Can rocks breaks? Let’s find out the answer. You have seen that the enormous forces locked within the Earth are enough to bend huge masses of rock. Sometimes the rocks may break and great blocks may move quite large distances. This is known as faulting.
A famous fault in Britain is the Great Glen Fault running across Scotland from the Island of Mull in the southwest the Moray First in the northeast. This is almost as though Scotland had broken into two pieces.
In fact, along this line, the northern part has moved south-westwards relative to the southern half by a distance of as much as 105 kilometers. This type of fault is called a tear fault, and they are very important on a global scale as we shall see later. A well-known example is the San Andreas fault in California.
There are many different types of faults, and faults, like folds have their own special names. If you could stretch a bed of rock until it broke, you would be quite steep, and one block would move downwards along this plane with respect to the other so that if the plane was inclined to the left the right-hand block would be the lower. Normal faults than are usually associated with crustal stretching.
On the other hand, reverse faults are associated with crustal shortening. In the above situation, a reverse fault would have the lower block on the left. In some cases, reverse faults occur where the fault plane is almost horizontal. This type of fault is called a thrust.
As you can see, on any given fault plane there can be vertical movement, that is, up and down, and this is called the throw of the fault. There can also be horizontal movement, that is, from side to side, and this is called the heave of the fault.
Many of these strange terms that have become associated with faulting had their origins in the coal mines of the north of England where faults were particularly important. One way of discovering the presence of a fault is by noticing that there is a sudden change in the beds of rock from one side of the fault to the other.
For example, on one side of a fault, there might be carboniferous limestone’s and on the other older Devonian sandstones. This would mean that the younger limestones had dropped down in relation to the sandstones. Other miners terms associated with faulting include the word hade. The fault plane of a normal fault, for example, slopes towards the fallen block of rock, that is, it ‘hade’s to the downthrow’.
Where there are combinations of reverse and normal faults, whole blocks of rocks may be moved up or down. A block which has moved up is called a horst, and a block that has moved down is called a graben.
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