Subduction Zones and Plate Tectonics: Earth’s Dynamic Processes
Subduction Zones: Collision Between Plates
Subduction of Oceanic Lithosphere Under Oceanic Lithosphere
The result is the formation of a deep oceanic trench. When the subducting plate is partially melted in the mantle, magma originates. Part of it rises to the surface through cracks and reaches the subducted plate, forming an arched archipelago of islands with great volcanic activity.
Subduction of Oceanic Lithosphere Under Continental Lithosphere
This phenomenon produces an active continental margin orogen with great seismic and volcanic activity (e.g., Andes). Orogens are mountain ranges extending hundreds of thousands of kilometers along the converging plate edges. They arise through orogeny, a process consisting of the folding of large quantities of sediments from the erosion of nearby continents, accumulated in deep oceanic trenches.
When the subducting plate reaches a certain depth, it can melt, forming magma. Part of this magma escapes through fractures, originating a volcanic arc in continental sediments. The sediments accumulated in the oceanic trench are compressed, fractured, folded, and pushed up the lithospheric plate. All of this forms a pericontinental orogen, a continental ridge with intrusions of volcanoes and seismic activity, such as the Andes.
Intercontinental Collision
As the subduction process progresses, the mixed plate (with oceanic and continental lithosphere) is consumed. The ocean located between the two continental masses disappears, shrinking until the two masses collide, resulting in an intercontinental orogen. The accumulated sediments fold, fracture, and form an accretionary prism, whose size increases until it emerges and gives rise to a mountain range.
Initially, part of the subducted materials melts, transforming into magma. However, when the collision between the continents occurs, there is no volcanism in this type of mountain range, although seismic activity does take place.
Continental Drift
Alfred Wegener hypothesized continental drift based on the displacement of continents. He presented consistent evidence (paleontological, paleoclimatic, and geographic). The theory of plate tectonics supports Wegener’s theory.
The Wilson Cycle
The Wilson cycle explains an evolutionary cycle of opening and closing of oceanic basins and changes in the distribution of continents and oceans over time.
- The same tectonic processes that provoke the fragmentation of a supercontinent and its dispersion into erratic continental blocks cause the reunification into a new supercontinent. These cycles last approximately 500 million years.
- There was a large supercontinent called Rodinia that fragmented. The resulting fragments reunified 250 million years ago into another supercontinent, Pangea, which gave rise to the current distribution of continents.
- The theory of plate tectonics explains the movement of plates relative to each other and shows how continents are transported by the expansion of the ocean floor and the disappearance of oceanic lithosphere in subduction areas. Fossil evidence is used as proof of these events, along with paleogeographic data.
Plate Tectonics
Resources Generated by Internal Dynamics
- Mineral Deposits: Part of the exploited minerals (gold, silver, copper, zinc) are located in subduction zones or hot spots. Magmatic fluids dissolve minerals, concentrating them in the Earth’s crust. The magmatism of oceanic ridges also contributes to the creation of mineral deposits.
- Fossil Fuels:
- Coal: Formed by the burial of plant remains in sedimentary basins without oxygen, under high pressure and temperature due to inundations.
- Oil and Gas: Formed by the accumulation and burial of plankton in sediments without oxygen, under high pressure and temperature.
- Geothermal Energy: Used in places with volcanic activity, such as Iceland, to supply energy to the population.
Paleomagnetism
Paleomagnetism is the ancestral magnetism preserved in primitive rocks formed at the same time as volcanic rocks. Volcanic rocks are the best for paleomagnetic study because when some lavas solidify rapidly, their minerals are magnetized according to the direction of the Earth’s magnetic field (e.g., basalts).
This evidence justifies the theory of seafloor spreading based on the symmetry of magnetic bands. The magnetic signature of the basaltic blocks on either side of an oceanic ridge axis indicates that they were formed at the same time. This highlights that the basaltic rocks that constitute the ocean floor are formed at the ridges.