2 minute read

Magma

In geology, magma refers to molten rock deep within Earth that consists of liquids, gases, and particles of rocks and crystals. Magma has been observed in the form of hot lava and the various rocks made from the solidification of magma. Geologists have created magmas (artificial melts) in the laboratory to learn more about the physical conditions in which magma originated and its composition. Magma is the source of igneous rocks; it can intrude or force itself into surrounding rock where it cools and eventually hardens. These rocks are called intrusive igneous rocks. If magma rises all the way to Earth's surface it will extrude (push out), flowing or erupting out at the surface as lava, forming extrusive igneous rock (also called volcanic rock). Magma and the rocks it creates have similar chemical compositions.

Magma is generated within Earth's mantle, the thick layer between Earth's crust and outer core. Rock found deep within the crust is extremely hot, soft, and pliable, but rock does not become liquid until much deeper in the upper mantle. Pockets, or chambers of magma, can originate at various depths within the earth. The composition of the magma varies and indicates the source materials and depth from which they originated. Silicon dioxide (SiO2) is the predominant ingredient in magma. Other ingredients include aluminum oxide, iron, magnesium, calcium, sodium, potassium, titanium, manganese, phosphorus, and water.

There are three basic types of magma, each having a characteristic origin and composition: basaltic (the most common, originating in the lower crust/upper mantle), rhyolitic (originates in the oceanic crust), and andesitic (most originate is the continental crust). New magma is formed by rocks melting when they sink deep into the mantle at subduction zones. The chemical composition, temperature, and the amount of dissolved liquids and gases determine the viscosity of magma. The more fluid a lava mixture is, the lower the viscosity. As magma or a lava flow cools, the mixture becomes more viscous, making it move slowly. Magmas having a higher silica (SiO2) content are very viscous and move very slowly.

Magma has the tendency to rise because it weighs less than surrounding hard rock (liquids are less dense than solids) and because of the pressure caused by extreme temperature. The pressure is reduced as magma rises toward the surface. Dissolved gases come out of solution and form bubbles. The bubbles expand, making the magma even less dense, causing the magma to rise faster. The magma exerts a great deal of pressure on weak spots and fills up any cracks produced by the continual shifting of the earth's crust. On its way up toward the surface, magma can melt adjacent rock, which provides a suitable environment for the development of metamorphic rocks. When magma erupts as lava, its gases are released at the surface into the atmosphere or can be trapped in the molten rock and cause "air bubbles" in rock. The gases can also create violent explosions, throwing debris for miles around.

See also Volcano.


Resources

Books

Hamblin, W.K., and Christiansen, E.H. Earth's Dynamic Systems. 9th ed. Upper Saddle River: Prentice Hall, 2001.

Hancock P.L., and Skinner B.J., eds. The Oxford Companion to the Earth. Oxford: Oxford University Press, 2000.

Woodhead, James A. Geology. Boston: Salem Press, 1999.

Periodicals

Petford, N., Cruden, A., McCaffrey, K., and Vigneresse, J-L. "Granite Magma Formation, Transport and Emplacement in the Earth's Crust." Nature (December 2000): 669-673.

Additional topics

Science EncyclopediaScience & Philosophy: Macrofauna to Mathematics