Who first hypothesized the impact scenario?

In 1980, the American physicist Luis Alvarez (1911-1988) proposed that a large asteroid or comet hit Earth about 65 million years ago. His son, geologist Walter Alvarez (1940-), discovered a high concentration of iridium (an element associated with extraterrestrial impacts) at the Cretaceous-Tertiary (nicknamed K/T) boundary in Italy. Because of this find, and the realization that dinosaurs and many other species died out at the end of the Cretaceous, Luis and Walter Alvarez, along with colleagues Frank Asaro and Helen Michel, proposed that the extinctions at the K/T boundary were caused by the impact of a large space object. The iridium anomaly has since been found in over 50 K/T boundary sites around the world.

What are impact craters and are there any on Earth?

An impact crater is a large hole in the surface of a planet that is most often caused by a collision with a large space object, such as a comet or an asteroid. All planets and most satellites have impact craters, and even asteroids have impact craters. The Moon is our most obvious example of impacts on a planetary body, as the surface is dotted with hundreds of craters.

To date, scientists have identified over 150 impact craters on Earth. Most have been found on the surface; fewer than a dozen or so are buried or found deep in the oceans. There were probably many more craters, but erosion from wind, water, or the movement of the continental plates has erased any evidence of their existence. In addition, there may be many more craters under the thick, vegetative growth of the jungles, in high mountains, or buried deep under sediment on land or in the oceans. The largest known crater, the Vredefort crater in South Africa, is also one of the oldest, with an age of over two billion years. Another large impact crater, the Sudbury in Canada, is a major source of certain metals. Craters on the planet Mars dwarf Earths craters. The largest impact crater (also called a basin) on the red planet is Hellas Planitia, measuring 1,243 miles (2,000 kilometers) in diameter.

Large Impact Craters






South Africa



Ontario, Canada



Yucatan, Mexico



Quebec, Canada






South Australia, Australia


Chesapeake Bay

Virginia, USA






South Africa






Montana, USA


*Crater thought to be associated, or at least partially associated, with the extinction of the dinosaurs.

What evidence supports the impact theory?

The most compelling evidence for this theory is the Chicxulub crater near the Yucatan Peninsula in Mexico, an impact crater that was discovered by geologists in 1992. This almost 180-mile- (110-kilometer-) wide crater is thought to be the result of a 6-mile- (10-kilometer-) diameter asteroid. The crater was created approximately 64.98 million years ago, which would be the right time frame for the extinction of the dinosaurs. The crater is buried, and was actually found in the 1960s during a subsurface survey taken by an oil company. It took years before a geologist looking at the data noticed the circularity of the feature and brought the impact crater to the attention of the scientific community.

Why was the impact at Chicxulub so devastating?

The impact at Chicxulub around 65 million years ago would have been devastating enough, but the effects were worse because the asteroid traveled at such a shallow angle. The approximately 6-mile- (10-kilometer-) wide object struck the Yucatan at an angle of approximately 20 degrees above the southeast horizon. If the impact had occurred straight down (perpendicular to Earth) most of the energy would have been directed into the planets interior. But the shallow impact angle of the Chicxulub object meant that debris in the form of vaporized and molten rock was scattered forward towards the northwest. This instantly destroyed all living organisms, including dinosaurs, over western North America. The material forced into the upper atmosphere by this angled impact would have cooled the climate over a period of months, effectively killing off the remaining dinosaurs.

Because of erosion and other active geological processes on Earth, impact craters such as Barringer Meteorite Crater in Arizona, are uncommon. The theory that a devastating impact from an asteroid killed off the dinosaurs at first seemed like an unlikely explanation to many scientists (iStock).

How many impact craters are associated with dinosaur extinction?

Currently, there is only one known Earth impact crater associated with dinosaur extinction: the Chicxulub crater in Mexico. In addition, tiny glass fragments from the impact ejecta (the rock and soil that sprayed from the crater when it formed) were found in 1990 in the Caribbean Ocean on the island of Haiti. The ejecta debris appears to fall in line with the Chicxulub crater. Another crater, the Manson structure in Iowa, was once thought to have formed at the end of the Cretaceous, but subsequent studies show it is not the correct age. Two impact crater sites in Belize and Mexico, about 290 miles (480 kilometers) and 140 miles (230 kilometers) from the Chicxulub crater, respectively are thought to be from the ejecta of Chicxulub the material thrown up from the crater that landed nearby.

How long after the asteroid impact did the dinosaurs become extinct?

No one truly knows how long after the asteroid impact the dinosaurs became extinct, but several studies in 2009 point to a possible number: around 300,000 years after the impact. For example, scientists uncovered marine sediments in Mexico that seem to contain at least 52 species of various animals, creatures that were found in younger rock. Another scientist claimed to have unearthed dinosaur bones in the American Southwest that date almost a half million years after the Yucatan impact. If this is true, scientists dont believe that the dinosaurs actually survived in areas around the impact, but instead the creatures that survived in relatively unscathed areas later moved and recolonized in areas closer to the impact.

Has there been any evidence of a recent killer asteroid?

Yes, scientists believe they have found evidence of a more recent killer asteroid. This huge rock smashed into southeastern Argentina about 3.3 million years ago, possibly killing off 36 species of animals because of the local climate change. Neither the asteroid itself nor a crater was found, but glassy fragments (called escoria) were dug up from the soil. These fragments indicate intense heating that could only have come from a space object striking Earth.

Unlike the asteroid that supposedly struck at the end of the Cretaceous period, the Argentina hit did not affect the global climate. But the strike apparently did change the local climate, cooling down the temperatures. This killed off many animals in the region, including giant armadillos, ground sloths, and a large-beaked carnivorous bird.

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