All About Impact Craters

An impact crater is a depression in the ground which is relatively circular. It is caused by some object hitting the earth at a high speed. Impact craters are found on every planet and moon in the solar system. Craters are different from volcanoes. Craters have raised rims around the circular edge. Their floors are lower than the land or ground around them. Volcanoes are caused by an eruption or earthquake and rise above the ground rather than make the ground lower.

Impact craters are more common on certain planets and moons than others. The planets Earth, Venus and Mars, and the moons, Europa and Titan, have fewer craters. More geological activities occur on these, and the existing craters become covered over in time.

On Earth, about 190 impact craters have been discovered. Some are recent and some quite old. They range greatly in size and are found in the interior regions of continents. Very few underwater craters have been found due to the problem of examining the depths of the sea. Also, the floor of the sea bottom changes continually.

In the late 1800's, Daniel Barringer was one of the first scientists to discover a crater in the United States, the Meteor Crater in Arizona. It was renamed the Barringer Crater. Others still did not accept the theory of the causes of an impact crater. They thought they were the result of volcanoes. Walter Bucher, in the 1920's, studied several other craters but still believed they were the result of some explosion. In 1936, two other scientists went back and studied Bucher's work and agreed that they were probably the result of a very powerful impact. It was not until the 1960's that scientists had clear evidence of the cause of the craters. Carlyle Beals of Canada and Wolf von Engelhart of Germany made a detailed study of fifty craters around the world by 1970.

The high speed of the impact upon the body causes shock waves in that body. Both parts are decompressed. This sets in motion an explosion, which then produces an impact crater. Because the craters are caused by explosions, their shapes are always circular. The damage caused by the shock wave produces a higher temperature in both bodies. Usually, the change is enough to melt the impactor.

After the impact, the target is pushed downwards, then outwards and upwards. The flow of material produces a cavity in the shape of a semi-circle. Eventually, the flow creates a bowl-shaped crater where the center has been pushed down. A crater rim gets pushed up. After this cavity is finished expanding, it is called a transient cavity. The depth of this cavity is about one-quarter to one-third of its diameter. Usually, the transient cavity collapses due to the force of gravity.

Specific shock-metamorphic effects on rocks distinguish an impact crater. These include shattered rocks, crystal rocks, and crystal deformations. Sometimes these are deeply buried and hard to identify in simple craters. The center of a complex crater can reveal such evidence.

On Earth, impact craters have produced valuable minerals. Some of these minerals were formed due to the impact. Some were brought to the surface by the impact itself. Iron, uranium, gold, copper, and nickel are some minerals produced from effects of an impact event. In North America each year, it is thought that five million dollars' worth of minerals is mined due to impact events. The largest goldfield in the world is called the Vredeford Dome. It has supplied forty percent of all the gold ever mined in the world. The gold field was a result of an impact event.

A: 103
B: 190
C: 570
D: 247

A: Impact craters are not found on moons.
B: Impact craters are found on most moons and planets.
C: The shape of an impact crater is usually irregular.
D: A volcano is an impact crater.

A: The force of the impact usually melts what it hits.
B: A transient cavity is the inside of a volcano.
C: All impact craters are about the same size.
D: Many impact craters are found under the sea.

A: Bucher
B: Von Engelhart
C: Barringer
D: Beals

A: Vredeford Crater
B: Meteor Crater
C: Bucher Crater
D: Beals Crater

A: Total depression
B: Complete crater
C: Transient cavity
D: Depression cavity

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