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

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?

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

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).