Tectonic Effects on Volcanism

Global plate tectonics occur on such a grand scale and occur so slowly it seems that they may not have much effect on volcanism, but that is absolutely wrong. In fact, almost all examples of volcanism on our planet are the direct result of plate tectonics!

It's hard to believe that global plate tectonics is such a new idea; heck, it only became popular in the '60s! This modest scientific theory has become a robust geological fact and is the new cornerstone for a modern science. It is the shifting of large rigid land bodies atop a plastic ocean of lithosphere that results in the majority of the large-scale features we see: mountain belts, ocean basins, most topographic highs, and most topographic lows. Tectonics can even affect regional weather by producing mountains belts and valleys! The boundaries between these plates interact in one or two of three manners: they may slide past each other, they may collide, or they may move away from each other. The latter two are the ones we will concern ourselves with here.

Basaltic Volcanism and Divergent Boundaries

Basaltic volcanism (silica poor) occurs predominantly along spreading boundaries, termed Divergent. At the divergent boundary, the plates are splitting apart, thinning the overlying crust and allowing the magma to move up toward the surface. Active divergent boundaries all occur in ocean basins for reasons that will be outlined at the end of this section and have all been thoroughly mapped. The magma traveling up toward the surface needn't travel through much solid crust as there is not that much above it; merely a few kilometers. For this reason, the extruded lava at the surface is of roughly the same composition as the mantle which supplied it- that is, it is relatively poor in silica and rich in magnesium-oxide. The melt is basaltic and thus highly fluid, forming pillow lavas, submarine sheet flows, and in rare instances oceanic islands. Shortly after extrusion, however, the material begins undergoing a process known as Isostatic Adjustment. What this means is that denser material begins to sink while lighter material rises. The areas close to the rift are higher because they are warmer and thus less dense. As the plate carries material away from the ridge, however, the crust cools and becomes more dense causing it to sink and drop away from the ridge. Basalt is very dense and so it remains low and, at the other end of the plate, is usually subducted beneath a continental plate.

Intraplate Basaltic Volcanism

Not all basaltic volcanism occurs at plate boundaries, however. Intraplate volcanism- that is, activity not near a boundary- occurs due to massive magma plumes rising up from the mantle. These are known as hot spots and their origins are still being debated and studied, but their existence is undeniable. The rising magma reaches the surface and produces an enormous amount of extruded lava, which comes in the form of island chains. The plume does not move in any direction but up while the plate continuously moves laterally overtop of it. The result is a chain of islands that trace out the nearly exact direction and speed the plate moved over it. The most well known example is the Hawaiian-Emperor chain. Looking at the Hawaiian islands, you can see an obvious trail and if you follow this trail you can find a much longer train of submerged seamounts going on for quite some distance (submerged due to isostatic adjustment). Examples of hotspot volcanism include the Hawaiian Islands and Iceland, although Iceland also lies on a divergent boundary! The composition of hotspot volcanoes differs slightly from that of normal divergent boundaries, but it is not significant enough to bare mentioning here.

Convergent Boundaries

Along convergent boundaries, you have one plate being subducted beneath another and being remelted. There are a number of aspects which affect this remelting of the subducted crust. First of all, the subducted slab is likely to consist mostly of basalt with an overlying layer of sediments. Moreover, hydrothermal activity early in its life may have metamorphosed the slab, thus creating hydrous minerals, which will release water into the melt. When the slab is reheated, the silica (having a lower melting point than MgO) is more prone to melting. In addition to this, the presence of water actually lowers the melting point of minerals allowing more melt to be released at lower temperatures. As the melt rises through the overlying crust, it remelts crustal rocks and simultaneously begins to cool. This cooling crystallizes the magnesium out of the mix, thus creating a water-rich silica-rich melt whose silica content is rising as the melt does. For this reason, volcanoes over convergent boundaries are usually silicic and explosive (though exceptions do exist).

This, of course, is just a brief overview of the effects global plate tectonics has on volcanic activity. Everything can be looked at more intensively, but that will be the task of other articles. You should now, after reading this, have an idea of where you can generally find various styles of volcanism globally, and why. For more information on the specifics of each style, check the article list on the MIVO main page.