A block mountain (also called a horst) is formed as a result of faulting. Specifically, it is produced when tectonic forces create two roughly parallel faults in the Earth's crust, and the block of land between or beside them is either pushed upward or remains elevated while the adjacent blocks subside. The upstanding block that results is the block mountain.
The process involves compressional or tensional forces in the Earth's crust that fracture rock along lines of weakness (faults). When the central block is pushed upward relative to the flanking blocks, it produces a horst. When the central block drops down instead, the resulting depression is called a graben or rift valley. Both features result from the same faulting mechanism.
Examples of block mountains include the Vosges and Black Forest mountains flanking the Rhine Rift Valley in Europe, and the Ruwenzori Mountains in East Africa.
Differential erosion shapes existing landforms but does not create block mountains. A fissure is simply a crack in rock. Vulcanicity produces volcanic mountains (such as Mount Kilimanjaro), which are fundamentally different in origin from block mountains.
