Discovery Science: Earth – Fracture Tectonics
Due to the movement of its plates, the Earth’s crust is always subject to pressure and shear forces. When the tension in a mass of rock becomes so great that it exceeds the rock’s resistance, breaks occur at the weak points.
Fractures in a rock mass that are only a few inches to a yard long are called crevices. Longer breaks are called divergent boundaries. When a fracture occurs, cold, brittle rock masses in the Earth’s crust may be compressed with lateral pressure, for example, if crustal movement causes two such sections to collide. The result is that one section is pushed upward. In this way a ridge block, or massif, is lifted vertically along the fault line. Fractures can also occur if the rock expands instead of being compressed.
If the angle at which the rock section is lifted is less than 45°, the occurrence is called a thrust. The term subsidence refers to the sinking of a massif. A rift, or graben, is formed if rock breaks along two parallel boundaries and the crustal block caves in, leaving behind the steep flanks formed by the fracture. Generally, these flanks break down further into so-called staggered fractures. If rift formation occurs on a large scale (for example, if sections of the crust are drifting apart on diverging plates), elongated rifts such as the Upper Rhine Valley are formed.
The opposite occurs if two crustal regions are moving closer together. Fractured massifs are then lifted to become horsts, or raised blocks. The Black Forest and the Front Range of the Rocky Mountains in Colorado were formed by such a process. Mountain ranges created by fracture tectonics are called fault-block mountains. In addition to the forces of compression and expansion, shearing forces also impact rock.
These forces are the result of sections of crusts moving past one another. Unlike the effects of thrust or subsidence, no difference in elevation occurs at these boundaries; there is, on the other hand, a lateral displacement
BASICS
FOLDS are created in the same way as fractures. They are generated by the lateral constriction of sedimentary strata, which results in a sequence of anticlines and troughs.