magma

magma A hot, silicate, carbonate, or sulphide melt containing dissolved volatiles and suspended crystals, which is generated by partial melting of the Earth's crust or mantle and is the raw material for all igneous processes. The melt component of silicate magmas, by far the most common magma type, comprises a disordered mixture of single Si—O tetrahedra and chains, branching chains, and rings of Si—O tetrahedra, between which are located randomly positioned cations (e.g. Ca²⁺, Mg²⁺, Fe²⁺, and Na⁺) and anions (e.g. OH⁻, F⁻, Cl⁻, and S⁻) loosely co-ordinated with the oxygens in the silicate tetrahedra. The greater the silica content of the magma, the more chains and rings of silicate tetrahedra there are to impede each other and hence increase the viscosity of the magma. The pressure regime and composition of the magma control which minerals nucleate and crystallize from a magma when it cools.

magma Molten material that is the source of all igneous rocks. Magma refers to this material while it is still under the Earth's crust. In addition to its complex silicate composition, magma contains gases and water vapour.