This is still a controversial question. One of the leading theories is that the earth was struck by a meteorite of considerable size, and this impact threw up immense amounts of dust and other particulates into the atmosphere. These particles changed the atmosphere so that a greater percentage of sunlight was reflected back into space, causing global cooling. Because this happened quickly, species were unable to adapt and many died off.
Several other theories also depend on climate change, but cite different possible reasons for the changing climate: continental drift, volcanoes changing the composition of the atmosphere, or changes in the earth's orbit have all been suggested.
Another theory is that mammals were actually responsible because they ate the dinosaur's eggs.
There are many theories as to why dinosaurs disappeared from the Earth about 65 million years ago. Scientists are divided between two primary schools of thought on the subject: the gradualists believe that the dinosaurs' extinction was a gradual process, and the catastrophists believe the dinosaurs were wiped out by a single catastrophic event.
The gradualists propose that the dinosaur population steadily declined at the end of the Cretaceous Period (144 to 65 million years ago) for a variety of reasons. Some claim the dinosaurs underwent biological changes which made them less competitive with other organisms, especially the mammals that were just beginning to appear. An alternate theory is that the dinosaurs suffered the effects of overpopulation. A third theory is that mammals drove dinosaurs to extinction by eating dinosaur eggs. Others believe that a variety of diseases wiped out the dinosaurs. Environmental factors also have been held responsible, such as changes in climate; continental drift (the gradual movement of land masses); volcanic eruptions; and shifts in the Earth's axis, orbit, and/or magnetic field (the portion of space near a magnetic body or current-carrying body, in which a magnetic force exists).
The catastrophists argue that a single disastrous event caused the extinction, not only of the dinosaurs, but also of a large number of other species that coexisted with them. In 1980, American physicist (a scientist specializing in the interaction between energy and matter) Luis Alvarez (1911-1988) and his geologist (a scientist specializing in the origin, history, and structure of the Earth) son, Walter Alvarez (1940—), proposed that a large comet or meteoroid (a large chunk of rock or metal from space) struck the Earth 65 million years ago. They pointed out that there is a high concentration of the element iridium in the sediments deposited at the end of the Cretaceous Period.
Iridium is rare on Earth, which makes it likely that such a large amount of the element had to come from outer space. Iridium deposits have since been discovered at more than 50 sites around the world. In 1990, tiny glass fragments, which could have been caused by the extreme heat of an impact with an extraterrestrial (beyond the Earth or its atmosphere) object, were identified in Haiti. The space object is theorized to have struck off the coast of Mexico's Yucatan Peninsula, where a 110 mile- (177 kilometer-) wide crater exists. This crater, covered by many layers of sediments, has been dated to 64.98 million years ago.
A hit by a large extraterrestrial object, perhaps as great as 6 miles (9.3 kilometers) wide, would have had a catastrophic effect upon the world's climate. Huge amounts of dust and debris would have been thrown into the atmosphere, reducing the amount of sunlight reaching the surface. Heat from the blast may also have caused large forest fires which would have added smoke and ash to the air. The lack of sunlight would have killed off plants, bringing about the starvation of herbivores (plant-eating animals) and carnivores (animals that eat other animals)—including the dinosaurs in both categories.
It is possible that the reason for the dinosaurs' extinction may have been due to a combination of gradual and catastrophic factors. The popula-tion of dinosaurs may have been gradually declining, for whatever reason, and the impact of a large object from space may have merely delivered the final blow.
The extinction of the dinosaurs has been used to argue that di-nosaurs were somehow inferior to humans, or were evolutionary failures. However, dinosaurs flourished for 150 million years. By comparison, the earliest ancestors of modern-day humans appeared only about 4 million years ago.
Sources: Barnes-Svarney, Patricia. The New York Public Library Science Desk Reference, p. 405; Golob, Richard. Almanac of Science and Technology: What's New and What's Known, pp. 73-84; Michard, Jean-Guy. The Reign of the Dinosaurs, pp. 88-94; Norman, David. Dinosaur! pp. 144-59.