The very bottom of the Grand CanyonThis rock sits in a very cool place; this is the single oldest ro

The very bottom of the Grand CanyonThis rock sits in a very cool place; this is the single oldest rock unit exposed in the Grand Canyon. This picture shows the Elves Chasm gneiss, a rock unit only exposed in a small, ~10 kilometer window deep in the canyon’s inner gorge. This rock is considered part of the larger Vishnu Schist formation that makes up the deepest part of the canyon.This rock unit formed around 1,840 million years ago: 1.84 billion years. That age makes it at least 80 million years older than any other rocks exposed in the Vishnu Schist.It is a highly metamorphosed rock – the term “gneiss” suggests a metamorphic rock that was exposed to temperatures and pressures long enough to grow large, new minerals.The Elves Chasm rocks are classified based on their chemistry as a “tonalite”. This name means that they likely used to be igneous rocks, similar to granites but without as much potassium. These types of rocks often are found as large bodies within the crust; in fact, in some areas they make up the crust itself. It has been suggested based on the chemistry and by how the other rocks nearby fit together that this rock was part of the original crust in this area and that all of the other rocks found in the canyon area either intruded through or were deposited on top of this deepest rock.-JBBImage credit: NPShttp://www.nature.nps.gov/geology/parks/grca/age/17-Karpilo_GRCA_Elves_Chasm_Gneiss-004WEB.jpgRead more:http://rruff.info/doclib/MinMag/Volume_42/42-322-199.pdfhttp://www.agu.org/books/ft/v115_315/FT115p0090/FT115p0090.pdfhttp://www.gcrg.org/docs/gtslib/karlstrom-06.pdfhttp://www.nature.nps.gov/geology/parks/grca/age/ -- source link

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