Heat rises. This is the reason, in a nutshell, for the presence of volcanoes on the earth. Mountainous manifestations of vents and cracks in the earth's crust, volcanoes provide release for the heat that builds up in the earth's core. A volcanic eruption occurs when the heat pressure underneath a particular vent becomes too strong for the crust to contain, and depending on the size of the vent and the amount of pressure an eruption can be minor (just a dribble of lava that seals off the volcano when it dries) or cataclysmic (such as the Krakatoa or Vesuvius eruptions).