In 1980 the father-and-son team of scientists Luis and Walter Alvarez proposed that the mass extinction of the dinosaurs and most other living things on Earth was caused by the impact of a large asteroid on the Earth sixty-five million years ago, called the Cretaceous–Paleogene extinction event. In the last 20 years, evidence indicates that the asteroid fell in the Yucatán Peninsula, at Chicxulub, Mexico.
A picture of the father-son team is below. In the picture, they are showing the layer they discovered of sedimentary material found all over the world at the Cretaceous–Paleogene boundary (Cretaceous–Tertiary boundary or K–T boundary. Upon examination, this layer contains a concentration of iridium hundreds of times greater than normal. Iridium is extremely rare in the Earth's crust because it is very dense, and therefore most of it sank into the Earth's core while the earth was still molten. The Alvarez team suggested that the material must have come from an asteroid that struck the earth at the time of the Cretaceous–Paleogene boundary.
Since the theory was developed, scientists have continued looking for more evidence. They have successfully found shocked quartz granules, glass spherules and tektites, indicative of an impact event, are common in the Cretaceous–Paleogene boundary, especially in deposits from around the Caribbean.The location of the impact was unknown when the Alvarez team developed their hypothesis, but later scientists discovered the Chicxulub Crater in the Yucatán Peninsula, now considered the likely impact site. The crater is more than 110 miles (180 kilometers) wide.
- Using estimates of the total amount of iridium in the K–Pg layer, and assuming that the asteroid contained the normal percentage of iridium found in chondrites, the Alvarez team went on to calculate the size of the asteroid. The answer was about 10 kilometers (6 mi) in diameter, about the size of Manhattan.
- The collision would have released the same energy as 100 teratonnes of TNT (420 ZJ), over a billion times the energy of the atomic bombs dropped on Hiroshima and Nagasaki
- Scientists believe that this asteroid impact caused the death of 99 percent of the organisms on Earth (85 percent of species). The impact would have caused a vast dust cloud that would have blocked sunlight and prevented photosynthesis for a few years. This would have caused the extinction of plants and phytoplankton and all of the organisms dependent on them (including predatory animals as well as herbivores). It is estimated that sulfuric acid aerosols were injected into the stratosphere, leading to a 10–20% reduction of solar transmission normal for that period. It would have taken at least ten years for those aerosols to dissipate.
- Life, of course, recovered. Small creatures whose food chains were based on detritus would have had a reasonable chance of survival. Today there are over 10,000 species of birds, 14,000 amphibian and reptile species, 5,000 mammalian species, plus probably millions of species of plants, microorganisms, and invertebrates. Everything living on Earth today had an ancestor that survived the asteroid impact 65 million years ago.
- In 2005 the U.S. government funded NASA to identify 90 percent of near-Earth asteroids by 2020.
- In March 2010 an international panel of scientists endorsed the asteroid hypothesis, specifically the Chicxulub impact, as being the cause of the extinction. A team of 41 scientists reviewed 20 years of scientific literature and in so doing also ruled out other theories such as massive volcanism.
CJLL Wright , http://commons.wikimedia.org/wiki/File:Yucat%C3%A1n_Peninsula.png