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Vineyards, Rocks, and SoilsThe Wine Lover's Guide to Geology$
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Alex Maltman

Print publication date: 2018

Print ISBN-13: 9780190863289

Published to Oxford Scholarship Online: November 2020

DOI: 10.1093/oso/9780190863289.001.0001

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PRINTED FROM OXFORD SCHOLARSHIP ONLINE (oxford.universitypressscholarship.com). (c) Copyright Oxford University Press, 2021. All Rights Reserved. An individual user may print out a PDF of a single chapter of a monograph in OSO for personal use. date: 19 September 2021

The Rocks Change: Shape Folds, Faults, and Joints

The Rocks Change: Shape Folds, Faults, and Joints

(p.112) 7 The Rocks Change: Shape Folds, Faults, and Joints
Vineyards, Rocks, and Soils

Alex Maltman

Oxford University Press

You may have looked at some rocky cliff and noticed sedimentary strata bent into huge curves, the shapes that geologists call folds. You may even have heard of terms like anticline and syncline. Almost certainly you will have heard of geological faults: the San Andreas Fault in California must be one of the best-known geological features there is. They are all examples of what geologists call geological structures. They can affect vineyards, and the names of examples appear around the world on wine labels. So how do these structures come about, and what decides whether rocks make folds or faults? We introduced the concept of tectonic stresses in the previous chapter. We learned that because they act in a particular direction they can induce foliations within metamorphic rocks, but of relevance here is that they can also cause rocks to change their overall shape. That is, the rocks deform, which gives rise to various geological structures. Any solid matter (unlike a liquid) that feels stresses, of whatever origin, will resist them up to a point before it starts to change shape. That point is what defines the strength of the material. The same principles apply when stresses are applied to a sediment or a soil, though rocks, with their constituent minerals firmly bonded together, resist much greater levels of stress before they deform. As one wag put it, the difference between a rock and a soil is that when you kick them a rock hurts your foot . . . So, focusing in on rocks, we see two ways in which they can deform: by flow and by fracture. Looking ahead to where this is going to lead, it’s flow that gives rise to folds, and faults result from fracture. A good analogy for the flow of rocks is glacial ice. The ice is solid to us, but given time, it can flow, to give the “river of ice” that is a glacier. If you leave a ball of silicone putty on a table top, after a few days it will have flowed, while still being a solid, to make a pool.

Keywords:   San Andreas fault, décollement, earthquake, fold mountains, gneissose banding, hillslope, horst, rift valley

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