Plate Tectonics: Convection, Subduction, and Plate Dynamics
The Model of Convection
He believed that the mantle was smooth and capable of convection currents pushing from below the lithospheric plates, which float. The hot and less dense material of the lower mantle would rise to the surface areas of the lithosphere, and as the material cooled, it would sink back down to the mantle zone where it would heat up again. The areas where convection currents rise would coincide with the ridges, and the areas where they decrease would coincide with subduction zones. This model was discarded when it was discovered that the mantle is solid but plastic.
Asthenosphere Model
This model considered the existence of a sublithospheric layer, the asthenosphere, where partially melted rocks allow the lithosphere to glide slowly over it. It proposed that convection currents only occur in the asthenosphere and provoke the rise of molten rock at the ridges. As this rock solidifies in the dorsal axis, it would cause a lateral thrust on the plates and their mobility. This model was discarded when it was determined that the asthenosphere, as described, does not exist.
The Model of Deep Subduction
This model poses a diffuse convection that affects the whole mantle. The downward flow would be formed by the cold, dense oceanic lithosphere that is introduced in subduction zones to the core-mantle boundary. In doing so, it pulls the plate and causes movement. The upward flow would be formed by plumes of hot material that would rise to the lithosphere, causing hot spots. In this model, the ridges are not the source of plate motion but the consequence of their movement.
Convergent-Type Boundaries
- Ocean-Ocean Convergence: The older, denser plate subducts at a steep angle. Earthquakes that occur are of low intensity, but there is very intense magmatic activity. The accumulation of volcanic products forms aligned groups of volcanic islands shaped like the arc of the Aleutians.
- Ocean-Continent Convergence: The oceanic plate subducts under the continental plate at a tight angle and with strong resistance, producing large earthquakes in the region. This usually results in a mountain range parallel to the edge, formed by the folding and rising of sediments accumulated in the trench.
- Continent-Continent Convergence: This may occur once the oceanic lithosphere that existed between them has been completely subducted. Neither of the two plates enters the mantle due to the low density of the continental lithosphere.
Dynamics of Transform Faults
Transform faults are characterized by high seismic activity due to the large amount of elastic energy associated with the friction of the two plates. Although the majority of transform faults are on the ocean floor, some, like the San Andreas Fault, affect the continental lithosphere.
Internal Dynamics of Plates
Most geological processes are linked to the interactions produced at the edges of the plates. However, there are some events taking place in areas far from these edges. To explain these, plate tectonics proposes the existence of hot spots, which implies the presence of hot material from the mantle. The partial melting of the hot rock as it enters an area close to the surface can form magmas that give rise to volcanic islands.