Dating terrestrial impact structures

Posted by / 15-Jul-2017 00:01

Dating terrestrial impact structures

Sometimes the impact site reveals an abundance of siderophile elements such as iridium, osmium, platinum, and palladium.

Other tell-tale signs of an impact are products of shock metamorphism such as shatter cones, planar features in quartz and feldspar, diaplectic glass, and a high-pressure form of quartz called stishovite.

Stishovite has never been found anywhere else but impact sites. Simple craters are relatively small with a smooth bowl shape, and exhibit a depth-to-diameter ratio of about one-to-six.

Meteor Crater in Arizona is a good example of a simple impact crater.

While shatter cones are easily visible to the naked eye, other effects of shock can be observed only through microscopic and X-ray studies, such as shock deformation lamellae in quartz (Carter 1965; Engelhardt and Bertsch 1969).

At such speeds, the projecticle explodes on impact and carves out a round bowl-shaped depression on the surface. How can you distinguish an impact crater from a volcanic crater?

Volcano craters are above the surrounding area on mountaintops while the craters from impacts are below the surrounding area with raised rims.

These shock metamorphic effects are characteristic of impact and do not occur in natural materials formed by any other process.

In addition, geochemical methods are used to find traces of the meteoritic projectile in impact melt rocks and glasses.

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The most obvious criterion is the presence of shatter cones, conical fractures with diverging striations along the length of the cone.

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