Mountain formation refers to the geological processes that underlie the formation of mountains. These processes are associated with large-scale movements of the Earth's crust. Folding, faulting, volcanic activity, igneous intrusion and metamorphism can all be parts of the orogenic process of mountain building. The formation of mountains is not necessarily related to the geological structures found on it. The understanding of specific landscape features in terms of the underlying tectonic processes is called tectonic geomorphology, and the study of geologically young or ongoing processes is called neotectonics. From the late 18th century until its replacement by plate tectonics in the 1960s, geosyncline theory was used to explain much mountain-building.
Types of mountains
There are five main types of mountains: volcanic, fold, plateau, fault-block and dome. A more detailed classification useful on a local scale predates plate tectonics and adds to these categories.
Volcanic mountains
Movements of tectonic plates create volcanoes along the plate boundaries, which erupt and form mountains. A volcanic arc system is a series of volcanoes that form near a subduction zone where the crust of a sinking oceanic plate melts and drags water down with the subducting crust. mountain next to Sofia Most volcanoes occur in a band encircling the Pacific Ocean, and in another that extends from the Mediterranean across Asia to join the Pacific band in the Indonesian Archipelago. The most important types of volcanic mountain are composite cones or stratovolcanoes and shield volcanoes. A shield volcano has a gently sloping cone due to the low viscosity of the emitted material, primarily basalt. Mauna Loa is the classic example, with a slope of 4°-6°. The composite volcano or stratovolcano has a more steeply rising cone, due to the higher viscosity of the emitted material, and eruptions are more violent and less frequent than for shield volcanoes. Besides the examples already mentioned are Mount Shasta, Mount Hood and Mount Rainier. Vitosha - the domed mountain next to Sofia, capital of Bulgaria, is also formed by volcanic activity.
When a fault block is raised or tilted, block mountains can result. Higher blocks are called horsts and troughs are called grabens. A spreading apart of the surface causes tensional forces. When the tensional forces are strong enough to cause a plate to split apart, it does so such that a center block drops down relative to its flanking blocks. An example of this is the Sierra Nevada Range, where delamination created a block 650 km long and 80 km wide that consists of many individual portions tipped gently west, with east facing slips rising abruptly to produce the highest mountain front in the continental United States. Another good example is the Rila - Rhodopemountain Massif in Bulgaria, Southeast Europe, including the well defined horsts of Belasitsa, Rila mountain and Pirin mountain - a horst forming a massive anticline situated between the complex graben valleys of Struma and that of Mesta.
Hotspots are supplied by a magma source in the Earth's mantle called a mantle plume. Although originally attributed to a melting of subducted oceanic crust, recent evidence belies this connection. The mechanism for plume formation remains a research topic.
Fault blocks
Several movements of the Earth's crust that lead to mountains are associated with faults. These movements actually are amenable to analysis that can predict, for example, the height of a raised block and the width of an intervening rift between blocks using the rheology of the layers and the forces of isostasy. Early bent plate models predicting fractures and fault movements have evolved into today's kinematic and flexural models.
