Earth’s Internal Processes and Mountain Formation
Earth’s Internal Processes
Internal processes are those directly or indirectly caused by the Earth’s internal heat:
- Metamorphism
- Magmatism
- Isostasy (epirogenic movements)
- Orogenesis (orogenic movements)
Magmatism and Plate Tectonics
Rock melting can be produced by the following processes:
- A raise in the rock temperature
- A drop in the confining pressure
- The presence of fluids (water)
According to plate tectonics, three igneous environments exist:
- Continental rifts, mid-ocean ridges: Mantle rocks melt by a drop in pressure.
- Ocean trenches: Crustal rocks melt by a raise in temperature (friction) and a presence of fluids produced by dehydration of the subducting plate.
- Hot spots (intraplate volcanism): Deep mantle rocks melt by an increase in temperature (mantle plumes).
Continental rifting, ocean ridges, and hot spots show iron-rich, magnesium-rich, and fluid-poor magmatism, since it comes from mantle melting. However, magmatism related to ocean trenches presents silica-rich, aluminum-rich, and fluid-rich magmas, produced by melting of crustal rocks.
Metamorphism and Plate Tectonics
Metamorphism is a geological process in which pre-existent rocks reorganize their minerals and texture, according to an increase in temperature and/or pressure inside the Earth. Rocks, however, remain basically with the same global chemical composition, and that’s why metamorphism is an isochemical process. When it is clear that an exchange of elements between the previous rock and internal fluids has taken place, that process is called metasomatism, or metasomatic metamorphism.
The metamorphic areas develop in plate boundaries:
| Boundaries | Types of Metamorphism |
|---|---|
| Mid-ocean ridges | “Sea floor metamorphism”: metasomatic thermal metamorphism |
| Transform faults | Dynamic metamorphism |
| Subduction zones | “Pair metamorphic belts”:
|
| Obduction zones | Regional metamorphism |
Isostatic Movements
Movements Related to Glaciations
When a continent is charged with an ice sheet, it sinks. When there is a climate change and the glaciers melt, the lithosphere goes up to regain the lost equilibrium (Note that the ice melting is much faster than the lithospheric movement. That’s why there still are fiords).
Movements Related to Erosion and Deposition
When a sedimentary basin undergoes a process of sedimentation, it sinks. This process is called subsidence. On the other hand, when a mountain chain is eroded, it tends to rise.
Genesis of Mountain Ranges
Several theories have been developed to explain the genesis of mountain ranges. They can be classified into two main groups:
- Fixist theories: they assume that continents are not able to move
- Contractionist theory
- Geosynclinal theory
- Mobilist theories: they assume that continents move
- Wegener’s Continental Drifting
- Plate Tectonics
Subduction Orogens
- Volcanic island arcs
- Continental volcanic arcs (peri-continental mountain ranges)
Obduction Orogens
- Collisional orogens (inter-continental mountain ranges)
- Terranes accretional orogens