For any one species, extinction may seem catastrophic. But over the grand sweep of life on Earth, extinction is business as usual. Extinctions occur continually, generating a "turnover" of the species living on Earth. This normal process is called background extinction. Sometimes, however, extinction rates rise suddenly for a relatively short time an event known as a mass extinction. Mass extinctions kill off many species, but the empty niches left behind may allow other lineages to radiate into new roles, shaping the diversification of life on Earth.
The K-T extinction
There is very good evidence that a giant asteroid hit Earth at the same time as the K-T extinction. The smoking gun is the 100-mile wide crater it left behind off the coast of Mexico, along with disturbed geologic deposits (iridium and shocked quartz) consistent with an asteroid impact. The impact probably caused tidal waves, earthquakes, and clouds of dust so thick that they blotted out the sun for months. Such a disaster is certainly capable of causing a mass extinction. However, many lineages were on the wane, experiencing a lot of extinction, even before the asteroid hit. These pre-impact extinctions must have other causes.
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