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Once the strength of a material is exceeded by an applied stress, it will respond by one of three forms of strain, elastic, plastic or brittle. In elastic strain, the applied energy is absorbed, the material is deformed (deformation is any change in either size and/or shape) and stored as long as the stress is applied. Upon the release of the stress, the material will return to its original size and shape and release the stored energy. In plastic strain, the applied energy is absorbed and consumed permanently deforming the material. In brittle strain, the applied energy is absorbed, the material is deformed and the energy is stored up to the elastic limit after which it breaks, releasing the energy. The question is “Do rocks respond by elastic, plastic and brittle strain?’. The answer is yes in all three cases. Consider the ice sheet that formed during the Ice Age over Hudson Bay. The lithosphere was bowed down several hundred feet without breaking. Elastic or plastic? Following the retreat of the ice 7,500 years ago, seawater inundated basin, forming Hudson Bay. With the weight of the ice removed, the lithosphere under Hudson Bay has been slowly rising and in time, the land will return to its original elevation. Rocks do respond elastically. Do they respond plastically? During an orogeny, rocks are deformed into anticlinal and synclinal folds that are permanent structures. Rocks do respond plastically. Another response of rocks subjected to tensional and compressive forces is that the commonly exceed their elastic limit and break creating fractures along which there is no movement called joints and fractures along which there is movement called faults. .

Keywords

rock stress response, elastic, plastic, brittle

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