Fault-block mountains form when Earth's crust pushes material up and down near a fault line. Rocks that get pushed upward eventually form mountains if there is enough seismic activity. The higher part of a fault-block area is called a horst, while the lower part is called a graben, according to Ducksters.
A fault-block mountain occurs when one side of a fault in the Earth's crust slides over another. The material from one side gets pushed upward while the other slides downward. These uplifted rocks form mountains over time and after a tremendous amount of intense pressure.
The force of seismic activity causes large blocks to thrust up and even sideways on one side of the fault. The other side forms a depression into the ground. This steep depression erodes over time and flattens out as material from the uplifted portion falls into the depression.
The Sierra Nevada Mountains of the Western United States are an example of fault-block mountains. This mountain range formed between 130 and 400 million years ago as the Pacific Plate lowered underneath the North American Plate, a process known as subduction.
There are three major types of mountains including the fault-block variety. Volcanic mountains form after magma reaches the surface and hardens over time. Fold mountains occur when two plates collide and Earth's crust crumples and folds upward.