Joints are classified by their geometry or by the processes that formed them. The presence of the first set strongly affects the stress orientation in the rock layer, often causing subsequent sets to form at a high angle, often 90°, to the first set.
Continued deformation may lead to development of one or more additional joint sets. This leads to the development of a single sub-parallel joint set. When this happens the rock fractures in a plane parallel to the maximum principal stress and perpendicular to the minimum principal stress (the direction in which the rock is being stretched). When tensional stresses stretch a body or layer of rock such that its tensile strength is exceeded, it breaks. This stress may be imposed from outside for example, by the stretching of layers, the rise of pore fluid pressure, or shrinkage caused by the cooling or desiccation of a rock body or layer whose outside boundaries remained fixed. Joints arise from brittle fracture of a rock or layer due to tensile stress. Joints infilled by precipitated minerals are called veins and joints filled by solidified magma are called dikes. Joints may be open fractures or filled by various materials. The most prominent joints occur in the most well-consolidated, lithified, and highly competent rocks, such as sandstone, limestone, quartzite, and granite. Often, the specific origin of the stresses that created certain joints and associated joint sets can be quite ambiguous, unclear, and sometimes controversial. They vary greatly in appearance, dimensions, and arrangement, and occur in quite different tectonic environments. Joints are among the most universal geologic structures, found in almost every exposure of rock. Thus a joint may be created by either strict movement of a rock layer or body perpendicular to the fracture or by varying degrees of lateral displacement parallel to the surface (plane) of the fracture that remains “invisible” at the scale of observation. Faults differ from joints in that they exhibit visible or measurable lateral movement between the opposite surfaces of the fracture ("Mode 2" and "Mode 3" Fractures). The distinction between joints and faults hinges on the terms visible or measurable, a difference that depends on the scale of observation. A joint system consists of two or more intersecting joint sets. A joint set is a family of parallel, evenly spaced joints that can be identified through mapping and analysis of their orientations, spacing, and physical properties. Although joints can occur singly, they most frequently appear as joint sets and systems. Joint spacing in mechanically stronger limestone beds shows increase with bed thickness, Lilstock Bay, SomersetĪ joint is a break ( fracture) of natural origin in a layer or body of rock that lacks visible or measurable movement parallel to the surface (plane) of the fracture ("Mode 1" Fracture).
0 kommentar(er)
