Welcome to Earth Science Week, everyone! Why not start off with a bang?
At the end of last week, there was some buzz in the geoblogosphere and Twitter about a new paper in Nature that claims to have solved a riddle that has plagued igneous petrology for quite some time. That question is "why are volcanic arc above subduction zones so narrow and what controls their location relative to the trench." (OK, so, that is really two questions.) "Melting above the anhydrous solidus controls the location of volcanic arcs" by Philip England and Richard Katz in the latest issue of Nature tries to solve these conundrums.
However, before we get into those two questions, maybe some brief review on subduction zones. Subduction zones are convergent tectonic boundaries where one tectonic plate is shoved under another. The downgoing plate being subducted is always oceanic due to its denser, thinner nature. The overriding plate can be oceanic or continental. The current theory is that as the downgoing slab is subducted, it heats up as it is shoved into the mantle, so at a certain depth, the slab starts to dehydrate. All the water contained in the sediment and hydrous minerals (like amphibole, serpentine, talc and more that make up altered oceanic basalts) is released, adding water to the overlying mantle wedge (see below), causing the melting point of that mantle to go down - and causing it to melt and form magmas (flux melting). These magmas rise into the lithosphere and crust and eventually erupt on the surface as arc-related magmas. Most arc magmas are what we would classify as calc-alkaline magma (where the magma's CaO content is less than total K2O and Na2O). So, subducting slab releases water as the slab goes down, this prompts melting in the overlying mantle and magmas rise into the overriding plate.
