What causes a volcano?

Answer 1

Under what is the crust on earth is the mantle, under that the outer core which contains magma. Now, when that magma builds up pressure under the crust, it has to go somewhere, so it goes up and slowly pushes some of the crust up, that's what makes the mountainous part, after that the magma goes out (that's called lava once it is out) and all of that pressure blows off the top and lava comes out. Also, the sides can have lava come out of them, those are known as vents. Tectonic plates usually spread apart and that's what lets it through. When they go away from each other, that a divergent boundary, when they go together, a convergent boundary. When one plate goes under another, that's called the subduction zone, usually found by the ocean. This is what created the Ring Of Fire, a string of volcanoes by the ocean. Divergent boundrys mostly make new ocean floor. I hope I answered your question.

Answer 2

all the presure that the volcano has inside and when the lava is about to erupt
Magma from the mantle rises into the mountain and breaks through the cap of the mountain forming lava and ashes