Block mountains are distinct geological features created by the fracturing and vertical movement of large rock sections, not by the gentle folding of the Earth’s crust. This process results in a rugged, alternating landscape of elevated mountain blocks and sunken valleys. Unlike the tall, often curved peaks of fold mountains, block mountains typically exhibit straight, steep sides and flatter summits. Their formation is driven by powerful forces that cause the rigid upper crust to break apart along fault lines.
The Extensional Forces
The formation of block mountains is primarily driven by extensional stress acting upon the lithosphere. This is a pulling force that stretches and thins the crust, often occurring in areas where tectonic plates are moving away from each other, known as divergent plate boundaries. The pulling motion causes the Earth’s rigid, upper crust to behave in a brittle manner, meaning it fractures instead of bending or folding.
This stretching leads to the development of deep cracks known as faults. These faults are planes of weakness along which the rock blocks can move relative to one another. The resulting landscape is a direct consequence of this brittle deformation.
Anatomy of Horst and Graben Structures
The key mechanism in block mountain formation involves a specific type of fracture called a normal fault, which is characteristic of extensional environments. In a normal fault, the block of crust that sits above the fault plane, called the hanging wall, slides downward relative to the block below it, known as the footwall. This movement allows the crust to stretch and accommodate the pulling forces.
When the crust fractures into parallel blocks, the resulting vertical movements create two distinct landforms: the horst and the graben. The horst is the uplifted block of crust that forms the block mountain, elevated relative to the surrounding land. These blocks are bounded by two parallel normal faults whose planes dip away from each other.
Conversely, the graben is the down-dropped block, a sunken section of crust that forms a valley or trough. The graben subsides between two parallel normal faults that dip toward each other. When a graben is particularly long and narrow, it is often referred to as a rift valley.
The block mountain system is defined by the alternating pattern of these structures, where the high horsts are separated by the low grabens. The vertical displacement along these faults can range significantly, sometimes resulting in vertical drops of several thousand meters. This process of differential vertical movement along normal faults gives block mountains their characteristic steep, linear slopes, often featuring a distinct fault scarp.
Global Manifestations
Block mountain systems are found across the globe, providing evidence of past or ongoing extensional tectonics. One of the most extensive examples is the Basin and Range Province in the western United States, which covers much of Nevada and parts of surrounding states. Here, hundreds of parallel mountain ranges (horsts) alternate with wide, flat-bottomed valleys (grabens), creating a landscape shaped by crustal stretching.
The Sierra Nevada range in California is a prominent block mountain, formed by the uplift of a massive crustal block along a fault. Its eastern side features a steep, cliff-like face, a classic example of a fault-bounded escarpment. In Europe, the Vosges Mountains in France and the Black Forest in Germany are separated by the Rhine Valley, which is a large graben structure.
Another significant example is the East African Rift Valley system, which is an ongoing rift zone where the crust is actively being pulled apart. The valley floor is essentially a graben, flanked by elevated plateaus and mountains that represent the horsts.