Plate Tectonics: Mantle Convection, Subduction, and Orogeny Formation
Mantle Dynamics: Convection and Thermal Plumes
Convective movements consist of updrafts and downdrafts within the fluid mantle. These currents form when the fluid becomes unstable because its top is more dense and tends to sink, while the bottom is less dense and tends to rise to the surface.
Rifting and Thermal Plumes
Rifting is the rupture of a continental plate caused by the action of a thermal plume. Archipelagos of volcanic points, such as the Azores and Hawaii, are often associated with hot oceanic lithosphere, though not all are related to classic hot spots.
Convective flows downstream: The thermal plume is cooled by two primary processes:
- Conduction of heat in the lithosphere: This process involves magnetism, which gives rise to volcanism.
- Expansion of materials: The rocks forming the thermal plume expand as they reach areas of lower pressure. This expansion cools the material, similar to how the expansion of a gas lowers its temperature.
When the material sinks back into the mantle as descending currents, these currents become more diffuse upward because the radial flow disrupts the thermal plume.
Fragments from the thermal plume reaching the Repetti discontinuity do not sink easily and can be supported on the discontinuity due to pressure until the material is compacted.
Subduction Zones and Orogenesis
Subduction is the formation of convective currents created by descending oceanic lithosphere. A subduction zone is the area where an oceanic plate bends and sinks into the mantle.
Characteristics of Subduction Zones
- They are typically found in the oceans.
- The overriding plate (the plate rider) may be oceanic or continental.
- In the area where the subducting plate bends, a deep and elongated oceanic trench forms.
- The oceanic trench accumulates a large thickness of sediments. If these sediments are compressed against the overriding plate, they form a cliff-shaped accretionary prism.
- These are areas of high seismicity; earthquakes are distributed according to an incline called the Benioff plane.
- Subduction produces plutonic intrusions, seismicity, volcanism, and the formation of orogenic belts.
Orogens are mountain ranges, and their origin is linked to subduction zones and plate movements. According to their origin, four types can be distinguished: island arcs, thermal orogeny, collision orogeny, and intraplate orogeny.
Four Types of Orogeny
Island Arcs
If the overriding plate is composed of oceanic lithosphere, the resulting orogenic belt forms a linear archipelago of volcanic islands called an island arc. Island arcs are areas of intense volcanic and seismic activity. The oceanic trenches formed along island arcs are often the deepest on Earth. The high volcanic and seismic risk in these regions also brings other risks, such as tsunamis (highly destructive tidal waves).
Thermal Orogeny
If the overriding plate is formed by continental lithosphere, the resulting volcanic relief at the edge is called Thermal Orogeny. In this subduction zone, a trench forms that is shallower than those associated with island arcs. Sediments are tightly compressed against the continental slope and become attached to form an accretionary prism composed of metamorphic and sedimentary rocks. The oceanic crust of the subducting plate, just as occurs in island arcs, undergoes fusion when inserted into the mantle. The resulting magmas tend to rise, leading to volcanic eruptions and also forming plutonic rocks housed within the continental crust.
Orogenic Collision
When a collision occurs between two continental plates, neither plate can be easily subducted, and an Orogenic Collision occurs. In collision orogeny, unlike thermal or island arc settings, the predominant process is tectonic folding rather than volcanism. The relief is not an alignment of volcanoes, but is formed by the folding of the materials from both plates and the sediments that had accumulated between them before the collision. Magmatism occurs between the two plates, originating plutonic rocks.
Intraplate Orogeny
When a collision occurs between continents, the compression is transmitted into the interior of both plates. This often forms large faults, which in turn cause earthquakes. These orogenic belts do not necessarily have the linear form of those formed along the edge of a continent, but vaguely retain the shape of the sedimentary basin from which they originated. The folding of materials is less intense than in collision orogeny, and the resulting relief is also of lesser height.