GeoTrivia: Augen – The Eyes Have itIf you’ve ever been looking at a metamorphic rock in the field an
GeoTrivia: Augen – The Eyes Have itIf you’ve ever been looking at a metamorphic rock in the field and have the distinct impression that it is staring back at you, it’s most likely that you’ve found augen gneiss. “Augen,” the German word for “Eyes,” refers to a kind of rock deformation structure that is, not surprisingly, eye-shaped. It is formed when a more competent clot of denser minerals gets stuck within a band of flowing minerals (yes, this happens in temperature-pressure conditions high enough to make rocks flow). When augen are found in a rock, it leaves no doubt as to the rheological environment in which the rock formed. It also leaves no doubt about which way the rock around it was flowing: just like it looks, the rock was flowing around the large part of the eye and towards the streamlined end. Most common to gneiss, augen are also found in other rock types that are deformed in sufficiently awful conditions such as, in this photo, amphibolite. This particular “eye” was found staring at us from a zone where a large piece of oceanic lithosphere was mushing up and over an old continental margin at relatively high temperature ~900C. Note: Due to the rheological conditions of flowing rocks, there will only be right eyes or only left eyes, but no pairs of eyes, so never any crossed eyes. If augen wink at you, you’ve been out in the field too long.Annie R.Photo: Amphibolite sole, Liagouna Pindos.If you really need references, start here:http://www.maden.hacettepe.edu.tr/dmmrt/dmmrt66.htmlhttp://mrdata.usgs.gov/geology/state/sgmc-lith.php?text=augen+gneisshttp://www.tekphys.geo.uni-mainz.de/publications_PDF/12-PasschierSimpson86.pdf -- source link
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