FREQUENCY: 100 MILLION OR MORE YEARS
Asteroids large enough to cause widespread extinctions collide with Earth very rarely. These asteroids measure more than 10 kilometers (six miles) across. Because they're so large, astronomers think they've spotted all of the ones that could cross Earth's orbit during the next two centuries.
All of Earth's most devastating impacts happened many millions or even billions of years ago. Researchers have found about a dozen of the colossal craters that resulted-one impact 35 million years ago gouged open the mouth of Maryland's Chesapeake Bay. But the best-known impact happened 65 million years ago.
Did an impact do in the dinosaurs?
A thin layer of grayish clay marks the geologic boundary between the period when the dinosaurs flourished and the time when mammals rose to dominance. That clay boundary between the Cretaceous and Tertiary periods represents a mass extinction, in which 70 percent of all species died out-including the nonavian dinosaurs.
In 1980 researchers found evidence in that layer of a giant meteorite or comet impact. They found high levels of iridium-an element common in meteorites yet very rare in Earth's crust-and tiny spheres of glass that must have condensed from rock vaporized during the impact. After a decade of intense searching for this impact's crater, researchers hit pay dirt at the northern tip of Mexico's Yucatán Peninsula. A crater 185 kilometers (115 miles) wide had formed at essentially the same time the extinction took place.
In the fraction of a second after the impact, both the meteorite and the rocks beneath it turn from solid into liquid and vapor. The superheated plume of vapor shoots up above the atmosphere and spreads around the globe. Heat from the falling molten and gaseous rock ignites fires across the planet. Dust, soot and smog block out sunlight for months. This combined destruction kills off many plant species and leads to months or years of global cooling.
Topic: Earth Science
Keywords: Asteroids--Collisions with Earth, Astrogeology, Astrophysics, Collisions (Astrophysics), Dinosaurs, Geology, Stratigraphic, Mass extinctions, Meteorite craters, Meteorites, Natural disasters, Near-Earth asteroids, Paleontology