FAULTS IN GEOLOGY
What Are Faults?
Earth is made up of many tectonic plates.The plates float around on the mantle like ice floats on a pond. The movement of the plates is incredibly slow, but since the plates are so big, when they bump into and rub against each other, we get massive events like volcanoes and earthquakes. And along these plate boundaries, we find faults. Faults are cracks in the earth’s crust where movement occurs on at least one side. So, in order for a crack in the ground to be a fault, one side or the other has to move, but sometimes both sides move, too!
What Causes Faults?
Faults are classified by how they move, and there are three main types of stress that cause movement along faults. The stress occurs because, as mentioned before, the plates fit together really well, but also float around on the mantle and rub against each other.
Tensional stress is when slabs of rock are pulled apart. Imagine stretching a rope out all the way and then continuing to pull on it from both ends. You’re putting tension on the rope because it’s being pulled in opposite directions. The same is true for tensional stress in rocks.
Compressional stress is when slabs of rock are pushed together. They are literally being compressed into one another. This is like when two cars crash into each other — they’re compressed into smaller, crumpled versions of what they were before.
Shear stress is when slabs of rock slide past each other horizontally in opposite directions. The rocks are not smashed into each other or pulled apart, but their edges slide along each other with a lot of friction. This is like when you rub your hands together to warm them up. One hand goes forward and the other goes backward, rubbing against each other.
Types of Faults
Before we study types we should understand two terms HORST and GRABEN
a Horst is the raised fault block bounded by normal faults or Graben . A Horst is a raised block of the Earth’s crust that has lifted, or has remained stationary, while the land on either side has subsided.
a Graben is a depressed block of land bordered by parallel faults. Graben is German for ditch or trench. The plural form is either Graben or Grabens. A Graben is a valley with a distinct escarpment on each side caused by the displacement of a block of land downward.
The type of fault we get depends on the type of stress that caused it, which also tells us about how the fault moves. Faults have two sides: the hanging wall and the footwall. These terms were coined by miners because you can stand with your feet on the footwall and hang a lamp on the hanging wall on the opposite side. And the reverse situation would be impossible!
normal fault — a dip-slip fault in which the block above the fault has moved downward relative to the block below. This type of faulting occurs in response to extension and is often observed in the Western United States Basin and Range Province and along oceanic ridge systems.
Reverse faults are exactly the opposite of normal faults. If the hanging wall rises relative to the footwall, you have a reverse fault. Reverse faults occur in areas undergoing compression (squishing).
thrust fault — a dip-slip fault in which the upper block, above the fault plane, moves up and over the lower block. This type of faulting is common in areas of compression, such as regions where one plate is being subducted under another as in Japan. When the dip angle is shallow, a reverse fault is often described as a thrust fault.
strike-slip fault — a fault on which the two blocks slide past one another. The San Andreas Fault is an example of a right lateral fault.
