Why Does a Volcano Erupt?
A volcano erupts when the pressure within the molten magma below the surface becomes stronger than the strength of the rocks on the surfacethat make up the volcano. The magma then finds the path of least resistance to the surface and erupts. That pressure comes both from the buoyancy of the magma and from gas bubbles inside the magma that want to expand. The magma pressure comes from the expansion of gas bubbles that are within the magma. The heat source for all erupted materials comes from natural radioactive decay within the Earth. The concentration of radioactive elements in the Earth is not really very high, but the volume of the Earth is great enough so that there is enough of a contribution of heat from the decay to produce melting. The material that melts (magma) has a lower density and it migrates to the surface to erupt.
