Which statement best defines a transform boundary?

• A. A boundary where two plates slide past each other horizontally.
• B. A boundary where two plates collide, forming mountains.
• C. A boundary where plates move apart.
• D. A boundary where one plate sinks beneath another.

Answer: (A)

Movement of tectonic plates relative to each other defines the boundary type. A transform boundary features horizontal sliding of adjacent plates past one another, producing shear stress and earthquakes along faults, with little formation or destruction of crust. This is why the description of plates sliding past each other horizontally is the best definition. For context, an example is the San Andreas Fault. In contrast, moving apart describes a divergent boundary where new crust forms at mid-ocean ridges or rifts; plates colliding describes a convergent boundary that builds mountains; and one plate sinking beneath another describes subduction, also a feature of convergent boundaries.