Magma is molten or partially molten rock material located beneath Earth's surface, generally containing a mixture of liquid silicate melt, solid crystals and dissolved or exsolved gases. It is not simply a large underground pool of completely liquid rock. Many magmas are crystal rich and behave as complex multiphase materials whose physical properties depend on temperature, pressure, composition, crystal content and volatile concentration.
The liquid component consists predominantly of silicate material, with major elements including oxygen and silicon together with aluminium, iron, magnesium, calcium, sodium and potassium. Dissolved gases can include water vapour, carbon dioxide and sulfur species. Magma can form in the mantle or crust through partial melting, and it can evolve chemically while stored and transported. When magma reaches Earth's surface and emerges as molten rock, it is generally called lava.
Magma formation requires partial melting because Earth's mantle and crust are usually solid under normal conditions. Partial melting can occur when temperature increases, pressure decreases, or volatile substances such as water alter melting conditions. At divergent boundaries, decompression melting is especially important because rising mantle experiences lower pressure. At subduction zones, water released from the descending slab promotes melting in the mantle above it. Heat transfer from hot magma into surrounding crust can also generate crustal melts.
The composition of the resulting magma depends on the source rock, degree of partial melting and physical conditions. A small degree of partial melting can produce a melt composition different from the bulk composition of the source because different minerals enter the melt preferentially. Magma composition can subsequently change through crystallisation, mixing and interaction with surrounding rocks.
Temperature is one of the fundamental controls on magma properties. Many common magmas erupt at temperatures roughly between 700 and 1200 degrees Celsius, although the exact range varies with composition and volatile content. Mafic basaltic magmas are commonly hotter, often around 1000 to 1200 degrees Celsius, whereas silica rich rhyolitic magmas can be substantially cooler, sometimes around 650 to 900 degrees Celsius. Temperature alone does not determine viscosity.
Silica content, polymerisation of the silicate melt, crystal content and dissolved gases also strongly influence resistance to flow. Basaltic magma generally has relatively low viscosity, while rhyolitic magma is much more viscous. Viscosity affects whether magma can flow readily, whether gases can escape and whether eruptions are likely to be comparatively effusive or explosive.
Magma contains dissolved volatile substances whose behaviour becomes critical as pressure falls. Water, carbon dioxide and sulfur gases can remain dissolved in magma at high pressure but become less soluble as magma rises toward the surface. This can cause exsolution, in which bubbles form and grow within the melt. Gas expansion can dramatically reduce the density of magma and can drive rapid ascent or fragmentation. If bubbles can escape efficiently, magma may erupt relatively gently as lava.
If highly viscous magma traps expanding bubbles, pressure can build until the magma fragments explosively into ash, pumice and other pyroclastic material. Volatile content therefore contributes strongly to eruptive behaviour. Carbon dioxide is generally less soluble in silicate melt than water and may begin exsolving at greater depths, potentially influencing magma storage and ascent even before eruption.
Magma evolves through several important processes. Fractional crystallisation occurs when minerals crystallise from cooling magma and are physically or chemically separated from the remaining melt, changing the composition of the liquid. Assimilation occurs when magma incorporates material from surrounding rocks. Magma mixing happens when two magmas with different compositions or temperatures interact. These processes can produce complex chemical trends and contribute to the diversity of igneous rocks.
Bowen's reaction series describes the general order in which common silicate minerals crystallise from cooling magma under idealised conditions, with minerals such as olivine and pyroxene forming earlier at higher temperatures and minerals such as quartz and potassium feldspar crystallising later under suitable compositions. Real magma systems are more complicated because pressure, water content, oxygen conditions and chemical composition influence crystallisation.
Magma can occur in different geological settings and compositions. Basaltic magma is typically relatively low in silica and rich in iron and magnesium compared with felsic magma. Andesitic magma has intermediate characteristics and is common in volcanic arcs. Rhyolitic magma is silica rich and commonly highly viscous. Terms such as mafic, intermediate and felsic describe broad compositional categories, although they should not be treated as exact equivalents of individual magma types.
Magma can also be classified according to trace elements and isotopic composition, which provide information about its source and evolutionary history. Geologists analyse erupted volcanic rocks because they are solidified records of magma, using mineral textures, major element chemistry, trace elements and isotopes to reconstruct processes that occurred underground.
Magma does not always reach the surface. If it cools and crystallises underground, it forms intrusive igneous rock, whereas magma that erupts becomes the source of extrusive volcanic rocks. The rate of cooling has major effects on texture. Magma that cools slowly underground allows crystals to grow relatively large, producing coarse grained rocks such as granite or gabbro depending on composition. Lava that cools rapidly at the surface usually produces much finer grained rocks such as basalt or rhyolite. Extremely rapid cooling can prevent crystals from forming effectively and produce volcanic glass such as obsidian. Magma may also crystallise at different depths and times, creating rocks with large crystals embedded in a finer groundmass, a texture known as porphyritic texture.
A common misconception is that magma exists as a universally continuous underground ocean beneath Earth's crust. In reality, most of the mantle is solid, and magma occurs in localised or interconnected regions where conditions permit partial melting. Magma reservoirs can be transient and structurally complex, and many contain abundant crystals rather than being entirely liquid. The term magma specifically refers to molten rock beneath the surface, while lava refers to magma after it reaches the surface.
Magma is central to the rock cycle because it can generate igneous rocks and contribute indirectly to sedimentary and metamorphic processes through uplift, erosion, burial and heat transfer. It is also a major agent of geological differentiation, transporting heat and chemical elements from Earth's interior toward the crust and surface.
