Above: Normal fault. A fault is a fracture or break in the rock along which the two blocks of rock have moved past each other. For a normal fault, the block of rock above the fault (hanging-wall block) moves down relative to the block of rock above the fault (footwall block).

Below: Formation of normal faults. Experimental models allow us to determine how faults may form. In the models, a flexible rubber sheet (blue) separates two metal plates (gray). As the mobile plate is pulled away, the rubber sheet stretches. To simulate faulting, wet clay (yellow) is placed over the metal sheets and rubber sheet. As the rubber sheet stretches, the overlying clay also stretches and thins, forming normal faults.  Similarly, as the crust is stretched during rifting, normal faults form.  From Schlische and Withjack (2001).

Top: Normal faults in cross-sectional view. Sketch of normal faults produced in experimental model of stretching.  The layers were all originally flat-lying prior to stretching. Bottom: Normal faults in plan view.  Photograph of the top surface of the clay.  Faults inclined toward the light source are bright; faults inclined away from the light source are dark. From Schlische and Withjack (2001).