Final answer:
Fault-block mountains form at divergent plate boundaries, where tectonic plates move apart and blocks of Earth's crust create linear patterns of mountains and valleys.
Step-by-step explanation:
The type of mountain that forms because of a divergent plate boundary is a fault-block mountain. At a divergent boundary, tectonic plates move away from each other, and tensional forces can cause large blocks of the Earth's crust to drop down relative to other blocks, forming mountains and valleys aligned in a linear pattern. This process is quite distinct from the mountain-building events seen at convergent plate boundaries, such as the formation of arc-continent or Andean type mountains. The Cascades in the Pacific Northwest, for instance, resulted from subduction-related processes at a convergent plate boundary, which is not the scenario for divergent boundaries that create fault-block mountains.