Of the two major types of Volcanoes, andestic and basaltic, the two typical volcanoes begin life when a mass of low-density magma forces its way to the surface. When the density of the rising magma is the same as that of the surrounding rock, it gathers in a magma chamber. Any rise in pressure in the chamber may now push the magma upwards through cracks in the overlying rock. As the magma traveling up a crack approaches the surface, the pressure from the overlying rocks reduces; gases are released from the magma and expand so suddenly that an explosion rips open a funnel shaped vent (called a diatreme) to the surface. The lava that blasts
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