Photoglossary

Faults   Faults are fractures in the crust of a planetary body along which some movement has occurred. Displacement along the fault may be caused by extremely slow, continuous movement, or by rapid fracturing as built up stress is suddenly released. Movement along a fault can be vertical, horizontal, or lateral. Two types of force or stress that can be applied to rock are tensional force, which results in the pulling apart of crustal rocks, and compressional force, which squeezes rocks together. Another kind of force is shearing, which causes the rocks to slide past each other. Faults are classified by the way in which the rocks on either side of the fault move in relation to one another. In a normal fault, the hanging wall drops down relative to the footwall. This downward movement results in a steep cliff or fault scarp. (How can you tell the difference between the hanging wall and the footwall? The footwall is the one that is roughly shaped like a foot--wider at the base and sloping inward as it moves up.) These faults are called "normal" faults because downward motion is the normal response to gravity.   Movement along a reverse fault is opposite that of a normal fault. The hanging wall moves upward in relation to the footwall. This is the reverse of the normal response to gravity, and hence the name, reverse fault. If a reverse fault is low angle, 45 degrees or less, it is called a thrust fault. Reverse faults occur where opposing horizontal forces compress the crust enough to cause it to fracture, and force one crustal block to rise over the other.     Illustration Credit: USGS       Io Home Page Photoglossary Home Page