why did dinosaurs become extinct?

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.