What is the connection between volcanoes and earthquakes?
Volcanoes are the vents where the earth is letting out molten magma from the hot interior of the earth. The magma is contained in huge pressurized chambers that are sealed up - usually. When the seal breaks the magma which is under intense pressure bursts through the opening - like when you pop a balloon with a needle. Or if enough pressure builds up it can just explode through the weakest point - like when you overinflate a balloon to the point of popping. Earthquakes are caused by the movement of the huge plates of the earth's crust along the top of the magma. At one end of a plate the magma is constantly being released - puching the plates apart at that point. At the other end, the plate material is being pushed underground deeper into the magma to be melted.The crust itself is pretty irregular. Sometimes the pressure from the movement of the plates gets "stuck" and sits for some time until the pressure from the expanding end of the plate overcomes the resistance and it breaks free suddenly. What should have been a steady 1-3 inches per year of movement can build up to moving 6, 12, 18 inches or more of movement in an instant causing an earthquake. The amount of sudden slip has a lot to do with the severity of the quake. The shockwave of such a massive quick movement ripples through the land outward from the center of the slip (epicenter). This shockwave can ooccasionally break open an otherwise sealed vent to a magma chamber allowing a volcano which was previously inactive to suddenly become active again.
The location of earthquakes and volcanoes support the theory of plate tectonics because the areas that are seismically active occur along the edges of the Earth's plates. This is because lava can push to the surface between the plates, and plates that overlap can cause both volcanoes and earthquakes.