Hi James, In the most simple terms, when rock melts in the deep crust to form magma, this magma is less dense than the surrounding rock and the magma eventually pushes it's way to the surface through fissures and cracks in the surrounding rock. As the magma rises, pressure decreases and gas bubbles may begin to form and escape from the magma, further driving the rise of the magma to the surface, resulting in an explosion or extrusion of lava at a volcanic vent. Our challenge at the observatory is to determine "when". To learn about volcanic eruption precursors and monitoring techniques, visit this site: http://volcanoes.usgs.gov/about/index.php -Janet -------- JamesM. Kenyon wrote: What is the current thinking on how Volcano`s erupt and why?