One exciting theory is that an extraterrestrial body hit the earth, causing the Late Cretaceous extinctions. Walter and Luis Alvarez and their coworkers found the evidence for this collision during a study of some Cretaceous clay from northern Italy. To their surprise, they found that the clay was rich in the element iridium. Iridium is rare on earth but is more common in extraterrestrial bodies such as meteorites and comets. After further studies, the Alvarezes found the iridium only in a narrow layer. To their amazement, this iridium layer almost exactly matches the Cretaceous-Tertiary boundary. This led them to suggest that a large extraterrestrial body had hit the earth, which caused the extinction of dinosaurs in the Cretaceous.
Since this discovery, scientists have found the "iridium datum plane" (the iridium layer) at the Cretaceous-Tertiary boundary at over 50 sites worldwide. There is other evidence that a large body from outer space hit the earth. For instance, when an asteroid hits the earth, intense heat and pressure develop. The heat and pressure cause changes in the rocks where the comet hit. One of these changes is shock-fractured quartz grains. Bits of quartz (a common mineral in the earth's crust) will break in an unusual way only from intense heat and pressure. The only other place shock-fractured quartz is found is at ground zero of atomic explosions (where a nuclear bomb is exploded). Common elements also act differently when under intense pressure. For example, nitrogen, a usually harmless gas, may have condensed and rained back as nitric acid, a deadly acid rain.
The impact of an asteroid would be a major event in the history of the earth. The iridium layer over the world shows that the comet or meteorite must have been over six miles wide. When it crashed to earth, it would have been traveling 12 miles per second, creating a crater about 100 miles wide.
Because of its speed, the asteroid would have ripped a giant hole in the earth's atmosphere. Parts of the earth's crust would have been blown into the upper atmosphere when the asteroid hit. Later, this would rain down as tiny glass beads, ash, shock-fractured quartz, and parts of the asteroid.
A large amount of dust would have covered the earth. The amount of dust caused by the explosion of Tambora, a volcano in Indonesia, in 1815 caused climate changes worldwide for several years. The dust and debris that would have covered the earth following a meteorite hit of the size suggested by the Alvarezes would have been greater than any volcano.
The dust cloud would have taken weeks or months to settle. First, the temperature on earth would have dropped to below freezing because the dust clouds would have stopped the sun's rays from reaching the earth. This would have harmed the green plants and ocean plankton. Plankton and green plants form the bottom of the world's food chain. They also change carbon dioxide to oxygen.
Late Cretaceous animals might have suffocated because of a lack of oxygen or starved to death. This would have begun with the plant-eaters and carried through to the meat-eaters. After that, the dust cloud would have caused global warming because the heat of the earth would have been trapped. It could not escape through the thick layer of dust in the upper atmosphere.
Since the first Alvarez study, many lines of evidence associated with the iridium layer have all lent support to the contention that an extraterrestrial impact was associated with the end of the Cretaceous. Evidence includes the discovery of an impact structure off the Yucatan peninsula and the discovery of shock-fractured quartz grains. A preponderance of fern spores was also discovered; ferns are usually the first plants to recolonize an area that has been devegetated by a natural disaster.
The dust cloud proposed by the Alvarezes would have killed all plant and animal life-not just dinosaurs. And paleontologists think the extinctions in the marine world lasted thousands of years. This may mean that the marine and land extinctions did not happen at the same time. This has led some scientists to suggest the possibility that many smaller meteors or comets hit the earth over a longer period of time.