Attila Kilinc, head of the geology department at the University of Cincinnati, offers this answer. Professor Kilinc has been studying volcanoes in Hawaii and Montserrat.
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Click on Image to Enlarge Image: Hawaii Center for Volcanology MOLTEN ROCK, or magma, and gasses spew from the mouth of a volcano in the Hawaiian islands. |
As rock inside the earth melts, its mass remains the same while its volume increases--producing a melt that is less dense than the surrounding rock. This lighter magma then rises toward the surface by virtue of its buoyancy. If the density of the magma is less than that of the surrounding and overlying rocks, the magma erupts onto the surface either by pushing through weak spots or emerging through existing openings.
Magmas also contain dissolved volatiles such as water, sulfur dioxide and carbon dioxide. When magma is six kilometers below the surface, about 5 percent of its weight is dissolved water. As this magma moves toward the surface, excess water separates from the magma in the form of gas bubbles. As the magma moves closer to the surface, more and more water exsolves from the magma. If the magma is collecting in a location beneath the earth's surface without a vent, like a magma chamber or conduit, then the gas/magma ratio increases. When the volume of gas bubbles reaches about 75 percent, magma disintegrates into pyroclasts (partially molten and solid fragments) and erupts explosively.
The third process that causes volcanic eruptions is an injection of new magma into a chamber that is already filled with magma of similar or different composition. This injection forces some of the magma in the chamber to move up in the conduit and erupt at the surface.
