Photoglossary

Magma and Lava   Magma is hot, molten rock that forms beneath the Earth's surface in the lower crust and upper mantle. Once magma erupts onto the surface it is referred to as lava. Magma is not just liquid rock material, it also contains solid mineral crystals as well as dissolved gases, such as water vapor and carbon dioxide. The most important chemical compound in magma is silica (SiO2). The amount of silica in a magma greatly determines its viscosity (resistance to flow), and viscosity influences the type of eruption that may occur when the magma reaches the surface. Even before solid crystals begin to form in liquid magma, silica molecules begin linking together, and these links increase its viscosity. Therefore, the higher the silica content in a magma, the greater its viscosity. Temperature is also a factor - the hotter the magma, the less viscous it will be. There are two basic types of magmas--basaltic and granitic. Basaltic magmas are comprised of only about 50 % silica, whereas granitic magmas may contain as much as 70% silica. Basaltic magmas, which have the lower silica content, are more fluid and can flow for considerable distances before solidifying. Granitic magmas, which have a higher silica content, are thick and sluggish and tend to plug the vent from which they are extruded. Dissolved gases such as water vapor and carbon dioxide can comprise as much as 14% of the volume of a magma. The amount of these gases present in a magma also influences the nature of a volcanic eruption. Hot, fluid, basaltic magmas allow gases to readily escape, and these lavas flow freely onto the surface from fissures in long streams or sheets. The cooler, more viscous, granitic magmas, however, inhibit the escape of dissolved gas so tremendous pressure builds up within them. This results in more violent eruptions.   Photo Credit: J.D. Griggs, USGS   Io Home Page Photoglossary Home Page