You've probably heard how dinosaurs went extinct when a comet or asteroid the size of 100,000 Superdomes slammed into the Earth around 65 million years ago. But this description is not quite right.
Dinosaurs are not extinct! Birds are living dinosaurs that survived even though their relatives didn't.
The 5 Great Mass Extinctions
Mass extinctions—when a significant number of species go extinct within a short interval of time—have happened throughout the history of life on Earth. The biggest mass extinctions—when at least half of all species die out in a relatively short time—have happened only a handful of times over the course of our planet's history. The largest mass extinction event occurred around 250 million years ago, when perhaps 95 percent of all species went extinct.
- ORDOVICIAN-SILURIAN EXTINCTION: Small marine organisms died out. (440 mya)
- DEVONIAN EXTINCTION: Many tropical marine species went extinct. (365 mya)
- PERMIAN-TRIASSIC EXTINCTION: The largest mass extinction event in Earth's history affected a range of species, including many vertebrates. (250 mya)
- TRIASSIC-JURASSIC EXTINCTION: The extinction of other vertebrate species on land allowed dinosaurs to flourish. (210 mya)
- CRETACEOUS-TERTIARY EXTINCTION: (65.5 mya) wipes out more than 50% of plants and animals. It is a major turning point in Earth's history, marking the end of the geologic period known as the Cretaceous and the beginning of the Tertiary period.
The Cretaceous-Tertiary extinction was key to the rise of mammals. What were some possible causes for it?
Collision with Large Meteorite
The evidence for an impact by an enormous asteroid or comet comes from tiny particles of dust.
EVIDENCE: In the late 1970s a group of researchers led by physicist Luis Alvarez and geologist Walter Alvarez made an important discovery. While analyzing rock samples from 65 million years ago, the father-son team measured exceptionally high concentrations of a rare element, iridium.
INTERPRETATION: The element iridium is rare in Earth's crust but is more abundant in the interior of our planet and in certain asteroids and comets. The Alvarez team argued that the iridium-rich layer of dust was the fallout from a massive asteroid or comet that vaporized upon impact with Earth, blasting huge quantities of dust and debris into the atmosphere.
This iridium layer has been found around the world, even in regions once covered by water. This, combined with other evidence, suggests the object that hit Earth was gigantic-probably around 10 kilometers (roughly six miles) across.
CONCLUSION: The consequences of a major impact-reduced sunlight, short-term cooling, acid rain, huge tsunamis, global forest fires and long-term greenhouse warming-would have been devastating to life on Earth. Because the iridium-rich layer corresponded with the mass extinction 65 million years ago, the Alvarez team concluded that the impact triggered the extinction. In 1991, other investigators proposed that a crater in Mexico, called Chicxulub, was the likely site of this impact.