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7.4. Rock deformation

Types of rock deformation

The rocks of the Earth 's crust are subjected to tensile compression of detente and shear. Although we cannot see how they are deformed, we can appreciate, in the rocks on the surface, how have been the efforts that caused their deformation and thus reconstruct and know the tectonic activity in the area.

Depending on the composition and physicochemical nature of the rocks, and the pressure and temperature conditions, rocks can react in three different ways to tectonic stresses:

  • Elastic deformation. The material deforms when a stress is applied, but when the stress ceases, it returns to its original shape. It is a reversible deformation , like that of a rubber band, for example, that recovers its shape after exertion.
  • Plastic deformation. They are irreversible deformations that remain after making the effort. It would be the cause of the folds that remain in the strata after being subjected to stress. An example of plastic deformation would be the behavior of plasticine that maintains its deformation after applying a stress to it.
  • Brittle deformation. When stress is applied, the material fractures. It is an irreversible deformation that occurs in rigid materials when the stress exceeds the deformation capacity of the material. The failures  are caused by fragile behavior of rocks. An example of brittle deformation is glass that breaks when stressed.

Deformación según el esfuerzo

By Moondoggy ([1]) [GFDL or CC-BY-SA-3.0], via Wikimedia Commons

When deformations affect terrestrial materials, characteristic geological structures originate:

  • FoldsThey are plastic deformations of the rocks when subjected to compressive stresses.
  • Fallas and jointsThey are brittle deformationsproduced when the stresses exceed the deformation capacity of the rocks. If there is displacement of the fragments, it is a fault, if there is no displacement, it is a joint.
  • Elastic deformation, as the affected materials regain their original shape, cannot be seen in geological structures. For example, it is common in earthquakes.