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for centuries, people have been puzzled over what caused dinosaurs to die off. today, there are several theories about what caused the extinction of dinosaurs. there are two primary theories, and scientists who study these theories fall into two main categories—intrinsic gradualists and extrinsic catastrophists.

the intrinsic gradualists believe that increased volcanic activity at the end of the cretaceous period led to the dinosaurs dying out. according to this theory, volcanoes all over the world were erupting at different intervals for a period of several million years. soot and ash from these eruptions may have gradually accumulated in the atmosphere and blocked the sunlight causing the earth's surface to cool. this climate change may have led to the extinction of dinosaurs. the volcanoes may have affected the earth's surface as well. there is a layer of rock in the earth's crust known as the cretaceous-tertiary (k-t) boundary. the layer is found worldwide, and it contains a high level of iridium, a type of metal. the k-t boundary marks the end of the cretaceous period and the beginning of the tertiary period. some scientists believe the high level of iridium found in the k-t boundary may have come from molten rock that spewed from the volcanoes.

the extrinsic catastrophists believe that a giant comet or meteor crashed into earth. the crash may have been the catalyst for the extinction of dinosaurs. the impact of such an object would have thrown large amounts of debris into the air all at once. the debris would have blocked out the sunlight and caused the planet to cool. the impact would also have caused earthquakes, tsunamis, and other natural disasters. some scientists argue that the high level of iridium in the k-t boundary may have come from the meteor rather than from the volcanoes. some of these scientists believe that several comets or meteors collided with earth over a relatively short period of time. these theorists think the extinction of dinosaurs resulted from the multiple impacts.

although some evidence supports each theory, neither of the theories is conclusive. none of the theories explain why the dinosaurs went extinct while many mammals and small reptiles were able to survive. also, fossil records found in some areas support a gradual decline while fossil records found in other areas support a sudden die out. for now, scientists must continue to search for answers about what caused the extinction of dinosaurs.