8.2.3: Directed Stress

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8.19.png — Figure 8.19: Directed stress causes deformation

Directed stress, sometimes called differential pressure, is also a force applied to an object, but the force is not the same in all directions. For example, when we squeeze a lemon, we are applying directed stress. When we stretch a rubber band, we are also applying directed stress. The drawing in Figure 8.19 shows greater stress being applied horizontally than vertically, causing compression in one dimension. Within Earth, directed stress is common due to plate tectonic processes that push large pieces of lithosphere together or pull pieces apart. Unlike lithostatic pressure, high levels of directed stresses are not sustained for long because rocks deform to reduce the stress. Directed stress, thus, is commonly associated with rock folding or faulting.

8.20.png — Figure 8.20: Gneiss may be created when directed stress is applied to a granite

Directed stress can cause new minerals to form within a rock, but much more commonly it produces deformation, fracturing, or textural changes only. Mineral grains may rotate, align, become distorted, or disintegrate. Figure 8.20 shows how directed stress can change granite (igneous rock) into gneiss (metamorphic rock).

Directed stress may also cause recrystallization as grains dissolve and regrow in other places, or combine to produce larger crystals. Sometimes, directed stress causes shearing and dynamic metamorphism when different parts of a rock slide past each other.

8.21.png — Figure 8.21: A mylonite, a highly deformed rock from Otrøy in the Western Gneiss Region, Norway

During shearing, mineral grains can become highly stretched in one direction, and fractures can develop that give a rock a planar texture. This figure (8.21) shows a rock called mylonite, a highly deformed kind of rock created when fine sheared material recrystallizes. The sample is from Norway’s Western Gneiss Region. Directed stress, parallel to the layering in this rock, caused feldspar (white) and biotite (black) grains to become elongated as shearing took place. While this was occurring, metamorphism produced wine-red garnet crystals – a single large one is near the left side of the photo and many small ones are scattered throughout. The 1-euro coin is 2.3 cm across, for scale.