Chapter 14: Mountain Building

Mountain belts develop primarily along convergent plate boundaries where compressional stress compresses and thickens the crust, generating folding, thrust faulting, and metamorphic transformations. The chapter maps major mountain systems including the American Cordillera, the Alpine-Himalayan belt, and Pacific island arc chains, while analyzing older eroded ranges that reveal ancient tectonic activity. Subduction zones drive mountain formation through four characteristic features: deep ocean trenches marking subduction sites, volcanic arcs formed by magma rising through the overlying plate, forearc regions accumulating sediment scraped from the descending plate, and back-arc basins created by extension behind the volcanic arc. Island arcs composed of igneous and metamorphic rocks contrast with larger Andean-type belts that form when oceanic lithosphere slides beneath continental margins, producing thickened crust, continental volcanic arcs, large intrusive bodies, and forearc basins. Collisional orogenies occur when buoyant crustal fragments collide with continents, with Cordilleran-type belts resulting from accreted terranes welded to continental margins, while Alpine-type belts form from continent-continent collisions exemplified by India's collision with Asia creating the Himalayas or Africa's collision with North America producing the Appalachians. Beyond compressional tectonics, fault-block mountains develop in extensional settings where normal faulting uplifts crustal blocks separated by down-dropped basins, as seen in the Basin and Range province. The chapter also addresses factors controlling mountain elevation, including isostasy, the principle that crust floats on the mantle with thickened crust developing deep roots and standing higher than surrounding terrain. Isostatic adjustment occurs through erosion, glacial loading, and post-glacial rebound, processes evident in uplifting Canadian landscapes. Gravitational collapse occurs when mountains reach such heights that their bases flow laterally, spreading the crust and reducing elevation. Mantle convection contributes significantly to vertical crustal motion through upwelling plumes that elevate regions and subducted slabs that depress surrounding crust. The Laramide Orogeny demonstrates how deformation extends far inland from plate boundaries, uplifting mountains hundreds of kilometers from active margins.