Lexikon

magma is molten rock that is found beneath the surface of the Earth. Geologically magma is a complex high-temperature silicate containing fluid substance, containing suspended crystals and gas bubbles. Magma, as liquid, preferentially forms in high temperature, low pressure environments within several kilometers of the Earth's surface. It has variable density, temperature and viscosity. Magma often collects in a magma chamber inside a volcano. When magma cools and solidifies beneath the Earth's surface, it forms what are known as intrusive igneous rocks. When it reaches the Earth's surface, it flows out as lava and forms (volcanic) rocks. Magma compositions may evolve after formation by fractional crystallization, contamination, and magma mixing. Fractional crystallization is one of the most important geochemical and physical processes operating within the Earth's crust and mantle. Fractional crystallization in silicate melts (magmas) is a very complex process compared to chemical systems in the laboratory because it is affected by a wide variety of phenomena. Prime amongst these is the composition, temperature and pressure of a magma during its cooling. According to the SiO₂ composition the magma types are as follows: 1) Ultramafic (picric) SiO₂ < 45%; 2) Mafic (basaltic) SiO₂ < 50%; 3) Intermediate (andesitic) SiO₂ ~ 60%; 4) Felsic (rhyolitic) SiO₂ > 70%. Viscosity is a key melt property in understanding the behaviour of magmas. More silica-rich melts are more viscous. Dissolution of water drastically reduces melt viscosity. Higher-temperature melts are less viscous. More mafic magmas, such as those that form basalt, are hotter and less viscous than more silica-rich magmas, such as those that form rhyolite. Low viscosity leads to gentler, less explosive eruptions. Solidification of magma results igneous rock. In terms of modes of occurrence, iigneous rocks can be either intrusive (plutonic igneous rocks), extrusive (volcanic) or hypabyssal (subvolcanic).