Which geologic setting is most likely to produce a normal fault?

• A. Transform boundary
• B. Convergent plate boundary
• C. Subduction zone
• D. Divergent plate boundary

Answer: (D)

Normal faults occur when rocks are pulled apart, creating tensile stress. As the crust stretches, blocks fracture and the hanging wall tends to slip downward relative to the footwall. This extensional setting is exactly what happens at divergent plate boundaries, where two plates move away from each other and the lithosphere is stretched and thinned. Gravity helps the pulled-apart blocks drop along the fault plane, forming normal faults and often basins or rift valleys.

At other plate boundaries the stresses are different: transform boundaries mainly produce sideways, strike-slip motion with little vertical displacement; convergent boundaries compress the crust and generate reverse or thrust faults with the hanging wall moving up; subduction zones involve strong compression and complex faulting. So the geologic setting most likely to produce a normal fault is a divergent plate boundary.