![]() Geologic maps are two dimensional (2D) representations of geologic formations and structures at the Earth’s surface, including formations, faults, folds, inclined strata, and rock types. Table showing relationship between factors operating on rock and the resulting strains: Factor On the other hand, heating materials make them more ductile and less brittle. ![]() Removing heat, or decreasing the temperature, makes materials more rigid and susceptible to brittle deformation. Shale has low strength and granite has high strength. Rock strength measures how easily a rock deforms under stress. For example, applying stress slowly makes it is easier to bend a piece of wood without breaking it. Strain rate measures how quickly a material is deformed. Pore pressure is exerted on the rock by fluids in the open spaces or pores embedded within rock or sediment. The type of deformation a rock undergoes depends on pore pressure, strain rate, rock strength, temperature, stress intensity, time, and confining pressure. Brittle deformation is another critical point of no return, when rock integrity fails and the rock fractures under increasing stress. In the figure, yield point is where the line transitions from elastic deformation to ductile deformation (the end of the dashed line). The point at which elastic deformation is surpassed and strain becomes permanent is called the yield point. For example, if you bend a metal bar too far, it can be permanently bent out of shape. Ductile deformation occurs when enough stress is applied to a material that the changes in its shape are permanent, and the material is no longer able to revert to its original shape. For example, when you stretch a rubber band, it elastically returns to its original shape after you release it. Elastic deformation is strain that is reversible after a stress is released. This change is generally called deformation. When rocks are stressed, the resulting strain can be elastic, ductile, or brittle. Material C undergoes significant plastic deformation before finally brittle failure. Material B only elastically deforms before brittle failure. Material A has relatively little deformation when undergoing large amounts of stress, before undergoing plastic deformation, and finally brittle failure. 9.2 Deformation Different materials deform differently when stress is applied. Table showing types of stress and resulting strain: Type of StressĪssociated Plate Boundary type (see Ch. Shear stress involves transverse forces the strain shows up as opposing blocks or regions of material moving past each other. ![]() Compressional stress involves forces pushing together, and compressional strain shows up as rock folding and thickening. Tensional stress involves forces pulling in opposite directions, which results in strain that stretches and thins rock. There are three types of stress: tensional, compressional, and shear. Strain in rocks can be represented as a change in rock volume and/or rock shape, as well as fracturing the rock. When applied stress is greater than the internal strength of rock, strain results in the form of deformation of the rock caused by the stress. Stress is the force exerted per unit area and strain is the physical change that results in response to that force. Clockwise from top left: tensional stress, compressional stress, and shear stress, and some examples of resulting strain. These forces are called stress, and the physical changes they create are called strain. Forces involved in tectonic processes as well as gravity and igneous pluton emplacement produce strains in rocks that include folds, fractures, and faults. When rock experiences large amounts of shear stress and breaks with rapid, brittle deformation, energy is released in the form of seismic waves, commonly known as an earthquake.
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