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Tectonic Plate Boundaries
In plate tectonics, subduction takes place at convergent boundaries when one tectonic plate moves under another and sinks into the Earth’s mantle. Any area of the Earth’s crust where two plates move towards each other and subduction can occur is called a subduction zone. The typical rate of subduction is 2-8 cm per year. Subduction zones usually involve a continental plate and a much lighter oceanic plate. These zones have are known for earthquakes, volcanoes, and the formation of mountains.
Subduction as it occurs today is fairly well understood, but its origin remains a matter of continued study. One recent hypothesis states that ‘mantle upwelling and similar thermal processes, combined with an impact from an extraterrestrial source, would give the early Earth the discontinuities in the crust for the subduction of the denser material underneath lighter material.’ Another model of the initiation of subduction presumes that the difference of density between two adjacent lithopsheric slabs is sufficient to lead to the initiation of subduction. The resulting strain would lead to the rotation of the contact zone between the slabs to dip towards the lighter slab, and the dip would be reduced until offset along the contact zone would be enabled. Supporting experiments suggested that the initiation of subduction started with the penetration of the denser ductile “lithosphere” below its lighter counterpart. Consequently, the lighter “lithosphere” was uplifted, then collapsed on the denser slab, increasing the load on its edge and driving the denser sequence further under the lighter slab. It was additionally presumed that once the denser “lithosphere” was set below the lighter one, it underwent conversion to eclogite which increased its density and drove it to subduct into the “asthenosphere”. The rate of this part of the subduction process was determined by friction. Reduction of slab friction in nature could result from serpentinization and other water-related processes.
The effects of subduction can be seen all over our planet. Mountain ranges that were formed when ancient oceans were farther inland, volcanic islands, and recent tsunamis are all examples of what can happen along a subduction zone.
We have written many articles about subduction for Universe Today. Here’s an article about the continental plates, and here’s an article about subduction zone.
If you’d like more info on subduction, check out the U.S. Geological Survey Website. And here’s a link to NASA’s Earth Observatory.
We’ve also recorded related episodes of Astronomy Cast about Plate Tectonics. Listen here, Episode 142: Plate Tectonics.
Sources:
http://en.wikipedia.org/wiki/Subduction
http://www.windows2universe.org/earth/interior/subduction.html
http://myweb.cwpost.liu.edu/vdivener/notes/subd_zone.htm
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