From the USGS:
Why did the dinosaurs die out?
There are dozens of theories to explain a probable cause or causes. Throughout the Mesozoic Era, individual dinosaur species were evolving and becoming extinct for various reasons. The unusually massive extinction at the end of the Cretaceous exterminated the last of the dinosaurs, the flying reptiles, and the large swimming reptiles, as well as many other marine animals. There is now widespread evidence that a meteorite impact was at least the partial cause for this extinction. Impact craters are visible on most planets in our solar system. A spectacular example of this was witnessed in 1994, when Jupiter was struck by a series of cometary fragments. Some of these impact blasts were larger than the Earth's diameter. Other factors such as extensive release of volcanic gases, climatic cooling (with related changes in ocean currents and weather patterns), sea-level change, low reproduction rates, poison gases from a comet, or changes in the Earth's orbit or magnetic field may have contributed to this extinction event.
Sixty-five million years ago, a comet or asteroid larger than Mt. Everest slammed into the Earth, causing an explosion equivalent to the detonation of a hundred million hydrogen bombs. Vaporized impactor and debris from the impact site were blasted out through the atmosphere, falling back to Earth all around the globe. Terrible environmental disasters ensued, including a giant tsunami, continent-scale wild-fires, darkness, and cold, followed by sweltering greenhouse heat. When conditions returned to normal, half the genera of plants and animals on Earth had perished. This horrific story is now widely accepted as the solution to a great scientific murder mystery-what caused the extinction of the dinosaurs? In
T. rex and the Crater of Doom, the story of the scientific detective work that went into solving the mystery is told by geologist Walter Alvarez, one of the four Berkeley scientists who discovered the first evidence of the giant impact...Controversial and widely attacked during the 1980s the impact theory received confirmation from the discovery of the giant impact crater it predicted, buried deep beneath younger strata at the north coast of the Yucatan Peninsula...Geology and paleontology, sciences that long held that all changes in Earth history have been calm and gradual, have now been forced to recognize the critical role played by rare but devastating catastrophes like the impact that killed the dinosaurs. - Book Jacket
Sixty-five million years ago, a comet or asteroid larger than Mt. Everest slammed into the Earth, causing an explosion equivalent to the detonation of a hundred million hydrogen bombs. Vaporized impactor and debris from the impact site were blasted out through the atmosphere, falling back to Earth all around the globe. Terrible environmental disasters ensued, including a giant tsunami, continent-scale wild-fires, darkness, and cold, followed by sweltering greenhouse heat. When conditions returned to normal, half the genera of plants and animals on Earth had perished. This horrific story is now widely accepted as the solution to a great scientific murder mystery-what caused the extinction of the dinosaurs? In T. rex and the Crater of Doom, the story of the scientific detective work that went into solving the mystery is told by geologist Walter Alvarez, one of the four Berkeley scientists who discovered the first evidence of the giant impact...Controversial and widely attacked during the 1980s the impact theory received confirmation from the discovery of the giant impact crater it predicted, buried deep beneath younger strata at the north coast of the Yucatan Peninsula...Geology and paleontology, sciences that long held that all changes in Earth history have been calm and gradual, have now been forced to recognize the critical role played by rare but devastating catastrophes like the impact that killed the dinosaurs. - Book Jacket
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