Question: What is the relationship between volcanoes, earthquakes, and plate-tectonics?

Kate Dobson answered on 8 Nov 2014:

Plate tectonics is the theory that describes how the rigid plates that make up the surface of the Earth move. If you look at a map of the tectonic plates, and one of the earthquakes and volcanos you’ll see that most earthquakes and volcanoes occur along the plate margins.

So why is this? The plates are rigid, so where they touch they do one of three things: move towards each other (destructive boundaries), move apart (constructive boundaries), or move past each other (conservative plate boundaries).

At constructive boundaries, plates move apart and hot material from the mantle beneath flows up to fill the gap creating long strings or chains of volcanoes (one example is the Mid Atlantic ridge). These chains of volcanoes make new crust on both plates; crust which is relatively thin and dense. As the plates continue to move apart and more material is added, the older material cools and sinks. The floors of most of the oceans are oceanic crust formed at constructive boundaries.

At destructive margins two plates are moving together. What happens depends on if the plates are made up of thin “oceanic” crust or thick “continental” crust. When both plates are oceanic the older, cooler, and thicker plate usually pushes down underneath the other. It is “subducted”. We get lots of earthquakes in this subducting pate as it bends and cracks as it sinks, and an “arc” of volcanoes along the edge of the overriding plate. The volcanoes of the Caribbean (including the volcanos on the British Overseas Territory of Montserrat).

If an oceanic plate and a continental plate collide the oceanic plate is subducted and sinks beneath the thick continental plate. We usually see a mountain belt and volcanos along the edge of the upper plate, and earthquakes in the down going plate. The Andes are this kind of plate boundary, there are lots of volcaoes and lots of earthquakes.

When two continental plates collide, neither can be subducted. As they collide they both scrunch up forming large mountain ranges like the Himalayas, and we get lots of earthquakes. In the Himalayas the crust can be over 70km thick. In the oceans it is only about 5-10km thick.
Finally, at conservative boundaries the plates slide past each other, but the “edges” of the plates are very jagged and rough so there is a lot of friction and the movement is jerky, so we have lots of earthquakes (one example is the San Andreas Fault in California).

You can find more information and some good pictures showing the structure of the crust and what happens at the plate boundaries here. http://www.geolsoc.org.uk/Plate-Tectonics/Chap3-Plate-Margins