Question: why do volcanos occur at plate boundaries that slide against each other?

James Verdon answered on 22 Jun 2012:

Hi mrspolly,
There are two main types of plate boundary that can create volcanoes. At ‘extensional’ boundaries, the plates are pulling apart. This leaves a gap between the plates, and magma can upwell into the gap, eventually reaching the surface and flowing out at a volcano. These volcanoes tend to be less violent. Ethiopia (the East African Rift) and Iceland are examples of extensional volcanoes.

Where plates collide, one plate can be forced below the other one. The plate that is forced underneath gets hotter as it is pushed down, until eventually it begins to melt. The molten material is less dense, so it rises through the overlying plate, punching its way to the surface through cracks and faults. These types of volcanoes are usually more violent and explosive! Examples of collisional volcanoes are the volcanoes in Italy (Vesuvius, Etna) and Greece (Santorini, Nisyros), where the African is being pushed under the European plate, as well as the Andes in South America, and Mount St Helens in USA, where the Pacific plate is being pushed under both North and South America.

A third type of volcano doesn’t happen at plate boundaries, but at ‘hotspots’, where heated plumes of mantle rise from the boundary with the core, and rise all the way up. Their heat is enough to melt the mantle, and again the molten material punches through the crust to form a volcano. Hawaii is probably the most well known example of a hotspot volcano