Exploring Rocks: Deformation

Earth's outer layer is a thin, rigid crust that is broken into a dozen large plates. The plates move, converging or sliding past one another, at the rate your fingernails grow. Mountains form where plates collide. Rock bodies in the crust change their shape, or deform, to accommodate plate motion commonly resulting in folds and faults. The photos illustrate how deformation in a sandstone increases toward a major fault


This sandstone is composed of sub-rounded quartz and feldspar grains. The slight horizontal alignment of grains is evidence that the sandstone is weakly deformed.


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Sandstone, JSmtland County, Sweden
Crossed polarizers with gypsum plate

As deformation increases, the quartz grains are stretched out parallel to the direction of flow in the rock.


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Protomylonite, JSmtland County, Sweden
Crossed polarizers with gypsum plate

Continued deformation leads to extreme elongation of the quartz. Some of the quartz ribbons have stretched so that their length is now over 100 times their width. Blocky rafts of feldspar float in this river of quartz because feldspar is harder to deform than quartz.


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Augen mylonite, JSmtland County, Sweden
Crossed polarizers with gypsum plate

Other nifty examples of deformation include: Crenulations, Folds and Folded Phlogopite.

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