What type of stress acts parallel to a surface?

Shear stress is the type that acts parallel to a surface. In geologic terms, shear stress arises when two surfaces slide past each other. This can be visualized as a force that tends to cause deformation by shifting one layer of material over another without changing the volume of the materials involved, often leading to faulting or folding in the earth's crust.

Shear stress is significant in understanding tectonic activity, as it can influence how rocks behave under stress. When rocks are subjected to shear forces, they can experience internal friction and result in faults such as strike-slip faults, which are characterized by horizontal movement.

In contrast, tension and compression stress act perpendicular to a surface, causing elongation and shortening, respectively. Normal stress also operates perpendicular to a surface, applying internal pressure. These different types of stress play distinct roles in geologic processes, but shear stress specifically involves the sliding parallel to surfaces, making it unique in this context.