz and, in some places, small weathered glass beads thought to be tektites.[1] Meteorite impact theory[edit]A team consisting of the physicist Luis Alvarez, his son, geologist Walter Alvarez, and chemists Frank Asaro and Helen Vaughn Michel were the first to link the extinction to an extraterrestrial impact event based on the observation that iridium is much more abundant in meteorites than it is on Earth.[2] This theory was later substantiated by other evidence, including the eventual discovery of the impact crater, known as Chicxulub, on the Yucatán Peninsula in Mexico. See also[edit]
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Why is iridium so rare on Earth?Iridium is the most corrosion-resistant metal on Earth. It is extremely difficult to melt, and because of its high melting point, it is hard to form, machine, or work iridium. It is also one of the rarest elements in Earth's crust.
Why is iridium common in asteroids?A very thin layer of iridium exists in the Earth's crust. It is thought that this was caused by a large meteor or asteroid hitting the Earth. Meteors and asteroids contain higher levels of iridium than the Earth's crust. The impact would have caused a huge dust cloud depositing the iridium all over the world.
Is iridium more common in space?Iridium is found in meteorites in much higher abundance than in the Earth's crust.
Where did the energy of impact come from?Where did the energy of impact come from? The kinetic energy of the asteroid was transformed by friction into heat while it passed through the Earth's atmosphere.
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Why does the abundance of iridium suggest that the iridium came from an impact?
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