Volcanic arcs: it’s all in the melt 8 October 2010Posted by admin in current research, geoscience, volcanology.
We all know that volcanic arcs are related to subduction zones: the areas of the Earth’s surface where an oceanic lithospheric plate comes up against another plate and moves beneath it, ultimately being re-absorbed into the mantle in a vast geological recycling scheme. The oceanic rock is full of water, which begins to be released as it reaches greater depths and becomes subject to increased heat and pressure, entering the overlying mantle wedge that lies between it and the continental lithosphere and lowering its melting point, producing melt that ascends to feed volcanoes above the subduction zone. In offshore island arcs and continental volcanic arcs the line of volcanoes is always offset some way from the point at which the subducted plate makes its dive, which is marked by an oceanic trench.
Various explanations have been put forward for why the volcanoes appear where they do, usually connected to theories as to the locations of the points of highest temperature in the zone where the upper surface of the subducted slab and the mantle wedge are in contact with each other. No-one has really got to the bottom of it, however (so to speak). But two researchers from the University of Oxford Department of Earth Sciences, Philip C. England and Richard F. Katz, have been working on this problem in a new way, by applying a mathematical model of heat transport in this zone to establish where the areas of highest temperature, and thus of melt production, occur. Their conclusion is that the locations of subduction zone volcanoes can only be explained if they emerge above regions in which mantle is melting in the absence of water. Hydrous melting — melting associated with the presence of water — pervades the mantle wedge, but the conventional models of arc formation suggest that there is a particular point at which the degree of hydrous melting increases rapidly because of conditions of temperature and/or pressure, and that the arc front forms above this point. Yet these processes of hydrous melting occur not in a restricted area of the wedge but across a broad region of the mantle core, raising the issue of how the melt then becomes concentrated to feed a relatively narrow volcanic arc. England and Katz avoid that difficulty by invoking the crystallization of rising magma as it reaches the thermal boundary layer at the top of the wedge. This deflects the magma back towards the trench and into the ‘nose’ of the mantle wedge, whence it makes its way upwards by thermal erosion, opening pathways for hydrous melt which drives the volcanism of the arc.
The paper, ‘Melting above the anhydrous solidus controls the location of volcanic arcs’ by can be found in the 7 October 2010 issue of Nature. It’s subscribers only of course: the link below will give you the abstract, to read further you need to log in or pay up.
- Philip C. England & Richard F. Katz, ‘Melting above the anhydrous solidus controls the location of volcanic arcs’, Nature, 700–703 (07 October 2010), doi:10.1038/nature09417. [abstract]