The Earth's crust features a 65-million-year-old boundary that separates the age of the dinosaurs from that of the mammals. Underneath this boundary lie rocks from the Cretaceous period and above lies material from the Paleogene period (formerly known as the Tertiary period), so it is known as the K-Pg layer (formerly known as the K-T layer). All fossilized dinosaur remains so far discovered lie below it, signifying a mass extinction occurred at that time in Earth's history. Some controversy surrounds the reason for this extinction.
Conditions in the Late Cretaceous Period
Two main schools of thought concerning the extinction of the dinosaurs exist, but proponents of each agree that conditions on Earth were far from ideal for the dinosaurs at the end of the Cretaceous period. There is evidence that the Earth's climate was cooling -- which could have been partially a result of a change in the planet's orbit. There is also evidence for a short-term disturbance that threw soot into the air, produced noxious gases and acid rain, and exacerbated the cooling effect. This disturbance deposited a thin layer of the rare metal iridium on the Earth's surface.
A Catastrophic Collision
The presence of iridium in the K-Pg layer prompted Berkeley planetary scientist Walter Alvarez -- among others -- to hypothesize that the extinction was caused by a catastrophic asteroid collision. Iridium is rare on Earth but common in extraterrestrial bodies, such as comets and asteroids. The presence of a layer of vaporized rock -- or tektites -- underneath the iridium layer, and other findings, led Alan Hildebrand from the University of Calgary to suggest that the impact created the Chicxulub crater in Mexico's Yucatan Peninsula. Chemical analysis of these rocks confirmed Dr. Hildebrand's findings. Discoveries of layers of soot at the K-Pg boundary, suggestive of widespread fires, lends further support to Alvarez's hypothesis that an asteroid impact did in the dinosaurs.
No Place for Dinosaurs
Before Alvarez proposed his hypothesis, scientists believed that the extinction was caused by a combination of volcanic activity and plate tectonics. There was an increase in volcanic activity in India during the late Cretaceous period, which could have produced enough soot to block out the sun and cool the Earth. Moreover, the volcanoes could also have produced the iridium layer, since that element is more abundant inside the Earth than on the surface. At the same time, plate movements were causing the oceans to recede from the land, which would also have had an effect on climate. These conditions would have caused a gradual extinction rather than a sudden one.
A Combination of Events
The volcanic theory was all but abandoned after the discovery of the Chicxulub crater, but it regained popularity amid discoveries that the dinosaur population may have been declining before the asteroid hit. Some scientists now envision the dinosaur extinction as having been caused by a combination of events, of which the asteroid impact was merely the coup-de-grace. Evidence indicating a 26-million-year periodicity in mass extinctions suggests that the Oort cloud -- a cloud of frozen objects on the fringes of the solar system -- may intersect Earth's orbit and rain comets onto the planet's surface at regular intervals. Finally, it's worth noting that not all dinosaurs went extinct; some evolved into the highly successful birds, members of which can be found on every continent today.
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