Break (geology)
A fraction ( English : fracture or rupture ) referred to in the tectonics a plane in the rock, along which a loss of cohesion has occurred and there is no cohesion between the opposing surfaces therefore. The process of breaking itself is also called breaking.
species
The opposite surfaces of a fracture can either touch or have an aperture . Gap (English joint ) is called a fracture has occurred along the little or no movement. If along the fracture a significant shift has taken place, one speaks of a rejection (English: fault ).
Types of fracture propagation
Depending on how the fracture surfaces move relative to one another during the fracture formation, three main types of fracture propagation are distinguished:
- Tensile fracture (English: tensile fracture ) or extension fracture (English: extension fracture ): The resulting fracture surfaces deviate perpendicular to the fracture plane, so they move away from each other. If at least one tensile stress (negative stress) is involved in the break, one speaks of tensile break, in the case of compressive stress (positive stress) of extension break. Rock has an average compressive strength that is ten times greater than its tensile strength.
- Shear fracture type A: The fracture surfaces move relative to each other parallel to the fracture and the relative movement takes place along the fracture plane.
- Shear fracture type B: The fracture surfaces move relative to each other parallel to the fracture and the relative movement takes place perpendicular to the fracture plane.
In nature, tensile stresses arise from the expansion and contraction of rock. In the vicinity of the earth's surface, natural erosion and artificial removal relieve the rock, which is relieved and expanded. Tensile stress can also arise from the drainage of sediments and the associated decrease in volume. Igneous rock loses volume as it cools down. On slopes or cliffs, tensile stress is created by the force of gravity .
Rock can also expand when certain minerals (such as anhydrite ) absorb water . In general, the state of stress in the earth's crust is compressive. There are three main stresses that are perpendicular to each other: The greatest horizontal stress S H and the smallest horizontal stress S h are parallel to the earth's surface. The vertical main voltage S V is oriented vertically . It is zero on the earth's surface and corresponds to the overburden pressure with increasing depth . The two horizontal stresses also increase with increasing depth. In addition to these gravitational stress states, tectonic stresses can occur.
In the upper areas of the earth's crust, S H , S h and S V can correspond to σ 1 , σ 2 and σ 3 in fracture mechanical models, depending on their size . At great depths, the orientations of the main stress directions can change; especially at convergent plate boundaries, main stress directions in the upper and lower plate can be oriented differently.
literature
Claus-Dieter Reuther: Basics of tectonics. Tracking down the forces and tensions of the earth. Springer Spectrum, Berlin / Heidelberg 2012, ISBN 978-3-8274-2065-7 , pp. 12-17.
supporting documents
- ^ Hans Murawski , Wilhelm Meyer : Geological dictionary. 12th edition. Springer, Berlin / Heidelberg 2017, ISBN 978-3-662-54050-3 , p. 23 ( limited preview in Google book search).