Geomorphology explores how the Earth’s surface features are shaped by immense forces within the crust. Grand structures such as broad plateaus and sharply defined mountains are the physical results of powerful, slow-moving tectonic stresses. Understanding the differences between these features requires examining the distinct geological processes that create them. These processes dictate whether a large region will become a sweeping, high plain or a series of angular, jagged peaks. The final form a landscape takes is a direct consequence of the type of stress—compression, tension, or broad uplift—applied to the crustal rocks over millions of years.
The Nature and Formation of Plateaus
A plateau is characterized as a large, elevated expanse of land featuring a notably flat top, sharply rising from its surroundings on at least one side. These “tablelands” often form through two primary geological mechanisms that result in a broad, relatively undeformed structure.
One major cause is the slow, uniform vertical movement of a massive section of the Earth’s lithosphere, a process called tectonic uplift. This upward movement occurs without significant internal folding or fracturing of the rock layers, which preserves the horizontal orientation of the strata. The immense pressure from the collision of tectonic plates, such as the Indian and Eurasian plates that created the Tibetan Plateau, can cause large-scale crustal thickening and uplift across a wide area.
Another mechanism involves extensive volcanic activity, where highly fluid basaltic lava erupts repeatedly. These successive lava flows spread out and solidify into thick, flat sheets, gradually stacking up to form a volcanic plateau, like the Deccan Traps in India. In both cases, the defining feature is the uniformity of the elevation change, resulting in the signature flat summit of the plateau.
The Mechanics of Fault-Block Mountain Creation
Fault-block mountains are created by a different stress regime within the Earth’s crust. Their formation begins under tensional stress, where the crust is being pulled apart, causing it to stretch and fracture. This extension leads to the development of normal faults, which are fractures where the rock mass above the fault plane slides down relative to the rock mass below it. This fracturing breaks the crust into numerous distinct, vertical blocks.
When the crust extends, some blocks of land are uplifted while others subside, creating an alternating pattern of peaks and valleys. The raised blocks, which form the mountain ranges, are known as horsts. Conversely, the dropped blocks that form the intervening valleys or basins are called grabens.
The Basin and Range Province in the western United States is a prime example of this horst-and-graben topography, defined by parallel, linear ranges separated by long valleys. This localized, block-by-block movement gives fault-block mountains their characteristic asymmetrical, angular, and jagged appearance.
Key Distinctions in Structure and Appearance
The primary difference between a plateau and a fault-block mountain lies in the fundamental nature of the uplifting force and the resulting internal rock structure. Plateaus are defined by broad, uniform uplift across a vast region, which leaves the underlying sedimentary rock layers relatively horizontal and undeformed. The summit of a plateau maintains a table-like, flat surface that can span hundreds of miles, even if the edges drop off sharply as escarpments. The uplift is a regional, gentle rise of a large, coherent block of crust.
In contrast, fault-block mountains are the result of localized, differential vertical movement along distinct normal faults caused by crustal tension. This process fractures the crust into separate blocks that are tilted or displaced relative to their neighbors. Instead of a single flat summit, the topography consists of a series of parallel mountain peaks (horsts) alternating with sunken valleys (grabens). The internal structure of these mountains is highly fractured and displaced, making their appearance much more rugged, angular, and asymmetrical.