Thursday, January 01, 2009

Popigai crater

The Popigai crater in Siberia, Russia is tied with Manicouagan Reservoir as the 4th largest verified impact crater on Earth. A large bolide impact created the 100-kilometer diameter crater 35.7 ± 0.2 (2σ) million years ago during the late Eocene (Priabonian stage). The crater is just north of the Siberian city Norilsk, or 1 1/2 hours (by helicopter) from the outpost of Khatanga. It is designated by UNESCO as a Geopark, a site of special geological heritage.

For decades the Popigai crater has fascinated paleontologists and geologists, but the entire area was completely off limits because of the diamonds and the mines constructed by gulag prisoners under Stalin; however, a major investigatory expedition was undertaken in 1997 (IPEX 1997) which greatly advanced understanding of the enigmatic structure. The impactor in this event has been identified as either an eight-kilometer diameter chondrite asteroid, or a five-kilometer diameter stony asteroid.

The shock pressures from the impact instantaneously transformed graphite in the ground into diamonds within a 13.6 kilometer radius of the impact point. Diamonds are usually 0.5 to 2 millimeters in diameter; a few exceptional specimens are 10 millimeters in size. The diamonds not only inherit the tabular shape of the original graphite grains but they additionally preserve the original crystal's delicate striations.

Popigai is the best example yet of the formation of a crater of this type. Three other craters are larger, but they are either buried (Chicxulub), strongly deformed (Sudbury), or deformed and severely eroded (Vredefort).

There is a small possibility that Popigai impact crater formed simultaneous with the c. 35 million year old Chesapeake Bay and Toms Canyon impact craters.

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