Structures In Rocks Topics

Geologic Structures (Part 2)


• Faults are fractures (cracks) in rocks along which movement has occurred.  Faults are classified according to which direction the rocks moved along the fault: normal faults, reverse faults, thrust faults, and strike-slip faults. Faults also create environmental effects such as the movement of groundwater, and can cause hazards such as rock slides and earthquakes.

Fault Diagram(Diagrams by Phyllis Newbill)

In the above left diagram, the rock block is unfaulted, but contains a weakness.  The above right diagram shows the block after faulting.  The fault plane is the plane on which movement occurs.  The footwall is the part of the fault where your feet would be if you stood on the fault plane.  The hanging wall would be above you. 

Normal Faults

• If the hanging wall moves down relative to the footwall, the fault is a normal fault.  Normal faults are caused by tensional stress, or stress that pulls rocks apart..

Normal FaultNormal fault. Note that the right side of the fault is the hanging wall which has moved down relative to the left side. (Diagram by Phyllis Newbill)

Normal FaultNormal fault (Photograph by Stan Johnson)

MapThis small fault is a normal fault because the hanging wall has moved down relative to the footwall. This fault is in a quarry in Rockingham County.