Question: How and why do compressional forces cause folding?

Asked by 6fkabrea to Anna, Daniel, Denise, Jesse, Rehemat on 19 Jun 2014.

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  1. Photo: Jesse DavenportJesse Davenport
    answered on 19 Jun 2014:

    Hello 6fkabrea,

    Great question! When compressional forces act on a rock, the rock must fail and deform in a ductile manner, not a brittle manner! This video illustrates really well the difference between ductile and brittle deformation (https://www.youtube.com/watch?v=E99zyNZC7gQ). As the video says for ductile deformation to occur you usually need elevated temperatures and lots of pressure on the rock. So, when you have these conditions you can bend and fold the rock without breaking (brittle) and thus you can get some really beautiful folds in rocks such as these: https://www.google.fr/search?q=pictures+folded+rocks&client=firefox-a&hs=8zG&rls=org.mozilla:en-US:official&channel=sb&tbm=isch&tbo=u&source=univ&sa=X&ei=XcCiU8K-JeXS0QXLooCoBg&ved=0CB8QsAQ&biw=1280&bih=695

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  2. Photo: Rehemat BhatiaRehemat Bhatia
    answered on 19 Jun 2014:

    Hi 6fkabrea,

    That’s a really interesting question and I think Jesse has summed up the answer quite nicely.
    In sedimentary rocks, compressional forces result in the formation of fold sets which are oriented perpendicular to the stress direction.
    One of my favourite folds is this one – its a set of chevron style folds at Millook Haven in Cornwall, South West England. http://static.iwalknorthcornwall.co.uk/images/photos/large/millook_strata.jpg
    They were formed when the Variscan Orogeny took place about 290 million years ago.

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  3. Photo: Denise De GaetanoDenise De Gaetano
    answered on 19 Jun 2014:

    Hi 6fkabrea,

    What you are asking us is about forces that squeeze or push toward one another from opposite directions (cause shortening or flattening).

    A really useful short documentary is the following: http://www.youtube.com/watch?v=ryPHDedwo2I

    Hope this helps! 🙂 You should try it out with some plasticine!

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  4. Photo: Anna RabittiAnna Rabitti
    answered on 19 Jun 2014:

    Wow, amazing pictures the ones linked by Jesse! and indeed I agree with the last suggestion of Denise: try to copy some of those pictures playing with plasticine, you’ll see how strong Nature is to be able to fold rocks in those shapes!

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  5. Photo: Daniel Lao DavilaDaniel Lao Davila
    answered on 19 Jun 2014:

    Compressional forces caused by convergence of plates or by gravitational collapse will form folds in the crust. Usually this folds are formed by movement along thrust faults. These are low-dipping faults that move rocks upward. As rocks move over ramps (changes in the inclination of the fault), they will fold to accommodate the deformation. So folds form because there is not enough space laterally to accommodate the rocks when the rocks are shortened. Folds allow the strain to occur by thickening the crust. At deeper levels where the temperature is high and rocks flow, folds will form by dragging of the layers along the sides of shear zones. Rocks deformed this way are called mylonites and have some of the most interesting textures in deformed rocks.

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