Compression of the continental fault causes folds and thrust faults to form at continental-continental and continental-oceanic convergent plate borders. As a result, rock blocks are piled on top of each other. Some of the mountains in subduction zones, when an ocean plate slips beneath a continental plate or another oceanic plate, may be volcanoes. The downward-moving oceanic plate introduces water into the mantle, changing the chemical of the rock and lowering the melting point. The magma that results rises through the overlying plate, forming volcanoes.
The East African Rift is an example of a divergent plate boundary where horst and graben topography can form. Faults form as a continent separates, and graben, or rock blocks, fall below. Between the graben, other slabs of rock known as horsts stay as high terrain and eventually become mountains.