Faults and Faulting
FAULTS AND FAULTING:
A faults is a fracture in the crystal rock involving the displacement of rock on one side of the fracture with respect to the rock on the other side. On the other Hand, a fracture without displacements is called a joint. According to Arthur N.Strahler,
“ A fault is a break in the brittle surficial rocks of the Earth’s crust as a result of unequal stresses. Faulting is accompanied by a slippage or displacements along the plane of breakage. Faults are often of great horizontal extent so that the fault line can be traced along the ground of many miles sometimes 100 miles or more.”
The compressional stresses are strong where plates converge and rocks are crushed tightly together. As a result of the stresses, the rocks bend, fold, slide and squeeze upward or downward because the lithosphere occupies less horizontal space. The stress is tensional where the plates diverge and the crust is subjected to spreading processes else where.
Types of Faults:
There are three major types of faults as given below.
- Compressional faults.
- Tensional faults.
- Transcurrent faults.
(1). Compressional Faults:
These faults are produced as a result of the compressional forces and are of several types e,g., reverse fault, parallel fault (echelon) or thrust fault. The reverse fault takes place when crystal rocks is compressed into a smaller horizontal space. As a result, the crust is shortened because one block rides over the other along a steep fault plane. The block which moves up is termed as up-thrown and which moves down is called down-thrown. Such faults result in landslides and other mass movements. Similarly, when a strip of crystal rocks positioned between reverse faults assumets the shape of a block-like plateau it is called parallel faults. However, when angle of a faults plane in a compressional faults is very low, then the structure is called a thrust fault ( over-thrust fault).
(2). Tensional Faults:
In a tensional fault, a rock is pulled apart through tensional stresses. In such faults, there is more space of crystal material. Thus results one or more normal faults. A normal fault may be defined as a fault which has a moderately inclined fault plane separating a block from one that has been significantly down-thrown. The eastern front of the Grand Teton Mountains (USA) and western front of the Wasatch Mountain (Utah, USA) are excellent example of the tensional faults. East Africa is the best example of rift valley topography.
(3). Transcurrent Fault:
In a transcurrent fault, the blocks of the crystal rock move laterally and motion along the fault plane is horizontal not vertical. In this case, there is no up-thrown or down thrown blocks. San Andreas fault is the best example of transcurent faults (Srike-slip faults).
Types of faults on basis of stresses:
The faults have been divided into three categories on the orientation of stresses:
- Normal Faults
- Thrust Faults
- Strike-Slip Faults:
(1). Normal Faults:
It is a tensional fault exhibiting a moderately inclined-fault-plane which separates a block that has remained stationary from one that has been significantly down-thrown the normal fault result in a steep, straight fault scrap which has a height range between few to a few thousand feet. Normal faulting is the proof of tension in the Earth’s outer crust.
(2). Thrust Faults:
It is a compression fault in which the angle of the fault-plane is very low. It is also called an over burst fault:
(3). Strike-Slip Faults:
It is also called Transcurrent Fault. In these faults, the blocks of crystal rock move laterally and motion along the fault plane is horizontal, not vertical. Its best example is the San Andreas Fault which is a contact plane between the Pacific and the North American Plates:
Rift Valley (Graben):
Rift Valley is formed in a continental zone of plate divergence where tensional forces pull the thin crystal surface apart. In other words, rift valley is a trough that forms when the land sinks between parallel faults in strips. Usually, a rift valley is narrow and long. The Rhine valley in Germany, the Lakes of Africa, Red Sea and the Dead Sea of Israel are good example of Rift Valley.
It is a crystal block which had been raised between two reverse faults by compression forces. A horst is formed when the center block is up thrown and the side blocks are down-thrown in such a way that the raised mass looks like a dome. African’s great mountain Ruwenzori is a fine example of horst.
A block mountain is formed by the up-throwing of a block of land on one side of a pair of parallel faults. This raised mass looks like a mountain and is called a Block Mountain or a Monocline. Such mountains are found in the northern Pennine Chain Mountain and the Mountains of Western North America.