Meteorite Monday: Impactites

A tektite and shatter cone from the Cascadia Meteorite Lab display case

My inspiration for this post came while I was working on the meteorite case for the lab. Initially we were going to put a relatively cheap meteorite in the display, but then we decided that wasn’t such a good idea. So, the next best idea was the throw in a few impactites. These are rocks that had the rather unfortunate (or fortunate depending on how you wanna look at it) luck to be in the zone of a rather large meteor impact. The extreme heat and pressure from these events really plays hell on the surrounding rocks.

An Australite tektite from the Australasian strewnfield (image from Wikipedia)

Impactites come in two flavors: tektites and shatter cones. Tektites are rocks that instantly melted and were flung into the air from the meteor impact. While airborne, the molten rock quickly cooled and took on an aerodynamic shape. Such shapes include tear drops, dumbells or even that of an inverted button found among Australites. Tektites are found only at four impact craters across the planet. As such, they can almost be as expensive as the meteorites themselves. The origin of tektites has been hotly debated in the past. In the 1950’s and 1960’s, researchers suggested that tektites were actually of lunar origin. However, tektites lack certain noble gases that are common to lunar meteorites (1).

Shatter cones are formed in the bedrock underneath the impact zone. They form at a relatively low pressure of less than 2 gigapascals (GPa). To put that into perspective, 2 gigapascal translates into approximately 290,000 psi. A shatter cone outcrops is a good sign that one is standing in an impact crater (2).

Shatter cone from the Wells Creek in Tennessee (Image from Wikipedia)

 

Sources:

1. Schneider, D.M., Tektites. The Meteoritical Society. Accessed on 10/17/2011

2. French, B.M. 1998. Traces of Catastrophe. Lunar and Planetary Institute. Retrieved on 10/17/2011