Cretaceous–Paleogene extinction event
The Cretaceous–Paleogene extinction event was a significant mass extinction event that occurred approximately 65 million years ago, marking the boundary between the Cretaceous period and the Paleogene period. This event is widely attributed to a cataclysmic impact from a large asteroid or comet, estimated to be about six miles wide, which struck the Earth at what is now the Yucatán Peninsula in Mexico. The impact generated immense energy, equivalent to about 100 million megatons of dynamite, leading to catastrophic consequences including massive tsunamis, firestorms, and drastic climate changes.
The environmental upheaval caused by the impact resulted in the extinction of roughly 75% of all species on Earth, notably including all non-avian dinosaurs, which had dominated terrestrial ecosystems for approximately 160 million years. These extinctions were compounded by further volcanic activity that released harmful ash and gases into the atmosphere, contributing to a drastic decline in sunlight and disrupting the food chain.
This extinction event not only reshaped the planet but also paved the way for the rise of mammals and other organisms, leading to a gradual recovery of biodiversity over millions of years. The discovery of the Chicxulub crater and associated geological evidence in the late 20th century provided critical support for the impact hypothesis, reshaping our understanding of this transformative moment in Earth's history.
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Cretaceous–Paleogene extinction event
The Cretaceous-Paleogene extinction event was a mass extinction that occurred on Earth about 65 million years ago. Scientists believe the extinction resulted from a large asteroid or comet striking Earth. The impact of this object is estimated to have released energy equivalent to about 100 million megatons of dynamite. The immediate destruction caused by the impact, as well as the catastrophic environmental damage that occurred over the following tens of thousands of years—including tsunamis, firestorms, extreme climate change, and the blocking of sunlight—killed about 75 percent of all living things on Earth. This notably included all non-avian dinosaurs, which had ruled Earth as the largest land animals for about 160 million years.
The exact nature of the Cretaceous-Paleogene extinction event eluded scientists for many years. Some initially believed the dinosaurs had starved to death, been infected by a plague, or grown too large for their small brains to operate. It was only in the 1980s that father-son geologists Luis Alvarez and Walter Alvarez proposed that an impact object caused the dinosaurs' destruction. The next decade, the Chicxulub crater, buried under the Yucatán Peninsula in Mexico, was named as the impact site of the asteroid or comet that struck Earth 65 million years ago and caused the Cretaceous-Paleogene extinction event.
Background
The Cretaceous period was the longest and last period of the larger Mesozoic era, a stage of geologic time that lasted from about 252 to 65 million years ago. The Cretaceous period began about 145 million years ago and continued for another 80 million years until the end of the Mesozoic era. The era gave way to the Cenozoic era and its first period, the Paleogene.
Dramatic geologic changes marked the Cretaceous period. One of the most momentous was the breakup of the supercontinent Pangaea. This was a colossal landmass made up of all seven contemporary continents on Earth. By the start of the Cretaceous period 145 million years ago, Pangaea had slowly been breaking into sections for about 55 million years. The separation of the continents into Earth's northern and southern hemispheres created new climates for existing plants and animals. This allowed many new organisms to thrive during the Cretaceous period.
Dinosaurs such as the massive sauropods—four-legged, long-necked herbivorous dinosaurs—and smaller, bipedal dinosaurs such as Iguanodon wandered the plains and forests of the slowly forming separate continents. The Cretaceous period also saw the rise of dinosaurs such as the carnivorous Tyrannosaurus rex and Spinosaurus, as well as new herbivores such as Triceratops. As the continents continued drifting apart through the Cretaceous period, larger coastlines also allowed early aquatic reptiles and amphibians such as frogs, salamanders, and snakes to thrive near the oceans.
One of the most revolutionary biological developments of the Cretaceous period was the evolution of flowering plants known as angiosperms. Plants such as sassafras, ficuses, and magnolias became widespread, assisted by pollinating insects such as wasps and bees. Some herbivorous dinosaurs likely ate angiosperms, but most simply ate twigs and leaves. These and other flowering plants survived to the present day. This was the state of life on Earth at the time of the Cretaceous-Paleogene extinction event about 65 million years ago.
Overview
The most popular scientific theory of the nature of the Cretaceous-Paleogene extinction event asserts that a colossal asteroid or comet collided with Earth about 65 million years ago at the modern-day Yucatán Peninsula in Mexico. According to the theory, this object, estimated to have been about six miles (9.6 kilometers) across, struck Earth at about 40,000 miles per hour (64,373 kilometers per hour). The force of impact likely released the equivalent of 100 million megatons of TNT and created a crater about 110 miles (177 kilometers) wide.
The impact created shockwaves that traveled around the world. The force produced massive earthquakes on land and ocean floors. The ocean tremors created immense tsunamis with waves measuring thousands of feet high. Meanwhile, the strength of the object's impact sent rock and other chunks of rubble into outer space. Much of the debris that had partially exited the atmosphere eventually returned to Earth. It caught fire upon reentry, resulting in deadly firestorms across the planet and a sudden increase in Earth's temperature. The strike also sent dust and other less solid debris into the air. This material covered Earth entirely, blocking the Sun and poisoning the air. This was all likely made worse by the sudden eruption of volcanoes around the world, as the ash released in the explosions was also poisonous for all animals to breathe.
Earth's lack of sunlight began a series of reactions that ultimately disrupted food chains across the planet. With no light, plants eventually died, and without plants, herbivorous dinosaurs and other organisms soon died, probably within weeks. The carnivores who fed on herbivores died months later. The destructive effects of the Cretaceous-Paleogene extinction event lasted tens of thousands of years, a relatively short span of geologic time. By the end of this period, about 75 percent of all organisms on Earth had become extinct. This included all non-avian dinosaurs. Without the threat of predatory dinosaurs, small mammals, amphibians, and reptiles that had survived the event were allowed to thrive and overtake Earth.
Various employees of the Mexican petroleum company Pemex first noticed the presence of a large impact crater on Mexico's Yucatán Peninsula in the 1970s. Magnetic survey maps showed a semicircular arc in the Gulf of Mexico off the northern coast of the peninsula. In the 1980s, American scientist Luis Alvarez and his son Walter hypothesized that a giant asteroid or comet impact killed the dinosaurs. They claimed this was evidenced in the levels of the metal iridium in the layer of Earth's rock from the end of the Cretaceous period. Iridium is rare on Earth's surface but abundant in space.
In the 1990s, the large impact crater in Yucatán, named the Chicxulub crater for a nearby settlement, was found to correlate with theories that a six-mile-wide object impacted Earth about 65 million years ago and ultimately killed the dinosaurs. Paleontologists have estimated that biodiversity, or the great variety of organisms brought on by gradual evolution, then took millions of years to return to normal levels on the various continents. This indicates how greatly the Cretaceous-Paleogene extinction event disrupted life on Earth.
Bibliography
Bagley, Mary. "Cretaceous Period: Animals, Plants & Extinction Event." Live Science, 7 Jan. 2016. www.livescience.com/29231-cretaceous-period.html. Accessed 23 Oct. 2017.
"Chicxulub Crater." Atlas Obscura. www.atlasobscura.com/places/chicxulub-crater. Accessed 23 Oct. 2017.
Choi, Charles Q. "Asteroid Impact that Killed the Dinosaurs: New Evidence." Live Science, 7 Feb. 2013. www.livescience.com/26933-chicxulub-cosmic-impact-dinosaurs.html. Accessed 23 Oct. 2017.
"Cretaceous Period." National Geographic. www.nationalgeographic.com/science/prehistoric-world/cretaceous/. Accessed 18 Nov. 2024.
"Dinosaur Extinction." National Geographic. www.nationalgeographic.com/science/prehistoric-world/dinosaur-extinction/. Accessed 18 Nov. 2024.
Kaplan, Sarah. "How Long Did It Take for Life to Rebound after the Death of the Dinosaurs?" Washington Post, 7 Nov. 2016. www.washingtonpost.com/news/speaking-of-science/wp/2016/11/07/how-long-did-it-take-for-life-to-rebound-after-the-death-of-the-dinosaurs/?utm‗term=.4327625dc4ab. Accessed 23 Oct. 2017.
Nace, Trevor. "Scientists to Drill 'Ground Zero' of Dinosaur Extinction." Forbes, 17 Mar. 2016. www.forbes.com/sites/trevornace/2016/03/17/scientists-drill-ground-zero-dinosaur-extinction/#e4a0b653dea0. Accessed 23 Oct. 2017.
"What Killed the Dinosaurs?" PBS. www.pbs.org/wgbh/evolution/extinction/dinosaurs/asteroid.html. Accessed 23 Oct. 2017.
"Why Did the Dinosaurs Die out?" History, 5 June 2023, www.history.com/topics/why-did-the-dinosaurs-die-out. Accessed 18 Nov. 2024.