Magma

Magma is the molten or semi-molten natural material from which all igneous rocks originate. Found beneath the Earth’s surface, it occurs within the crust and upper mantle and may also exist on other terrestrial planets and moons. In addition to molten rock, magma commonly contains suspended crystals and dissolved volcanic gases. Its formation, movement, and cooling constitute the basis of igneous processes, playing a critical role in shaping the Earth’s lithosphere and fuelling volcanic activity.

Formation and Occurrence

Magma is generated when mantle or crustal rocks undergo partial melting. This melting may occur in several tectonic environments, including subduction zones, mid-ocean ridges, continental rift systems, and mantle hotspots. At mid-ocean ridges, decompression melting forms large volumes of mafic magma, whereas subduction zones produce more silica-rich varieties through hydration melting and crustal assimilation. Hotspot environments can generate diverse magma types due to the thermal influence of upwelling mantle plumes.
Once formed, magma typically migrates upwards through the crust, driven by buoyancy. Magma may accumulate in magma chambers or in transcrustal crystal mush zones, where its composition can be modified through processes such as fractional crystallisation, magma mixing, crustal assimilation, and degassing. Depending on the environmental conditions, magma may either erupt at the surface as lava or remain underground, solidifying to form intrusive bodies including dykes, sills, laccoliths, plutons, and batholiths.
Direct scientific encounters with magma are rare; however, geothermal drilling projects in Iceland and Hawaii have intersected molten material in situ, providing valuable insights into high-temperature geological processes.

Physical and Chemical Properties

Magma exists as a multiphase mixture composed of liquid melt, solid crystals, and dissolved gases. As it rises towards the surface, decreasing pressure causes volatile gases, such as water vapour and carbon dioxide, to exsolve, resulting in a mixture increasingly dominated by gas bubbles. This change influences the eruptive style, magma viscosity, and the formation of pyroclastic materials.
Silicon dioxide (silica) is the dominant chemical component in most magmas. Silicate magmas also contain oxides of aluminium, magnesium, iron, calcium, sodium, and potassium. Petrologists describe magma composition by referring to the weight or molar proportions of these oxides. Silica content strongly correlates with physical characteristics such as viscosity and eruption temperature. On this basis, magmas are broadly categorised as felsic, intermediate, mafic, or ultramafic.

Felsic Magmas

Felsic magmas contain more than 63% silica and typically form rhyolite and dacite rocks. Their high silica content results in a highly polymerised melt, giving rise to extremely high viscosities ranging from 10⁵ to 10⁸ pascal-seconds. Because of these properties, felsic magmas often erupt explosively, generating extensive pyroclastic deposits. However, effusive eruptions can occur, producing lava domes, spines, coulees, and blocky flows that may contain volcanic glass such as obsidian.
Felsic lavas can erupt at relatively low temperatures, though unusually hot rhyolitic flows—reaching 950°C—are capable of travelling tens of kilometres. The Snake River Plain in the United States contains examples of long, fluid rhyolitic flows formed under such high-temperature conditions.

Intermediate Magmas

Intermediate magmas, with silica contents between 52% and 63%, commonly produce andesite lavas. These magmas form at higher temperatures than felsic varieties and contain greater proportions of magnesium and iron. Their viscosities, typically around 3,500 pascal-seconds, are significantly lower than those of felsic melts.
Intermediate lavas often produce blocky flows and volcanic domes and are frequently associated with steep-sided composite volcanoes, such as those in the Andes. They tend to crystallise minerals such as amphibole and pyroxene, producing a darker groundmass and prominent phenocrysts.

Mafic Magmas

Mafic magmas contain between 45% and 52% silica and are rich in iron and magnesium. They erupt at relatively high temperatures and possess comparatively low viscosities—similar to that of ketchup—which allow them to flow readily. Basaltic lava flows can travel great distances, forming broad shield volcanoes and extensive flood basalt provinces.
Typical surface expressions include smooth pāhoehoe and rough ‘a‘ā lavas. Submarine eruptions often generate pillow lavas due to rapid cooling under water. Basalt flows can inflate as molten material continues to feed into the interior beneath a solid outer crust, producing lava fields of considerable thickness.

Ultramafic Magmas

Ultramafic magmas, such as komatiite and picrite basalt, contain less than 45% silica and unusually high magnesium concentrations. Komatiitic magmas, in particular, may contain over 18% magnesium oxide and are thought to have erupted at temperatures around 1,600°C. Their extremely low viscosities, comparable to light motor oil, suggest an almost completely unpolymerised melt.
These magmas are rare in the modern geological record, as the Earth’s mantle has cooled since the Proterozoic. A few Phanerozoic examples from Central America are believed to have formed above exceptionally hot mantle plumes.

Alkaline Magmas

Alkaline magmas are distinguished by high concentrations of sodium and potassium oxides. They are commonly associated with continental rifting, subduction at depth, or intraplate hotspots. Their silica content ranges widely, encompassing ultramafic nephelinites and basanites through to felsic trachytes. Some, such as olivine nephelinite, originate from deep mantle sources, indicating complex mantle melting processes.

Nonsilicate Magmas

Although rare, nonsilicate magmas exist. Carbonatite lavas, notably at Ol Doinyo Lengai in Tanzania, comprise primarily carbonate minerals and have exceptionally low viscosities and low eruption temperatures—among the lowest recorded for any lava. Their compositions vary, including sodium and calcium carbonates, halides, fluorides, and sulphates. These lavas may form by immiscible separation from alkaline silicate magmas.
Iron oxide magmas are another unusual type, associated with deposits such as the Kiruna iron ore in Sweden and eruptive features at the El Laco complex of the Chile–Argentina border. These magmas may separate from calc-alkaline or alkaline parent magmas through immiscibility processes. In some volcanic settings, molten sulphur may erupt, forming distinctive sulphur flows at relatively low temperatures.

Magmatic Gases

Volcanic gases dissolved within magma exert significant influence on eruptive behaviour. Water vapour is typically the most abundant gas, followed by carbon dioxide and sulphur species. Gas solubility decreases as pressure falls during magma ascent, causing bubbles to form. High gas content in viscous magmas contributes to explosive eruptions, whereas low-viscosity, gas-poor magmas tend to erupt effusively.