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Impact Craters

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An impact crater is a site where a meteorite, comet or asteroid has crashed into a planet (i.e. Earth, Mars) or satellite (i.e. Moon). We will be concerned specifically with terrestrial or impact craters on Earth.

For a good overview into this topic let's visit Terrestrial Impact Craters , part of the Lunar and Planetary Institute's web site. Here we find that there are presently about 120 identified impact craters on earth. It is felt that in all probability there are many more but due to erosion, volcanic and tectonic activity, traces of them have been erased. So, how do you identify an impact crater? According to the LPI site, certain shock metamorphism structures have been associated with impact sites:

* an abundance of siderophile elements

* shatter cones

* shocked quartz

* diaplectic glass

* high-pressure mineral phases such as stishovite

Extremely high pressure produces shock effects. (Let's slip over to MIAC for a minute. Here you will find a picture and an explanation of how shatter cones are formed. The site states that the presence of shatter cones is sure evidence of an impact since the only other way that they can be formed is by nuclear explosion. Next, there is a picture of the feldspar mineral plagioclase , part of which has been changed to diaplectic glass, which happens at high pressure. There is also a picture of shocked quartz. Quartz does not show cleavage planes ordinarily but under high pressure can form parallel pseudo-cleavage planes, another indication that an impact has occurred.) Stishovite or silicon dioxide (SiO2) forms at very high pressures but not necessary high temperatures, the exact conditions that would exist at an impact site. Siderophile (having an affinity for metallic iron or iron-like ) elements (iron, cobalt, nickel, gold and palladium) are found most abundantly in the earth's core. (Take a peek at Observed relative abundances of elements for a brief discussion of how elements separate out in a gravitational field.) It is therefore theorized that a high concentration of siderophile elements at the earth's surface is probably due to an extra-terrestrial source, such as a meteorite.

Now that we know how to identify an impact site, what does an impact crater look like? Let's return to Terrestrial Impact Craters. Two types of crater have been identified, simple craters and complex craters. Complex craters are characterized by a central peak, which forms when the crater floor rebounds from the initial impact. Crater Formation shows a possible scenario for the formation of a complex crater.

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