Geological Strain

Close Up Of Rope
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"Strain" is a word widely used in geology, and it's an important concept. In everyday language, strain seems to signify tightness and tension, or effort expended against unyielding resistance. This is easy to confuse with stress, and indeed the dictionary definitions of the two words overlap. Physicists and geologists try to use the two terms more carefully. Stress is a force that affects an object, and strain is how the object responds to it.​

Various common forces that operate on Earth impose stress on geologic materials. Gravity does, and currents of water or air do, and the tectonic movements of the lithospheric plates do. The stress of gravity is called pressure. The stress of currents is called traction. Fortunately, tectonic stress is not called by another name. Stress is simple to express in calculations.

Deformation from Stress

Strain is not a force, but a deformation. Everything in the world—everything in the universe—deforms when subjected to stress, from the vaguest cloud of gas to the most rigid diamond. This is easy to appreciate with soft substances, where its change in shape is obvious. But even solid rock changes its shape when stressed; we just have to measure carefully to detect the strain.

Strain comes in two varieties. Elastic strain is the strain that we sense in our own bodies—it is stretching that bounces back when the stress is reduced.

Elastic strain is easy to appreciate in rubber or metal springs. Elastic strain is what makes balls bounce and the strings of musical instruments vibrate. Objects that undergo elastic strain are not harmed by it. In geology, elastic strain is responsible for the behavior of seismic waves in rock. Materials that are subjected to enough stress may deform beyond their elastic capacity, in which case they may rupture, or they may stretch which is the other kind of strain: plastic strain.

Plastic strain is deformation that is permanent. Bodies do not recover from plastic strain. This is the kind of strain we associate with substances like modeling clay, or bent metal. In geology, plastic strain is what results in landslides in sediment, particularly slumps and earthflows. Plastic strain is what makes metamorphic rocks so interesting. The alignment of recrystallized minerals—the metamorphic fabric of schist, for instance—is a plastic response to the stresses imposed by burial and tectonic activity.