Mountain building, or orogenesis, occurs when sediment is pushed upwards through tectonic processes of compression. The term "orogeny" stems from the Greek word for "mountain generating". In geological terms, mountain ranges that are severely eroded are also referred to as orogenic belts. Some of the more well-known mountain ranges include the Appalachians of North America and the Himalayas of Asia.
Processes of Orogeny
Mountain building processes are combination of deposition of rock, tectonic convergence of crustal plates and erosion. These processes occur in three steps:
- Accumulation of either sedimentary or volcanic igneous rock, or both, that are several kilometers thick.
- Convergent of two plate boundaries, leading to the orogenic period of deformation and crustal uplift.
- A period of erosion, which causes isostatic rebound and further uplift.
Accumulation of Crustal Material
Accumulation of the sedimentary rock occurs when inland marine environments deposit layer upon layer of sediment, such as sandstone and limestone. Eventually these layers become part of the crust, whether its a continental crust or an oceanic crust. Likewise, eruption of volcanic material adds igneous material to the crust. This igneous material can become part of a continental crust, or in the case of volcanic islands, a part of the oceanic crust that reaches above the surface of the water.
Convergent Plate Boundaries
Convergent plate boundaries occur when two plates are pushed towards each other. These plate boundaries can be continent-continent, arc-continent or ocean-continent. In the case of ocean-continent convergence, the result is often the subduction of the ocean plate under the continent plate. Ocean sediment at the point of subduction is scraped off the top of the ocean plate and accumulates at the trench.
Arc-continent convergence results in a volcanic island arc colliding with a continental plate. The ocean plate, on which the island arc resides, overrides the portion of the continental plate that is under the ocean. As subduction continues, the volcano island arc gets closer and closer to the continental shelf, where it soon meets continental crust.
Continent-continent convergence results in two continental plates colliding, often closing an ocean basin that had previously existed between the two continents. As these plates collide, sediment that accumulated under water, volcanic island arcs and continental crust are all smashed together to cause deformation and crustal uplift.
Isostatic Rebound
After millions of years of crustal uplift and deformation, the sediment and igneous rock that makes up these new mountain ranges is highly susceptible to eroding. Erosion of mountain materials causes it to lose mass and weight. A process called isostatic rebound occurs where the mantle below the thick mountain crust rises because of the decrease in weight above it. This results in further uplift of the mountain range. Eventually, however, erosion takes its toll and great mountain ranges are reduced to large hills. The height of the Appalachians once rivaled the Rocky Mountains, but millions of years of erosion have reduced the Appalachians the the remnants of the great mountain range it once was.
Related Articles:
Subduction Zone Volcanoes of the United States
Sources:
U.S. Geological Survey: Understanding Plate Motions
Skinner, Brian J., Porter, S.C., Park , J. Dynamic Earth: An Introduction to Physical Geology, John Wiley and Sons, Inc., 2004.
 |
 |
 |
