How do Volcanoes erupt?

Volcanoes are formed when there is a great stress inside the earth’s crust and the pressure of magma reaches so high that it finds its way upward causing an eruption. Sometimes the magma chamber breaks its sides and only reaches to the flank or summit of volcano and sometimes reaches to the surface causing an eruption. In the volcanoes which are not much active supply the magma from the deeper parts of the earth is not sufficient to cause eruption so most of the times crystals forms and they go down at the bottom releasing gas in the form of bubbles which come to the top of chamber and sometimes the pressure created by these gas bubbles are enough to erupt a volcano or just wait there for a little more push which they get from the next new magma formed within the earth. The heat just triggers the magma for eruption which was there for a long time.Magma chambers are located very close to the surface and usually just under the volcano. The pressure of the cooler earth at the crust keeps it there, but the magma is still hot. As mentioned before, some of what makes up magma are gasses. These gasses are constantly expanding, and since they are the least dense part of magma, they contribute the most pressure that presses against the earth. The gas forms bubbles that expand and rise to the top of a magma chamber. When the pressure from them gets to be more than that of the earth around the magma, cracks form and the magma is able to escape, causing an eruption. If the earth is very weak or thin, the pressure can cause it to explode, sending magma--called lava above ground--and rocks into the air. These are the more powerful eruptions that are seen. The magma dries quickly when exposed to the cool air, hardening and eventually stopping more magma from spewing out.