Formation Schistosity is developed at elevated temperature when the rock is more strongly compressed in one direction than in other directions (nonhydrostatic stress). Nonhydrostatic stress is characteristic of regional metamorphism where mountain building is taking place (an orogenic belt). The schistosity develops perpendicular to the direction of greatest compression, also called the shortening direction, as platy minerals are rotated or recrystallized into parallel layers. While platy or elongated minerals are most obviously reoriented, even quartz or calcite may take up preferred orientations. At the microscopic level, schistosity is divided into internal schistosity, in which inclusions within porphyroblasts take a preferred orientation, and external schistosity, which is the orientation of grains in the surrounding medium-grained rock.
formation schistosity is developed at elevated temperature when the rock is more strongly compressed in one direction than in other directions (nonhydrostatic stress) nonhydrostatic stress is characteristic of regional metamorphism where mountain building is taking place (an orogenic belt) the schistosity develops perpendicular to the direction of greatest compression also called the shortening direction as platy minerals are rotated or recrystallized into parallel layers while platy or elongated minerals are most obviously reoriented even quartz or calcite may take up preferred orientations at the microscopic level schistosity is divided into internal schistosity in which inclusions within porphyroblasts take a preferred orientation and external schistosity which is the orientation of grains in the surrounding medium-grained rock