Hypothesis: Origin and Evidence
the late 1970's geologist Walter Alvarez, and his father, Nobel-prize
winning physicist Luis Alvarez, identified an unusual clay layer
at the K/T boundary in Italy. This clay contained an unusually high
concentration of the rare-earth element iridium 30 times the level
typically found in the Earth's crust. Why was the discovery of iridium
so important? Although iridium is rare in the crust, it is abundant
in many meteorites and asteroids as well as the Earth's core. With
this evidence, Alvarez hypothesized that an asteroid must have struck
the Earth right at the K/T boundary. Further investigation has revealed
that this iridium-rich layer of clay occurs at more than 100 sites
around the world, providing evidence that this was truly a worldwide
It was estimated that to produce the amount of iridium in the clay layer, the impact
object would have been 10 km in diameter. Further evidence of an impact was discovered in the form of small grains of impact-shocked quartz and beads of impact glass (tektites) within the clay layer. Shocked quartz is formed by high-pressure shock waves, and is found at nuclear bomb sites and in meteor craters. Tektites are formed from the condensation of vaporized meteorite particles. Although shocked quartz has been found in K/T layers worldwide, tektites decrease in size with increasing distance from the impact site until they are altogether absent.
These pieces, along with high levels of iridium, provide evidence for an extraterrestrial impact at the end of the Cretaceous Period. Thus, the end of the dinosaurs’ reign may have been caused by an asteroid, not by sea level change or volcanism. Initially this theory was highly controversial, but today an extraterrestrial impact is considered to be a key factor in the K/T extinction event.
One of the main objections to the Alvarez theory was the absence of a 65-million-year-old crater anywhere on the Earth’s surface. Surely such an enormous asteroid impact would have left a sizable crater behind. In 1991, geologists discovered evidence for a huge crater at Chicxulub (pronounced CHIK-shoo-loob), on the Yucatan Peninsula in Mexico. Although the crater had long since been buried by hundreds of meters of sediment, surveys of magnetic and gravitational fields revealed its circular structure. In addition, recent sensitive topographic mapping has shown a low mound that represents part of the crater’s rim. At 180 km across, and dated to 65 mya, the crater is of the right size and age to have been caused by a 10 km asteroid hitting Earth at the end of the Cretaceous Period.