Volcanoic activity occurs when two tectonic plates collide, a subduction zone is created. Lower pressure above the submerged plate and below the top plate causes the rocks in the mantle zone to begin to melt. The lower density of the magma causes it to rise to the earth surface, pressure builds up around, this magma forces it's way through weak area of the earth, especially areas with vent and fissures and explosion occurs. The magma that eventually erupts to the earth surface is called lava.
Volcanic activity occurs due to partial melting of the rock in the earth's mantle. The melted part is a lower density than the rest of the mantle material, and it flows toward the surface causing a buildup of pressure which is forced out of the volcano and the area erupts and lava spews forth from the opening. Volcanoes erupt because of density and pressure.
The lower density of the magma is relative to the surrounding rocks, which causes it to rise. It will flow to the surface or to a depth that is determined by the density of the magma and the weight of rocks above it. As the magma rises, bubbles start to form from the gas dissolved in the magma. The gas bubbles exert tremendous pressure. This pressure helps bring magma up and forces it into the air.