The end of the era of the dinosaurs (the end of the Cretaceous period and the beginning of the Tertiary period, known as the K-T boundary) is thought to have been brought about by the impact of a large asteroid 6-12 miles wide and traveling at 15 miles per second. The asteroid landed at Chicxulub (can you pronounce that?), near the tip of the Yucatan peninsula in Mexico, 200 miles west of Cancun. [1]

The crater is not visible from the ground at all, but images from the space shuttle have provided the most striking visual evidence yet (see small diagram half way down this page: [2]). It’s 100-150 miles wide, buried under a half mile of sediment, and covered with jungle. It was detected while taking core samples and performing seismic monitoring while looking for oil. The crater contains a lot of iridium, an element that’s rare on earth but present in most asteroids, providing evidence that it is an asteroid impact crater.

The impact hypothesis was first proposed in 1980 but was very controversial. Then in the early 1990’s, the Chicxulub crater (one of many known impact structures on earth) was discovered to be the K-T boundary crater predicted by the impact hypothesis, because it occurred at that time. [3] Also, there is evidence all over the globe of a 100,000-years-thick layer of iridium, dating to the K-T period.

A drilling project has been going on at the crater site recently. There’s a diagram of the crater here: [4]. Despite this evidence, the impact hypothesis is still not the only theory of why the dinsaurs died out.

While it’s not known exactly how the asteroid killed the dinosaurs, many scientists believe the soot in the ground at the K-T boundary was deposited by a global firestorm in which 50% of the world's forests were burned. A recent study, however, suggests that no such firestorm took place. [5]

After the impact, dust blocked out the sun all over the world and reduced temperatures everywhere by 70 degrees Fahrenheit for several months or years. Once the dust settled, the earth then suffered the effects of a global warming of 10 degrees Fahrenheit—for over a hundred thousand years! This torturous sequence of events killed 50 percent of all the earth’s species. On land, no animal larger than 50 lbs. survived. [6]


Sources

[1] http://www.xrefer.com/entry.jsp?xrefid=388559&secid=.-&hh=1

[2] http://photojournal.jpl.nasa.gov/catalog/PIA03379

[3] http://www.spacedaily.com/news/deepimpact-01h.html

[4] http://www.spacedaily.com/news/deepimpact-02c.html

[5] http://news.bbc.co.uk/2/hi/science/nature/3295539.stm

[6] http://www.space.com/scienceastronomy/astronomy/dinosaurs_fry_991118.html