Orogenesis and Mountain Building Processes

Definition: Orogenesis is the set of processes involved in forming a mountain belt.

Mountains are typically defined by various elevation and slope criteria:
- Elevation of at least 2,500 m (8,200 ft);
- Elevation of at least 1,500 m (4,900 ft) with a slope greater than 2 degrees;
- Elevation of at least 1,000 m (3,300 ft) with a slope greater than 5 degrees;
- Elevation of at least 300 m (980 ft) with a 300 m (980 ft) elevation range within 7 km (4.3 mi).
Older mountain chains tend to erode and become less topographically prominent.

Comprised of large quantities of preexisting sedimentary and crystalline rocks that have been faulted and folded.

Subduction of Oceanic Lithosphere: The primary driving force behind orogenesis where denser oceanic plates subduct beneath lighter plates.
- Older Oceanic Plate Subduction: Occurs when two oceanic plates collide.
- Volcanic Island Arcs: Form where subduction occurs beneath oceanic lithosphere.
- Continental Volcanic Arcs: Form where subduction occurs below a continental plate and can generate mountainous topography along continents.

Build mountains through the combination of volcanic activity, the emplacement of plutons, and the accumulation of sediment from the subducting plate onto the upper plate.
- Micro-Continents: Large volcanic island arcs can be classified as micro-continents.

Form in Andean-type convergent zones. Before subduction, sediment accumulates on what is known as a Passive Continental Margin.
- Transformation to Active Continental Margin: Once a subduction zone forms, it transitions to an Active Continental Margin, which initiates deformation.
- Accretionary Wedge: An accumulation of sedimentary and metamorphic rocks that are scraped from the subducting plate.

Occurs in Pacific-like ocean basins where rapid seafloor spreading is balanced by rapid subduction.
- Terranes:
- Island arcs and crustal fragments (terranes) collide with continental margins.
- Some terranes may consist of microcontinents.
- Smaller terranes may be subducted while larger terranes are thrust onto the continent.

Alpine-Type Orogens: Result from continental collisions, where continental plates come together via the subduction of an oceanic plate attached to one of the continents.
- When two continental plates collide, the oceanic plate breaks off and submerges into the mantle.
- Example: The Himalayas, formed from the collision between the Indian and Eurasian plates.

Formed by orogenies lasting several hundred million years.
These mountains were developed during one of the stages in assembling the supercontinent Pangaea.
Resulted from three distinct stages of mountain-building processes.

These mountains are generated when a continent begins to rift, leading to portions of the ground dropping down, creating basins and ranges.