In structural geology, tensional forces that pull rocks apart normally result in the formation of which type of fault?

Introduction / Context:
Faults are fractures in the Earth crust along which blocks of rock have moved relative to each other. The type of fault that develops depends largely on the dominant stress regime in the region, such as tension, compression, or shear. Understanding which stresses produce which faults is important in geology, earthquake studies, and resource exploration. This question focuses on tensional forces, which pull rocks apart, and asks which kind of fault is normally produced under such conditions.

Given Data / Assumptions:

• The setting involves tectonic forces acting on rock masses.
• Tensional forces act to stretch and pull rocks apart, increasing crustal length.
• Fault types listed include normal, strike slip, thrust, and reverse faults.
• We assume standard textbook definitions of these fault types.

Concept / Approach:
In a normal fault, the hanging wall block moves downward relative to the footwall block along a steeply dipping fault plane. This movement is associated with crustal extension and tensional stress, which pulls the blocks apart and allows the upper block to drop. In contrast, reverse and thrust faults are associated with compressional forces that shorten and thicken the crust, causing the hanging wall to move upward. Strike slip faults accommodate horizontal shear motion without significant vertical displacement. Therefore, tensional forces are most closely linked with normal fault formation.

Step-by-Step Solution:
Step 1: Recall that tensional stress tends to pull rock masses away from each other, creating an extensional environment. Step 2: In such an environment, blocks of crust can move downward along steep fractures as the region stretches and subsides. Step 3: A normal fault is defined by downward movement of the hanging wall relative to the footwall, and it typically forms where the crust is being extended. Step 4: Reverse and thrust faults, by contrast, form where the crust is under compression, pushing blocks together rather than pulling them apart. Step 5: Strike slip faults involve mostly horizontal motion due to shear stress, not tensional stress that increases crust length.

Verification / Alternative check:
Geological maps of rift valleys, mid ocean ridges, and extensional basins show numerous normal faults bounding blocks that have dropped down to form grabens. These settings are known to be dominated by tensional forces as tectonic plates move apart. In contrast, mountain ranges formed by plate collisions display many reverse and thrust faults, consistent with compressional regimes. Major strike slip faults such as the San Andreas fault in California are associated with horizontal shear motion, reinforcing the connection between different stress types and fault geometries.

Why Other Options Are Wrong:
Strike slip faults with horizontal movement: These primarily result from shear stress where two blocks slide past each other horizontally, not from pure tension. Thrust faults with shallow angle compression: Thrust faults are a type of reverse fault formed by strong compression that shortens the crust, opposite of tensional extension. Reverse faults with hanging wall moving upward: Reverse faults also form in compressional environments where rocks are pushed together, not pulled apart.

Common Pitfalls:
Students may confuse the names normal and reverse, thinking that normal means common or typical and reverse is strange, without linking them to stress regimes. A more reliable approach is to remember that normal faults involve extension and downward movement of the hanging wall, while reverse and thrust faults involve compression and upward movement. Drawing simple block diagrams and arrows for tension and compression can help reinforce these connections for exam questions.

Final Answer:
Tensional forces that pull rocks apart normally cause normal faults where the hanging wall moves downward relative to the footwall.