Wednesday, November 28, 2007

Deskcrop #3 - Ventifacts

Today's deskcrop is a ventifact. A ventifact is a rock that has been abraded by the windblown particles: sandblasted if you will. They typically have a heavily grooved or polished surface. Large ventifacts can have the grooves aligned with the prevailing wind direction. Small ventifacts, like mine, appear to not remain stationary, and therefore can develop a "brainy" texture. Ventifacts are not all that common, reflecting the relatively small importance of wind erosion. Water (both solid and liquid) is far and away the most powerful erosive agent around; wind is much less significant. In order to see well developed ventifacts, it helps to look in places where the wind is absurdly strong, there are abundant loose and abrasive particles (i.e. sand), erosion from water is relatively minor, and you don't have to worry about plants or soil covering things up. My samples are from Ventifact Ridge, in Death Valley National Park. (legal note, these were collected by someone who had the proper permit to take a few chunks). The rock itself is a basalt, Pliocene in age I believe (but cannot confirm right now).

Besides general interest, ventifacts and other eolian-erosion related features are the best earth analogs we have for many of the images sent back from the different martian landers. Below are images taken by the Viking and Pathfinder landers thought to represent Martian ventifacts (from Greeley et al., 2002.) Greeley et al., is an interesting paper that compares images sent back from the Martian landers to eolian features in the Mojave Desert and in Iceland.

In addition to dry environments, ventifacts need time to form. I haven't found any references for the average amounts of time needed to create well-developed ventifacts, but I imagine it is strongly dependent upon wind speed, lithology, and the type of particulate matter being thrown through the air. References would of course be appreciated.

Greeley, R., Bridges, N.T., Kuzmin, R.O, and Laity, J.E., 2002, Terrestrial analogs to wind-related features at the Viking and Pathfinder landing sites on Mars. Journal of Geophysical Research, v. 107, n. E1, 10.1029/2000JE001481.

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