When dilatancy occurs in granitic rock, which statement best describes the change in the rock?

• A. The granite has cooled rapidly
• B. The granite becomes more dense
• C. The granite has been sheared and increases in volume
• D. The granite experiences no volume change

Answer: (C)

Dilatancy is the tendency of a rock to increase its volume when it deforms by shear, because cracks and voids open up within the rock. In granitic rock, this means that as the grain framework slides past one another, microcracks form and propagate, enlarging the pore space and causing the rock to dilate. That volume increase is the hallmark of dilatancy, so the statement describing the rock as having been sheared and increasing in volume best captures the behavior. By contrast, rapid cooling would typically cause contraction, increasing density would imply a smaller volume for the same mass, and no volume change would ignore the dilation that accompanies shear.