Do you know of any factors which affect the distribution of volcanoes?

Tay Ser

Certainly, plate tectonics affects the distribution of volcanoes. Volcanoes form where magma is produced, and these are very specific types of places. They include subduction zones, spreading centers, and hotspots. If you look at a map of the Earth's volcanoes you'll see that they aren't all over the place. Instead, they fall into very discrete zones. These also tend to correspond with zones of seismicity (although there are a number of areas that are seismically active but don't have volcanoes). The most simple answer is that volcanoes are caused by the fact that magma is produced in the upper mantle of the Earth. Magma is what geologists call molten rock before it makes it to the surface. Magma is produced in 3 main locations. One is subduction zones where one of the Earth's plates dives under another. The sediments and the plate itself give off gases which rise through the overlying mantle and help to cause it to partially melt, producing the magma that then erupts. These are often called arc volcanoes because they lie along arcuate lines that parallel the subduction zones. Another place where significant magma is produced is at spreading centers. Most of these are in the oceans, but some are on continents. Here, two tectonic plates spread apart. Mantle material wells up to fill the space, and in the process it partially melts in the lower pressure it finds itself in. This produces magma which can well up to the surface and erupt to produce new crust. Another place where lots of magma is produced is at hotspots. We don't really understand hotspots very well. We know that they don't care that the plates are moving around up on the surface so almost certainly they are deep structures. We also know that they can last a long time - the Hawaiian hotspot has been active for at least 70 million years. That's a quick answer to a very complex question. For more information you should check just about any introductory geology book or perhaps a volcanology book.

Sincerely,
Scott Rowland

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