Question: Do you know why volcanoes would be anywhere other than on plate boundaries? would i be right in saying that they usually are formed on plate boundaries?

David Pyle answered on 18 Mar 2011:

Yes, volcanoes are mostly on plate boundaries: where the plates pull apart (ocean ridges), and where the Earth melts by pressure-release as the mantle rises; and in subduction zones, where one plate subducts beneath another, and water is released into the mantle from the downgoing plate. The addition of water lowers the melting point of the mantle.

But we do see volcanoes away from plate boundaries: Hawaii being the best example. These are thought to form above ‘hot spots’ – parts of the convecting mantle which are a little hotter than the surrounding mantle, and which rise (because they are less dense) and melt by pressure-release melting. The hot spot idea has been around for a while, but is vigorously contested by some geoscientists. Whether ‘hot spots’ actually are hotter, or not, is still under some debate: some could simply be richer in water, for example. These density anomalies are usually thought to form at or near the bottom of the convecting mantle (i.e. at 3000 km depth, or so), so they will be distributed randomly, and not simply associated with plate boundaries.