5 EXTINCTION LEVEL EVENTS 730L

Transcription

1 5 EXTINCTION LEVEL EVENTS 730L

2 EXTINCTION LEVEL EVENTS By Cynthia Stokes Brown, adapted by Newsela

3 Life on Earth has seen several periods when a large portion of its species died off. Yet, life on our planet has always bounced back. Each time, a newly shaped tree of life has come into being. 2 3

4 FIve major extinction events An extinction event is when many species die off at once. It is normal for species to go extinct sometimes. But during an extinction event it happens more quickly than usual. Scientists now agree that five major extinction events have occurred. The crater of doom Sixty-five million years ago, dinosaurs were hunting and grazing around the world. An object the size of Mount Everest came flying through space and hit Earth. The huge object (a comet or asteroid) landed just off the coast of Mexico. It was traveling 150 times as fast as a jet. The impact made a hole the size of Belgium. Dust and debris rose high into the atmosphere and spread around the world. Forest fires filled the skies with smoke. The Sun s rays were blocked. Without sunlight, photosynthesis stopped or slowed. Temperatures cooled and rainfall decreased. Plants and animals died, including almost all the dinosaurs. About three-quarters of all species disappeared. Crocodiles, turtles, and small, rodent-like mammals survived. The small mammals are our ancestors. This is called the K-T event because it marked the end of the Cretaceous period and the beginning of the Tertiary period. (Cretaceous is spelled with a K in German). We understand the K-T event thanks to years of patient work by scientists. It began when young geologist Walter Alvarez made a discovery in the mountains of Italy. Volcanic activity is thought to have contributed to many of Earth s extinction events 4 5

5 Actually, a massive crater had been discovered decades earlier. Oil-company geologists had discovered and mapped a 150-mile crater off the coast of Mexico in K-T researchers contacted the oil-company geologists in The crater of doom had been found. Other extinction events Scientists have identified four other major extinction events. All came before the K-T extinction. Each is named for the geologic time it corresponds to. They are: the End-Triassic, the End-Permian, the Late Devonian and the Ordovician. An illustration of the K-T impact crater Alvarez found a thin layer of clay between the layers of Cretaceous and Tertiary limestone. The Cretaceous layer had many more fossils than the Tertiary layer. In the layer of clay was iridium. This element is very rare on Earth. It s more common in meteorites. This suggested an asteroid impact around the date of the extinction. Alvarez and his team announced in 1980 that an asteroid or comet had hit the Earth. They said that was the cause of a massive extinction. The End-Permian was the largest of the five major extinctions. It was the mother of all extinction events. About 95 percent of marine species were lost. Seventy percent of land species disappeared. The dying off lasted 165,000 years. Very few creatures made it through the End-Permian extinction. Cockroaches did. Ginkgo trees and horseshoe crabs did too. So did our ancestors, small protomammals (early mammals) that had evolved from reptiles. They were furry and warm-blooded, but still laid eggs. Most geologists accepted this hypothesis. Others were unsure. If a massive asteroid or comet had hit, where was the crater? There was no crater on land that was large enough. It seemed the crater must be underwater. Large objects that hit water can create huge tsunami waves. These waves leave behind unique signs in the rock record. Rocks on the shore of Texas, across from Mexico, showed these signs. 6 7

6 MASS EXTINCTIONS THE FIVE MAJOR MASS EXTINCTION EVENTS estimated % of species lost ORDOVICIAN EXTINCTION LATE DEVONIAN EXTINCTION END-PERMIAN EXTINCTION END-TRIASSIC EXTINCTION K-T EXTINCTION TODAY millions of years ago (MYA) 8 9

7 Possible causes of extinctions Most scientists agreed that an asteroid or comet caused the K-T extinction. Some of them hypothesized that all major extinctions were caused by space objects. That wasn t true. The other extinctions happened gradually. The K-T event happened suddenly. Scientists still aren t sure what caused the four other extinction events. Some possible explanations are: Volcanoes. Scientists have found evidence of large eruptions at the same time as extinctions. Volcanoes give off carbon dioxide, which causes global warming. They also send out dust that blocks the sun. Without sunlight, plants die and food chains collapse. A sixth major extinction? Many biologists argue that a sixth major extinction is happening today. This one is being caused by humans. We are destroying the habitats of other life forms. No one knows how many species currently exist on Earth. The best estimate is about 8.7 million. Experts believe that one in four known mammal species is threatened with extinction in the next several decades. One in eight known bird species is at risk. Biologists fear we could lose half of all known living species by the end of this century. Climate change. Impact or multiple-impact events. Ocean layers losing their oxygen (anoxic events). Changing position of oceans and continents (plate tectonics). Some combination of these may have taken place. Paleontologist Peter Ward made this hypothesis in 2006 to explain the four other major extinctions: Volcanoes caused a sudden increase in carbon dioxide and methane in the atmosphere. The warmer temperatures changed ocean currents. This caused some layers of the ocean to lose their oxygen. This allowed green sulfur bacteria to expand. The bacteria produced hydrogen sulfide. The hydrogen sulfide bubbled up, killing life on Earth and destroying the ozone layer. Without the ozone, life on Earth was unprotected from the Sun s ultraviolet rays. If we continue to put carbon dioxide into the atmosphere, we risk starting a similar chain of events

8 Sources Alvarez, Walter. T. Rex and the Crater of Doom. Princeton, NJ: Princeton University Press, Erwin, Douglas H. Extinction: How Life on Earth Nearly Ended 250 Million Years Ago. Princeton, NJ, and Oxford, UK: Princeton University Press, Ward, Peter D. Under a Green Sky: Global Warming, the Mass Extinctions of the Past, and What They Can Tell Us About Our Future. New York: Smithsonian/HarperCollins, Image credits Dinosaur skeleton in the desert Mark Garlick/Science Photo Library/CORBIS Sarychev Peak eruption, Kuril Islands, Russia Image Science & Analysis Laboratory, Johnson Space Center/NASA Artwork of the Chicxulub crater off the Yucatán Peninsula, Mexico Detlev van Ravenswaay/Photo Researchers, Inc

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