How Mountains Form

In general, it takes hundreds of millions of years for mountain belts to form, stabilize, and erode to become part of a stable craton. This evolution is marked by three stages: accumulation, orogeny, and uplift/block‐faulting.

Accumulation. Many mountains contain sequences of sedimentary and volcanic rocks that reach thicknesses of 2,000 to 3,000 meters. Most of this material was deposited in a passive or active continental marine environment during the accumulation stage. The sedimentary material typically weathers from the continental landmass or offshore island arc; deep marine sediments can also be scraped from the subducting plate and piled onto the accretionary wedge. Thick sequences of sandstone, shale, and limestone with minor volcanic material accumulate along passive continental margins, such as the eastern coast of the United States. Sediments that accumulate along a convergent boundary (active continental margin) are more varied than those along a passive margin and often contain up to 50 percent andesitic flows and tuffs. Limestones are rare to absent. Graywackes are common and represent a rapid accumulation of sediment from a nearby magmatic arc. These sedimentary and volcanic deposits along the continental margins have been pushed up into many of the mountain ranges we see today.

Mountain‐building convergence. Orogenesis is the mountain‐building and associated folding, faulting, deformation, and metamorphism that result from the onset of intense tectonic stress. Igneous intrusions are also common. The layered rocks are tightly compressed into folds that often result in thrust faulting. The deepest rocks are metamorphosed into schists, gneisses, and migmatites. Compression also results in vertical uplift of the deformed rock sequence. Block faulting can also occur after the forces have thrust the metamorphosed and deformed rocks upward and outward.

Ocean‐continent convergence deforms the accretionary wedge, metamorphoses rocks in the subduction zone, creates a mountainous magmatic arc, and develops fold‐and‐thrust belts on the backarc side of the magmatic arc. Ocean‐continent convergence results in the formation of the rugged topography of the deep ocean trenches and seamounts. The magmatic arc is elevated because of the massive igneous upwellings underneath it.

Arc‐continent convergence results when the intervening ocean is destroyed by subduction, welding an island arc to the continental edge. This convergence also results in deformation, uplift, and orogeny. Tectonic forces along the continental edge continue to generate heat, igneous intrusions, and compressional forces to the continental edge. It is probable that the northwestern United States was formed by a series of arcs that collided with and were welded to the North American craton.

The collision of two continental masses, or continent‐continent convergence, also results in the formation of mountain belts. The thick sedimentary sequences that formed on both continental edges are squeezed into intensely deformed mountain ranges that are some of the highest in the world, such as the Himalayas between India and the rest of Asia. The Himalayas are still rising because of continued compression along the suture boundary, and they host frequent earthquakes. Some geologists theorize that the Appalachian Mountains in the eastern United States were built as a result of the collision of the European and African plates with the North American plate that helped form the supercontinent Pangaea. The Appalachians and the Caledonian Mountains of Great Britain and Norway were all once joined along the same suture zone before Pangaea rifted into the continents that we see today.

Postconvergence mountain‐building. A mountain range undergoes additional uplift and block‐faulting after orogeny has ceased. Because the continental crust was thickened during mountain‐building, the gradual uplift over tens of millions of years is a result of isostatic adjustment. As material is eroded from the mountain belt, more uplift compensates for the loss of weight (mass) during erosion. The uplift creates vertical and extensional (tensional) stresses that result in the block‐faulting of mountain ranges along a series of normal faults. Fault blocks may also be tilted if the stresses are unevenly distributed. Scattered volcanic activity can also be part of this phase of mountain development. Fault‐blocked mountain ranges are usually separated by valleys filled with thousands of feet of sediment, such as those of the Great Basin (the Basin and Range region) in the western United States.