Meteorite
Impacts

The Barringer Crater shown in the adjacent image (Ref) is only the best-preserved of large meteor impacts. There is evidence for many more. Here is a link to some views of terrestrial impact craters, and here is a clickable map of terrestrial impact structure locations. Although in many instances these impact structures have been partially obliterated by erosion and tectonics, there is rather strong evidence that most are fossil impact craters.

Meteorite Impact Velocities

The average velocity of meteoroids entering our atmosphere is 10-70 km/second. The smaller ones that survive the trip to the Earth's surface are quickly slowed by atmospheric friction to speeds of a few hundred kilometers per hour, and so hit the Earth with no more speed than if they had been dropped from a tall building. For meteorites larger than a few hundred tons (which fortunately are quite rare), atmospheric friction has little effect on the velocity and they hit the Earth with the enormous speeds characteristic of their entry into our atmosphere. Thus, for example, it is estimated that the meteorite that produced the Barringer Crater was still travelling at 11 km/second when it struck what is now the Arizona desert 49,000 years ago. Such objects do enormous damage, because the kinetic energy carried by the meteorite is the product of the mass and the square of the velocity.

There is no documented record of a human being killed by a meteorite, but in recent years meteorites have crashed into bedrooms in Alabama, dining rooms in Connecticut, and a car in New York (more info).

The Peekskill Meteorite and other Fireballs

On October 9, 1992, a fireball was seen streaking across the sky from Kentucky to New York. At least 14 people captured part of the fireball on videotape; here is a spectacular move of the fireball (source). A 12-kilogram stony meteorite (chondrite) from the fireball fell in Peekskill, New York, smashed the trunk of a parked automobile, and came to rest beneath it. These are the first motion picture recordings of a fireball with an associated meteorite fall. Here is a short MPEG movie of a -4 magnitude Delta Capricornid fireball (253 kB MPEG) recorded near Hannover, Germany, on January 5, 1995.

The K-T Event and the Extinction of the Dinosaurs

Sixty-five million years ago, about 70 percent of all species then living on Earth disappeared within a very short period in what is termed the Cretaceous-Tertiary Mass Extinction---commonly known as the K-T Event (K is used to denote the Cretaceous period rather than C to avoid confusion with other periods such as the Cambrian). Among the species that disappeared were the last of the dinosaurs. The cause of this and other sudden species extinctions has long been an important and controversial topic.

In 1980, physicist Luis Alvarez and coworkers reported finding a very high concentration of the element iridium in the sedimentary clay layer laid down at the time of the K-T extinction. On Earth, iridium is very rare in the crust because it was concentrated in Earth's core when it was largely molten. However, chondritic meteorites often still have the primordial solar system abundances of these elements. This led Alvarez et al to suggest that a chondritic asteroid 10 kilometers in diameter that struck the Earth in the K-T period would contain enough iridium to account for the worldwide clay layer iridium enhancement, and that this meteor impact could also have triggered dramatic climatic changes that produced the K-T extinction.

The Crater Associated with the K-T Extinction?

Since the original proposal, additional evidence has accumulated that strongly supports the impact theory. However, the theory remains contentious, and opponents had long argued that there was no evidence for the requisite crater associated with the impact. In 1990, it was realized that geophysical data taken 10 years earlier in a search for oil in the Yucatan region of Mexico indicated a 180-kilometer diameter ring structure centered on the present coastline and lying beneath several hundred meters of sedimentary deposits called Chicxulub that had the requisite characteristics.

The Chicxulub structure has been age dated at 65 million years, and additional studies support its interpretation as a relic impact crater (here is a summary; here is a natural history museum exhibit concerning the event and recent ocean core sampling providing support for the K-T hypothesis). Maybeh the original crater has been eroded and covered with sediment, models suggest that shortly after formation the Chicxulub crater may have resembled the crater Strindberg on Mercury.

That being said, another theory gaining more recent acceptance is that there was no impact--rather it was an asteroid that burned up in the atmosphere and the sonic boom (shock wave) was the cause of the damage at the surface of Earth. This would avoid the need to explain the lack of evidence (or erosion) of an impact crater.

Summary: The Asteroid and the Dinosaur

Thus, it has been proposed that 65 million years ago in what is now Yucatan the impact at a velocity of 11 km/second of a 10-kilometer wide asteroid threw huge amounts of matter into the atmosphere (in addition to local phenomena like generating 2000 foot waves that may have emptied the Gulf of Mexico!). This created months of darkness (interfering with photosynthesis) and much cooler temperatures globally, and the resulting harsh conditions led to the extinction of many species, including the last of the dinosaurs.

Although this is a compelling hypothesis with substantial support, it remains controversial and has broad but not yet uniform acceptance within the scientific community. It is estimated that impacts of asteroids as large as the one thought responsible for the K-T extinction occur about once every hundred million years (more info).