Plate Tectonics: Convergence, Subduction, and Dynamics
Plate Convergence and Subduction Zones
Oceanic-Oceanic Plate Convergence
Occurs when the oceanic lithosphere cools and thickens as it moves away from the ridge. Eventually, it breaks and subducts.
Characteristics of subduction zones:
- Great bow.
- Intense magmatism, resulting in a string of volcanic islands on the overriding plate.
- Tectonic erosion.
- Melting of the oceanic crust as it enters the sub-lithospheric mantle.
- The overriding plate experiences relatively low pressure, which means:
- Sediments are subducted with ease.
- A very deep ocean trench forms.
Oceanic-Continental Plate Convergence
Characterized by:
- The subducting plate sinks at a shallower angle.
- Magmatism and volcanism occur.
- Obduction may occur.
- The subducting plate exerts great pressure on the overriding plate, so that:
- Sediments are not easily subducted.
- Seismicity is very high.
- Thickening of the continental plate occurs.
Orogen: A chain of mountains formed by the compression and thickening of the continental lithosphere.
Continental-Continental Plate Convergence
Features:
- Subduction is interrupted after the continental collision.
- Both continental lithospheres are embedded and override each other.
- An orogenic suture forms between the two continental plates.
- Ophiolites can be found.
- Major faults are present due to the rupture of the continental lithosphere.
- Intense deformation and metamorphism occur in the suture zone.
Sub-Lithospheric Dynamics
During oceanic subduction, two effects occur on the subducting plate:
- Dehydration: The plate loses almost all the water that soaked the subducted sediment.
- Partial melting: The materials with lower melting points become liquid and form a magma of similar composition to granite, which tends to rise to the surface.
Thermal Plumes and D” Layer
Thermal plumes are formed by columns of less-dense material rising through the lower mantle. When a thermal plume reaches the base of the lithosphere, it heats it, and a hot spot, an area of volcanism, becomes noticeable on the surface.
Basalt Plateaus
Volcanism occurring at a hotspot ejects basaltic lava at high temperatures. The areas covered with lava are called basaltic provinces or basalt plateaus.
Intraplate Volcanism Linked to Faults: The Canary Islands
- Volcanic activity is discontinuous, which wouldn’t be possible with a thermal plume beneath.
- Volcanic activity is unevenly distributed and doesn’t fit the model of extinct volcanoes.
- Seismic surveys haven’t detected the presence of hot material under the Canary Islands, unlike typical hot spots.
The most accepted hypothesis is that Canarian volcanism is related to a set of faults that traverse the Atlantic lithosphere. These failures are caused by the collision zone between the African and Eurasian plates, which led to the Atlas Mountains.
Geological Processes in Intraplate Continental Lithosphere
When a thermal plume is situated beneath the continental lithosphere (which is thicker and colder than oceanic lithosphere), it cannot easily puncture it. The continental lithosphere becomes less dense, expands, and begins to blister due to the pressure of the plume. This uplift causes stretching in the crust, and it begins to fracture.
In the fracture, or rift, basaltic magmas begin to inject, forming oceanic crust. The rift becomes an incipient ocean, and the process of extension begins.
The Wilson Cycle
In 1960, geologist J. Tuzo Wilson suggested that an ocean eventually closes. His theory was based on:
- Rifting: The beginning of the rupture of a continent.
- Opening: An ocean opens, separating a continent into two new continents.
- Subduction: The edges of the continents approach each other.
- Collision: The cycle ends with continental collision.
Rifting in the Iberian Peninsula
The Iberian Peninsula is currently undergoing a general uplift, visible both on the coast and inland. River terraces and raised erosive plains indicate that water systems are adjusting to ascending plains. There is a clear tendency towards fracturing.