Strike definition geology10/21/2023 ![]() ![]() Joints are perhaps the most common structural features of all types of rocks occurring everywhere in the world. In the core regions of folds where compressive forces are dominant, joints may be related to the compressive forces. Rocks may be compressed to crushing and numerous joints may result due to the compressive forces in this case. In folded rocks, these are located in axial regions. These are commonly observed in the vicinity of fault planes and shear zones where the relationship with shearing forces is clearly established (Fig. Joints produced in many rocks during the weathering of overlying strata and subsequent release of stresses by expansion is also thought to be due to the tensile forces (Fig. They are also produced in igneous rocks during their cooling. The most common location of such joints in folded sequence is on the outer margins of crests and troughs. Tension joints are those, which have developed due to the tensile forces acting on the rocks. In such cases, joints are classified into one of the following genetic types: Only in a few cases, the predominant force (compression or tension or shear) that has been responsible for the development of joints in the rocks can be established easily. As regards their origin it is often very difficult to attribute a particular type or group or system of joints to an exact cause of origin. Joints are very common and at the same time very complex structures in rocks. In these rocks all the joint systems traversing at any other angular inclination with the linear structures are described as diagonal joints (Fig. Longitudinal or S joints, are joints traversing parallel to the linear structure. Two terms are commonly used in such cases:Ĭross or Q joints, which are joints traversing the linear structures at right angles. In igneous and metamorphic rocks, the joints may be classified on the basis of their geometric relations with planar structures of those rocks such as lineation or cleavage etc. In these areas, joints can also be distinguished into dip joints, strike joints and diagonal joints. Similarly, joints running parallel to the layers are designated as C-joints. These cut the layers almost perpendicularly. The joints running parallel to b-axis are called radial joints. A line running parallel to the hinge-line is assumed as b-axis the a-axis is normal to it and the c-axis is normal to the plane containing the a and b axis. ![]() In the folded regions, joint orientation is conveniently described with reference to the hinge of the fold. These are simply referred as bedding joints. In stratified rocks, some joints may develop essentially parallel to the bedding planes. These are also called diagonal joints when they occur midway between the dip and strike of the layers. Oblique joints are those joints where the strike of the joints is at any angle between the dip and the strike of the layers. Dip joints in which the joint sets strike parallel to the dip direction of the rocks ģ. Strike joints in which the joint sets strike parallel to the strike of the rocks.Ģ. Three types recognized on this basis are (Fig. In stratified rocks, joints are generally classified on the basis of relationship of their attitude with that of the rocks in which they occur. At other times, the non-systematic joints may show no relationship with the systematic joints and their curved and rough surfaces may even cut across the former. In many cases these are related to the systematic joints in that these occur between them. They appear at random in the rocks and may have incompletely defined surfaces. The outer parts of rocks exposed to direct sun heat may develop cracks and fissures due to such repeated expansion and contraction.Īs the name implies, these joints do not possess any regularity in their occurrence and distribution. Such repeated expansion and contraction is characteristic of regions with dry hot (arid) climates where day and night temperatures on the one hand and summer and winter temperatures on the other hand vary within a very wide range – 50° to 60° C. Rocks, like many other solids, expand with rise in temperature and contract on cooling. Such contraction or shrinkage is generally accepted to be the cause of the vertical type of joints in granites and the so well-known columnar joints of basalts and other effusive rocks. Similarly, igneous rocks which form by cooling and crystallisation from an originally hot and molten material (magma or lava) also necessarily undergo considerable contraction during the cooling process giving rise to tensile forces strong enough to break the congealing masses into jointed blocks. Sedimentary rocks especially those of plastic nature and rich in moisture in the initial stages (clays, shales, limestones and dolomites etc.) undergo some contraction on drying up which might have resulted into irregular jointing.
0 Comments
Leave a Reply.AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |