The geologic record of life on Earth shows that there have been several sudden events that led to the extinction of large numbers of land and marine species within a short interval of time, and many of these are thought to have been caused by the impact of meteorites with Earth. Many of the boundaries between geologic time periods have been selected based on these mass extinction events. Some of the major mass extinctions include that between the Cretaceous and Tertiary Periods, marking the boundary between the Mesozoic and Cenozoic Eras. At this boundary occurring 66 million years ago dinosaurs, ammonites, many marine reptile species, and a large number of marine invertebrates suddenly died off, and the planet lost about 26 percent of all biological families and numerous species. At the boundary between the Permian and Triassic Periods (which is also the boundary between the Paleozoic and Meso-zoic eras), 245 million years ago, 96 percent of all species became extinct. Many of the hallmark life-forms of the Paleozoic era were lost, such as the rugose corals, trilobites, many types of brachiopods, and marine organisms including many foraminifer species. Several other examples of mass extinctions have been documented from the geological record, including one at the boundary between the Cambrian and Ordovician Periods at 505 million years ago, when more than half of all families disappeared forever.
These mass extinctions have several common features that point to a common origin. Impacts have been implicated as the cause of many of the mass extinction events in Earth history. The mass extinctions seem to have occurred on a geologically instantaneous timescale, with many species present in the rock record below a thin clay-rich layer, and dramatically fewer species present immediately above the layer. In the case of the Cretaceous-Tertiary extinction, some organisms were dying off slowly before the dramatic die-off, but a clear, sharp event occurred at the end of this time of environmental stress and gradual extinction. Iridium anomalies have been found along most of the clay layers, considered by many to be the "smoking gun" indicating an impact origin as the cause of the extinctions. one-half million tons (454,000 tonnes) of iridium are estimated to be in the Cretaceous-Tertiary boundary clay, equivalent to the amount that would be contained in a meteorite with a six-mile (10-km) diameter. Other scientists argue that volcanic processes within Earth can pro duce iridium and that impacts are not necessary. Still other theories about the mass extinctions and loss of the dinosaurs exist, including that they died off from disease, insect bites, and genetic evolution that led to a great dominance of male over female species. Other rare elements and geochemical anomalies are present along the Cretaceous-Tertiary boundary, however, supporting the idea that a huge meteorite hit Earth at this time and was related in some way to the extinction of the dinosaurs, no matter what else may have been contributing to their decline at the time of the impact.
other features have been found in the Chicxulub impact structure that support the impact origin for the mass extinctions. one of the most important is the presence of high-pressure minerals formed at pressures not reachable in the outer layers of Earth. The presence of the high-pressure mineral equivalents of quartz, including coesite, stishovite, and an extremely high-pressure phase known as diaplectic glass, strongly implicates an impacting meteorite, which can produce tremendous pressures during the passage of shock waves related to the force of the impact. Many of the clay layers associated with the iridium anomalies also have layers of tiny glass spherules thought to be remnants of melted rock produced during the impact that were thrown skyward, where they crystallized as tiny droplets that rained back on the planet's surface. They also have abundant microdiamonds similar to those produced during meteorite impact events. Layers of carbon-rich soot are also associated with some of the impact layers, and these are thought to represent remains of the global wildfires ignited by the impacts. Finally, some of the impact layers also record huge tsunamis that swept across coastal regions.
Many of these features are found around and associated with an impact crater recently discovered on Mexico's Yucatán Peninsula. The Chicxulub crater is about 66 million years old; half of it lies buried beneath the waters of the Gulf of Mexico, and half is on land. Tsunami deposits of the same age are found in inland Texas, much of the Gulf of Mexico, and the Caribbean, recording a huge tsunami, perhaps several hundred feet high (hundred m), generated by the impact. The crater is at the center of a huge field of scattered spherules that extends across Central America and through the southern United States. Chicxulub is a large structure, and is the right age to be the crater that records the impact at the Cretaceous-Tertiary boundary, thus fixing the extinction of the dinosaurs and other families.
The discovery and documentation of the meteorite crater on the Yucatán Peninsula, near the town of Chicxulub (meaning "tail of the devil") was a long process, but the crater is now widely regarded as the one that marks the site of the impact that caused the Cretaceous-Tertiary (K-T) mass extinction, including the loss of the dinosaurs. The crater was first discovered and thought to be an impact crater in the 1970s by Glen Penfield, a geologist working on oil exploration. He did not publish his results but presented ideas for the impact origin of the structure in scientific meetings in the 1980s. In 1981 University of Arizona geology graduate student Alan Hildebrand wrote about impact-related deposits around the Caribbean that had an age coincident with the Cretaceous-Tertiary boundary. These deposits include brown clay with an anomalous concentration of the metal iridium, thought to be from a meteorite, and some impact melts in the form of small beads called tektites. Hildebrand also wrote about evidence for a giant tsunami around the Caribbean at the K-T boundary but did not know the location of the crater. It was not until 1990 that Carlos Byars, a reporter for the Houston Chronicle, put the observations together and contacted Hildebrand, leading Hildebrand and Penfield to work together on realizing that they had located the crater and the deposits of the giant impact from the Cretaceous-Tertiary mass extinction.
Initially the Chixculub crater was thought to be about 110 miles (177 km) wide, but later studies have shown that it is a complex crater, and an additional ring was located outside the initial discovery. The crater is now regarded as being 190 miles (300 km) in diameter. The outermost known ring of the Chicxulub crater is marked by a line of sinkholes, where water moved along fractures and dissolved the underlying limestone.
The Chicxulub crater is buried under younger limestone, lying beneath 3,200 feet (1 km) of limestone that overlies 1,600 feet (500 m) of andes-itic glass and breccia found only within the circular impact structure. These igneous rocks are thought to be impact melts, generated by the melting of the surrounding rocks during the impact event, and the melts rose to fill the crater immediately after the impact. Supporting this interpretation is the presence of unusual minerals and quartz grains that show evidence of being shocked at high pressures, forming distinctive bands through the mineral grains. The center depression of the crater is about 2,000 to 3,600 feet (609.6 to 1,097.3 m) deep compared with the same layers outside the crater rim, although the surface expression is now minimal since the crater is buried so deeply by younger rocks. The band of sinkholes that marks the outer rim of the crater suggests that the interior of the crater may have been filled with water after the impact, forming a circular lake.
The size of the meteorite that hit Chixculub is estimated to have been six miles (10 km) in diameter, releasing an amount of energy equal to 1014 tons of TNT. Recent studies suggest that the type of meteorite that hit Chicxulub was a carbonaceous chondrite, based on the chemistry and large amount of carbonaceous material found in pieces of the meteorite recovered from the impact.
The impact at Chicxulub was devastating for both the local and the global environment. The impact hit near the break between the continental shelf and the continental slope, so it ejected huge amounts of dust into the atmosphere from the shelf, and caused a huge mass of the continental shelf to collapse into the Gulf of Mexico and Caribbean. This in turn generated one of the largest tsunamis known in the history of the planet. This tsunami was thousands of feet (hundreds of m) tall on the Yucatán Peninsula and was still 165-330 feet (50-100 m) tall as it swept into the present-day Texas coastline, reaching far inland.
As the impact excavated the crater at Chicxu-lub, probably in less than a second, the meteorite was vaporized and ejected huge amounts of dust, steam, and ash into the atmosphere. As this material reentered the atmosphere around the planet it would have been heated to incandescent temperatures, igniting global wildfires and quickly heating surfaces and waters. At the same time tremendous earthquake waves were generated, estimated to have reached magnitudes of 12 or 13 on the open-ended Richter scale. This is larger than any known earthquake since then, and would have caused seismic waves that uplifted and dropped the ground surface by hundreds to a thousand feet (up to 300 m) at a distance of 600 miles (965 km) from the crater.
The impact generated huge amounts of dust and particles that would have been caught in the atmosphere for months after the impact, along with the ash from the global fires. This would block the sunlight and create in ice condition across the planet. Countering this effect is the release of huge amounts of carbon dioxide by the vaporization of carbonate rocks during the impact, which would have helped induce a greenhouse warming of climate that could have lasted decades. Together these effects wreaked havoc on the terrestrial fauna and flora that survived the fires and impact-related effects.
The Chicxulub crater and impact are widely held to have caused the mass extinction and death of the dinosaurs at the Cretaceous-Tertiary boundary. The global environment was considerably stressed before the impact, however, with marine planktonic organisms experiencing a dramatic decline before the impact (and a more dramatic one after the impact), and global temperatures falling before the crash. Some scientists argue that not all fauna, such as frogs, went extinct during the impact, and they should have if the impact was the sole cause of mass extinction. Others have argued that the age of the impact is not exactly the same age as the extinction, and may actually predate the extinction by up to 300,000 years. Some models suggest that the global environment was already stressed, and the impact was the final blow to the environment that caused the global mass extinction.
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