What type of stress results in the thinning of rock in the middle?

The correct answer relates to the concept of tension in geology. Tension is a type of stress that occurs when forces act in opposite directions, effectively pulling apart the material. This pulling action causes the rock to elongate and thin out, particularly in the middle of a section that is under tension.

In geological terms, when rocks are subjected to tensile stress, they can crack or stretch, leading to the formation of features such as rift valleys or normal faults where the crust is being pulled apart. This is in contrast to compression, which leads to thickening of the crust, and shearing, which involves lateral displacement without significant thickening or thinning. Understanding how different types of stress operate helps in predicting geological formations and the behavior of the Earth's crust under various tectonic forces.