Plate Tectonics Questions Long
Back-arc basins are elongated, narrow, and deep oceanic basins that form behind volcanic arcs in subduction zones. They are typically found on the opposite side of the volcanic arc from the subduction zone and are associated with the extensional tectonic regime. The formation and characteristics of back-arc basins can be explained through the process of subduction and the interaction between tectonic plates.
Back-arc basins are formed as a result of the subduction of an oceanic plate beneath another oceanic or continental plate. When an oceanic plate subducts beneath a continental plate, the denser oceanic plate sinks into the mantle, creating a deep trench. This subduction process generates intense compressional forces that cause the overlying continental plate to deform and uplift, forming a volcanic arc on the continental side of the subduction zone.
As the oceanic plate continues to subduct, it undergoes partial melting due to the increasing temperature and pressure in the mantle. This molten material, known as magma, rises through the overlying mantle and crust, eventually reaching the surface and erupting as volcanoes along the volcanic arc. These volcanic eruptions are often associated with explosive activity and the formation of stratovolcanoes.
Simultaneously, the subduction process induces extensional forces behind the volcanic arc, leading to the formation of a back-arc basin. These extensional forces cause the lithosphere to stretch and thin, creating a region of crustal rifting. As the crustal rifting progresses, the lithosphere becomes progressively thinner, and the underlying asthenosphere rises closer to the surface. This uplift of the asthenosphere results in the formation of a new oceanic crust, which fills the gap created by the stretching and thinning of the lithosphere.
The characteristics of back-arc basins include their elongated shape, narrow width, and significant depth. They are often associated with a series of volcanic islands or seamounts parallel to the volcanic arc. The volcanic activity in back-arc basins is typically less explosive and more effusive compared to the volcanic arc. This is because the magma generated in back-arc basins is derived from partial melting of the mantle wedge above the subducting plate, which has a higher water content and lower viscosity compared to the magma generated in the volcanic arc.
Back-arc basins also exhibit distinct tectonic features such as normal faults, grabens, and horsts, which are a result of the extensional forces acting on the lithosphere. These features contribute to the overall basin shape and can be observed through seismic studies and bathymetric mapping.
In conclusion, back-arc basins form as a result of the subduction of an oceanic plate beneath another oceanic or continental plate. The extensional forces induced by the subduction process lead to the stretching and thinning of the lithosphere, creating a region of crustal rifting and the formation of a back-arc basin. These basins exhibit distinct characteristics such as their elongated shape, narrow width, significant depth, and volcanic activity that is less explosive compared to the volcanic arc.