KinkyThese rocks have had an interesting life. This photo shows layers of chert from the Franciscan

KinkyThese rocks have had an interesting life. This photo shows layers of chert from the Franciscan complex in California, and there are a few blades of grass growing in them to give some scale.These rocks have been tightly folded into kinks, in a pattern known as Chevron folding. These alternating V-shaped folds tend to form in layered sedimentary rocks where there is a difference in strength between the two layers; the layers slide past each other until and the weaker layer flows until the structure locks, producing a very tight fold hinge.Layered cherts like this are a good rock to produce this structure in. Chert is a silica-rich rock, typically produced within the ocean by chemical precipitation. Silica-rich layers are strong and hard to bend, and here they are inter-mixed with weaker layers of fine-grained sediment.The layers of sediments were originally deposited in the Pacific Ocean. The Franciscan complex in California is a highly folded and faulted mass of sediments left over from when the Farallon plate was subducting beneath North America. When an oceanic plate subducts, the sediments on top of it are often scraped off, as though the continent was sliding a spatula over the oceanic plate. Those sediments will then be compressed, folded, and faulted, just as happened to these rocks of the Franciscan.-JBBImage credit: Copyright © Michael Collier, shared for non-commercial purposes through http://www.earthscienceworld.org/ -- source link

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