Rock and rollWhen I look at a rock like this I find that I almost can feel the motion within. It’s l

Rock and rollWhen I look at a rock like this I find that I almost can feel the motion within. It’s like a snapshot, an action photograph that catches exactly what was happening to this rock. Can you feel the movement, the rolling of this grain?This is a classic texture found in metamorphic rocks – this one from the Ailao-shan Red River shear zone on the edge of Tibet. As the Himalayan Mountains grew, the rocks of eastern Asia were out of the way, forming large strike-slip faults. 20 million years ago these rocks were buried deep within one of those strike-slip faults where they were warm and ductile.The rocks above were faulting and sliding past each other while the rocks at the base sort of oozed past each other. But, not every mineral behaves the same way under metamorphic conditions. Some minerals, like quartz or calcite, become weak and able to flow at temperatures hundreds of degrees Celsius lower than other minerals.The pale layers in this rock are carbonate rocks, the remnant of limestones deposited along the edge of the Tethys seaway. They were buried tens of kilometers deep as the Himalayas grew and warmed by the heat of the earth until they were able to flow easily. The stress of associated with the fault far above stretched those layers, pulling them like taffy. But, within those layers also sat stronger minerals, like the black amphibole.That amphibole grain did not get hot enough to become ductile, instead it remained rigid and the calcite flowed around it. As the calcite flowed, the amphibole grain rolled and occasionally fractured. Pieces of it were pulled off and moved away, like tiny little faults within the rock. When you find a grain like this out in the field, it’s not only spectacular to see and worthy of sitting on a shelf; it also tells exactly how the rocks were moving. Can you figure out which way the rocks were moving, were the rocks on top moving to the right or to the left?Finding a rock like this preserved from an ancient shear zone gives geologists the ability to interpret the motion on faults that have been inactive for millions of years by answering the same question you just did.-JBBImage credit: Credit: Philippe Leloup via http://imaggeo.egu.eu/view/2362/Read more:http://www.sciencedirect.com/science/article/pii/S1342937X10001863https://books.google.com/books?isbn=3642684327http://www.science.marshall.edu/elshazly/Igmet/texture.doc -- source link

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