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YORK CASTLE HIGH SCHOOLYork Castle High School Geography Folding and faulting Grade 9 The main forces which cause faulting and foldingThe main forces that affect folding and faulting are (i) tensional force - when two forces act away from each other. Tension result in the extension of the surface of the earth and forms joints and normal faults. (ii) compressional force - when two forces act to-wards each other. Compression result in the contrac-tion of the earth’s surface and form folds, reversed fault and thrust faults. (iii) shearing force - when two forces act parallel to each other, though in opposite direction. Shear force result in two rock masses sliding pass each other. Parts of a faultFault plane - The plane that extends into the earth and along which slippage occurs. Fault scarp - The steep face of an exposed block. Fault line - The trace of the fault along the surface Up-throw - An upward displacement (movement) of rock on. Down-throw - The sudden drop or depression of the strata of rocks on one side of a fault. Throw - the vertical change of rocks as a result of their displacement by faulting. FaultingFractures of the rock strata of the Earth’s crust are called faults. They are generally caused by movement at plate margins, but can occur in other areas where the rocks are subject to great stresses. Faults occur in rocks when they are unable to resist the pressure to which they are subjected and crack, result-ing in fractures or joints. When a fault occurs, the rocks on either side of the fault are displaced. The two blocks slip relative to each other, on opposite sides, either vertically or horizon-tally, so that strata no longer match. Types of faultTensional or normal faultA tensional or normal fault results from two forces moving in a horizontal plane in opposite directions. The rock strata are under tension and finally break, causing the rocks to become jointed with normal faults occur-ring. The tension in the rocks is normally caused by tectonic plates moving apart. Forces move away from each other, causing compres-sion — divergent plate boundaries Compressional or reverse faultA Compressional or reverse fault is caused by com-pression of the rock strata in convergent plate zones. This can cause bending and folding of the rock layers and may result in the faulting of the rock strata. Faulting may accompany folding of the rock strata or may occur instead of folding. Forces move towards each other, causing compression and reverse faulting — convergent plate boundaries Tear fault or strike faultA tear fault or strike fault occurs when two plates slide past each other, along a transform zone bound-ary. The plates slide past each other, with no vertical movement. However, the evidence of horizontal displacement can be quite marked. Features form by faulting(1) Rift valleys or grabensThis is an elongated trough formed by the subsidence of a segment of the Earth’s crust between normal, faults. throw Fault scarp Fault line Up-throw BlockDown-throwBlock
YORK CASTLE HIGH SCHOOLA rift valley forms where the Earth’s crust, or outermost layer, is spreading or splitting apart. Rift valleys can be the result of normal faulting in which one side of a fault subsides relative to the other. The valleys are formed due to tensional forces in areas of divergent plate boundaries. Rift Valley formed by tensionRift valleys can also be formed by compressional forces which push up the land on each side of a central mass or block, resulting in the central mass subsiding. The uplifted area is called a block mountain or hoist. These are found in areas of convergent plate bounda-ries. Rift Valley formed by compressionExamples of Rift valleyThe most extensive continental rift valleys are those of: (a) the East African Rift System; (b) Russia's Baikal Rift Valley (c) and Germany's Rhine Rift Valley. Other Examples (d) Porus Graben - Jamaica (e) the Takutu Basin - Guyana (2) Block Mountain or Horst(a) A block mountain or horst is the raised fault block bounded by normal faults or rift valley. (b) An elongate (stretched out) blocks of the Earth’s crust that have been raised compared to their surrounding areas as a direct effect of faulting. How block mountains are formed The cracks formed on the Earth's surface are called "faults". Due to tectonic or seismic movements, there are movements along these faults. As a result, block of rocks are moved on either side of these faults. They can get tilted sideways or can be uplifted with incredible force. There are two formation theories of Block Mountains. Block Mountain formed by tension Rock layers being pulled apart by tenional forces going in opposite directions. Tensional forces cause rocks to break and normal faults develop. As a result, the side rocks went down. And in the mid-dle, the central block remained at an higher than the outer blocks. Block Mountains formed by compressionsAccording to this theory, the rock layers compressed the middle block. Due to this compression, the middle block rose forming the Block Mountains. The middle block remained at a higher elevation than the side rock blocks. Types of Block Mountains: LiftedLifted block mountains has two steep sides. Both side scarps are exposed. Example Sierra Nevada and Teton Mountains in Wyoming (North America) are the best examples of lifted type Block Mountains. TiltedTilted block mountains has one gentle slope. The other steep has an exposed scarp. These type of mountains are commonly seen in the Range and Basin region of the western United States, Rhine valley and south-central New England. 1 2 3 2 1 3 Horst 1 Tension 2 Tension Compression from the side Compression from the side