Faults are fractures in the Earth's crust. When rocks are subjected to stress and pressure, they may bend but eventually break. Sometimes they are displaced vertically or horizontally. The line of separation is called the fault plane. These movements can occur suddenly , like in an earthquake of slowly, over a period of time.
Types
Types
- Normal faults are caused by tension forces moving in opposite directions. It is sometimes called a dip-slip fault since one block moves downward or slips relative to the other block as seen in the animation below. The blocks of rock on the two sides of a non vertical fault are given special terms by geologists. The term given to the block that rests on top or above the fault plane is the hanging wall while the block that lies below the fault plane is called the foot wall.
- A reverse or thrust fault occurs when one block is pushed up and over another block as a result of compression forces as seen in the animation below.
- Tear faults or strike-slip faults occur when rocks slide past one another with little vertical movement as a result of shear stress as seen in the animation below.
These movements result in some unique features that we can observe in the landscape.
- Normal faulting causes one rock to be pushed downwards, forming a fault plane or scarp which is steep slope.
- Reverse faulting can lead to the formation of escarpments - long cliff features that have a steep slope or scarp on the side of the fault line where the rock slab has been pushed up; and a dip slope towards the other end.
Normal faulting - fault plane or scarp
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Reverse faulting - escarpment
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Block mountains and rift valleys
Orogenesis is the term used to describe mountain building processes such as folding and faulting. Sometimes parallel faults occur in rocks of the Earth's crust. When this happens, the land in between can either be pushed up higher than the surrounding land or pushed down.
When forces of tension pull the rocks apart , the land subsides to form rift valleys. A famous example is the East African Rift Valley that stretches from Lebanon to Mozambique and is approximately 6000 km long. It is believed that eventually the African continent will be split into two in a few million years! Smaller faulted valleys are called graben. One example is the Takutu Basin in Guyana.
When forces of compression cause a huge block of rock to be pushed up above the surrounding land, it is called a block mountain or horst. One example is the Sierra Nevada Mountains located in western USA.
Orogenesis is the term used to describe mountain building processes such as folding and faulting. Sometimes parallel faults occur in rocks of the Earth's crust. When this happens, the land in between can either be pushed up higher than the surrounding land or pushed down.
When forces of tension pull the rocks apart , the land subsides to form rift valleys. A famous example is the East African Rift Valley that stretches from Lebanon to Mozambique and is approximately 6000 km long. It is believed that eventually the African continent will be split into two in a few million years! Smaller faulted valleys are called graben. One example is the Takutu Basin in Guyana.
When forces of compression cause a huge block of rock to be pushed up above the surrounding land, it is called a block mountain or horst. One example is the Sierra Nevada Mountains located in western USA.
Click on the arrow to watch the You Tube video that depicts recent developments in the East African Rift Valley.