Under the ground, it is very hot, and there is a lot of pressure (the force that squeezes something into a space smaller than that thing is.) In fact, it is so hot, that rock melts into magma, which is what lava is called when it is underground. Also gases, such as air go up when theyre hot. Slowly the hot gases push the magma and the ground up, making a volcano. At one point there isso much pressure, and it is so hot inside, that the volcano explodes, in other words, erupts.

Well, this is a good question. Basically magma moves from depth to the surface because it is less dense than the overlying rocks. Because it is less dense it moves up, just like when you put Helium in a balloon. Helium is less dense than air...so when you fill the balloon, it is lighter than the air that surrounds it and so it goes up.

Magma (lava that hasn't yet exploded out of a volcano) is lighter than rocks, so it oozes up on top of the rocks and needs to escape, so volcanoes erupt to let the magma escape.