Earthquakes are caused by plates sliding past each other. This activity is characteristic of which boundary?

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

Answer: (B)

Earthquakes happen when rocks that are resting against each other along a boundary are stuck by friction, then suddenly slip and release energy. The key idea here is horizontal, side-by-side motion: the plates slide past one another rather than moving toward or away from each other. That sideways shear is exactly what occurs at transform boundaries, where two tectonic plates grind past each other along faults. This ongoing sliding explains the frequent earthquakes seen along transform boundaries like the San Andreas Fault. By contrast, divergent boundaries involve plates moving apart and creating new crust, and convergent/subduction boundaries involve collision where one plate slides beneath another—motions that produce earthquakes too, but the description of slipping past each other most directly points to a transform boundary.