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Table of Contents Enchanted Learning
All About Astronomy
Site Index
Our Solar System Stars Glossary Printables, Worksheets, and Activities
The Sun The Planets The Moon Asteroids Kuiper Belt Comets Meteors Astronomers

ASTEROIDS
Introduction Near-Earth Asteroids
(NEA)
Asteroids and Dinosaurs List of Some Asteroids Composition Activities,
Web Links

Dinosaurs and Asteroids
ASTEROIDS HITTING THE EARTH (METEORITES)

There are some asteroids that have orbits outside the asteroid belt. Some have orbits that cross the orbit of the Earth, and some have hit the Earth. A large asteroid could survive the trip trough Earth's atmosphere (called a meteor during its trip through the atmosphere) and form an impact crater.

An asteroid impact with the Earth may have caused the extinction of the dinosaurs. The Alvarez Asteroid Theory explains the huge K-T mass extinction 65 million years ago by a large asteroid hitting the Earth off the Mexican Yucatan peninsula. This impact would have caused severe climactic changes leading to the demise of many groups of organisms, including non-avian dinosaurs.

THE K-T MASS EXTINCTION
About 65 million years ago, at the end of the Cretaceous period, a large fraction of plant and animal families suddenly went extinct. In this Cretaceous-Tertiary or K-T mass extinction (K is for Kreide, meaning chalk in German, which describes the chalky sediment layer from that time; T is for Tertiary, the next geologic period), all land animals over about 55 pounds went extinct, as did many smaller organisms.

The K-T mass extinction obliterated the dinosaurs , pterosaurs, plesiosaurs, mosasaurs, ammonites, some families of birds and marsupial mammals, over half the plankton groups, many families of teleost (bony) fishes, bivalves, snails, sponges, sea urchins and others.

This catastrophe eventually led to the Age of Mammals.

THE ALVAREZ ASTEROID IMPACT THEORY
There are a lot of theories about why the K-T (Cretaceous-Tertiary) extinction occurred, but a widely accepted theory (proposed in 1980 by physicist Luis Alvarez and his son Walter Alvarez, a geologist), is that an asteroid 4-9 miles (6-15 km) in diameter hit the Earth about 65 million years ago. The impact would have penetrated the Earth's crust, scattering dust and debris into the atmosphere, and causing huge fires, volcanic activity , tsunamis, and severe storms with high windsand highly acidic rain . The impact could have caused chemical changes in the Earth's atmosphere, increasing concentrations of sulfuric acid, nitric acid, and fluoride compounds. The heat from the impact's blast wave would have incinerated all the life forms in its path.

The dust and debris thrust into the atmosphere would have blocked most of the sunlight for months, and lowered the temperature globally.

Those organisms that could not adapt to the temperature and light changes would die out. Since plants' energy is derived from the sun, they would likely be the first to be affected by changes in climate. Many families of phytoplankton and plants would die out, and the Earth's oxygen levels may well have dramatically decreased, both on land and in the oceans, suffocating those organisms which were unable to cope with the lower oxygen levels.

Major changes in the food chain would result from all of these these environmental upheavals. The herbivores (plant eaters) who ate those plants would starve soon after the plants died. Then, at the top of the food chain, the carnivores (meat eaters), having lost their prey, would have to eat each other, and eventually die out. Their large carcasses must have provided smaller animals with food for quite a while.

LOCATION OF THE IMPACT CRATER
There are many impact craters on Earth. A 120-mile-wide (180 km), 1-mile-deep (1600 m) impact crater, Chicxulub, is found at the tip of the Yucatán Peninsula, in the Gulf of Mexico. This crater dates back to 65 million years ago, and is probably the site of the K-T meteorite impact. Evidence of K-T period tsunamis have been found all around the Gulf of Mexico.

CHEMICAL EVIDENCE FOR THE THEORY
In the clay layer from the Cretaceous-Tertiary (K-T) boundary, scientists have found chemical evidence that supports the Alvarez impact theory. The K-T layer consists of the sedimentary deposits that accrued from the end of the Cretaceous period to the beginning of the Tertiary period. It is divided into two layers, the Magic Layer (3 mm thick) and the Ejecta Layer (2 cm thick).





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