Rocks are under
stress when they are subjected to a force at depth. When the rocks are exposed at the surface after uplift and erosion, the effects of the stress can be studied. Stressed rocks show varying degrees of
strain—the change in the volume and/or shape of the rock because of that stress. For example, a volcanic agglomerate may be compacted and its pyroclastic fragments stretched (strained) in response to a tectonic stress, such as compression.
Stresses. Three kinds of stress can be applied to rocks: tensional, compressive, and shear. Tensional stress occurs when a rock is subjected to forces that tend to elongate it or pull it apart; a rock that has experienced tensional stress tends to be narrower and longer than its original shape, like a piece of gum or taffy that has been pulled. A compressive stress on a rock is applied from opposite sides and has a tendency to shorten (compress) the rock between the opposing stresses, which may also stretch it parallel to the stress‐free direction. A shear stress results when forces from opposite directions create a shear plane in an area in which the forces run parallel to one another. The scale of shear stress can vary from a few centimeters to hundreds of meters.
Strains. When subjected to stress, a rock can undergo one of three kinds of deformation (strain): elastic, brittle, or plastic. Deformation is called elastic strain if the body of rock returns to its previous shape after the stress has been removed. A good example is the slow rebound of the North American crust after having been downwarped by the great weight of the Pleistocene glaciers. Brittle strain occurs when the stress is great enough to break (fracture) the rock. Plastic strain results in a permanent change in the shape of the rock. A ductile rock is one that “flows plastically” in response to stress. Whether the strain is plastic or brittle depends on both the magnitude of the stress and how quickly the stress is applied. A great stress that is slowly applied often folds rocks into tight, convoluted patterns without breaking them.