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Massive Meteorites in Geological History
| Article
# : |
17076 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
8 / 1990 |
3,286 Words |
| Author
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David Alt, James W. Sears, Donald W. Hyndman
David Alt, James W. Sears, Donald W. Hyndman are geologists at
the University of Montana at Missoula. |
A look at pictures of the moon reveals a surface pocked with thousands of impact craters that are just open holes, while a few of the larger craters have walls that rise steeply above flat floors covered with dark lava flows. In the midst of this saturated jumble of craters are smooth regions, the maria, which are actually the sites of the largest craters. These craters have been completely filled with lave flows, which in places spill beyond their edges into nearby low areas. Those are the dark patches that can be seen without the help of a telescope. Some of the Apollo missions landed on the maria floors, and their astronauts collected samples of the lava, called basalt. (Basalt is a type of lava that crystallizes to from coal black rock).
Craters are the dominant feature on the surface of the moon, because cratering is virtually the only process that operates there. In contrast, craters are only minor feature in landscapes of the earth because many complexly interactive geologic processes operate on the surface of the earth. Even so, geologists have now recognized dozens of impact craters on the earth, and the list grows.
The Manicouagan Crater of Quebec (diameter about 40 miles) and the Popigai Crater of Siberia (diameter about 60 miles) are the largest now generally recognized. Both have volcanic rocks in their floors.
A comparison of known impact craters on the earth and on the moon reveals a surprising anomaly. While the moon contains several craters with a diameter of more than about 120 miles, the earth contains none with a diameter of more than about 60 miles. Furthermore, while the moon contains several extensive maria, no maria have previously been identified on the earth.
If unrecognized maria exist on the earth, they may mask sites where massive meteorites have struck the earth.
Asteroids and Comets
Astronomers have so far found more than 80 asteroids with orbits that cross the earth's orbit, and the rate of discovery suggests that 10 times that many may finally be found. In addition, an unknown number of comets, probably hundreds, cross the earth's orbit. Anything in an orbit that crosses the earth's can collide with the earth.
The chance that an asteroid or comet will hit the earth this year is probably about the same as the
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