Magma

Magma is the parent material of all igneous rocks. While often used interchangeably with “lava,” the distinction is simple but significant: magma exists underground, while lava is magma that has breached the surface. This subterranean molten rock acts as the engine for volcanoes and is a key driver of the rock cycle, recycling elements between Earth’s interior and its surface over geological timescales.

The Three Components of Magma

Magma is rarely a simple homogeneous liquid. It is a complex, multiphase substance composed of:

1. The Melt: The liquid portion, made of mobile silicate ions—combinations of silicon, oxygen, aluminum, iron, magnesium, calcium, sodium, and potassium. The arrangement of these ions into polymer chains of varying length largely determines the viscosity of the magma.
2. Solids: Mineral crystals that have begun to freeze out of the melt as it cools. Early-forming crystals (such as olivine, pyroxene, or feldspar) may be suspended in the melt, giving magma a porridge-like consistency. A partially crystallized system with more than ~50% crystals is called a crystal mush.
3. Volatiles: Dissolved gases that remain trapped in the liquid at high pressure. The most common are water vapor (H₂O), carbon dioxide (CO₂), and sulfur dioxide (SO₂); smaller amounts of hydrogen chloride (HCl), hydrogen fluoride (HF), and noble gases are also present. When magma rises toward the surface and pressure decreases, these volatiles exsolve (come out of solution), nucleating as bubbles. This expansion of gas drives explosive eruptions.

Physicochemical Properties

• Temperature: Magma temperatures range from roughly 700°C for high-silica rhyolites to over 1,300°C for low-silica, iron-rich basalts. Ultra-high-temperature komatiitic lavas, now rare but abundant in the Archean eon, may have erupted at temperatures exceeding 1,600°C.
• Viscosity: This measures the magma’s resistance to flow and is the most important property governing eruption style. High-silica magma forms long, entangled silicate polymer chains (analogous to long-chain plastics) and is extremely sticky. Low-silica magma has shorter, less-connected chains and flows readily. Basaltic magma can be 10,000 to 100,000 times less viscous than rhyolitic magma. Viscosity is the primary factor determining whether a volcano erupts effusively (flows) or explosively (explodes into ash and pumice).
• Density: Magma is generally less dense than the surrounding solid rock, which is the ultimate reason it rises buoyantly through the crust. Basaltic magma has a density of approximately 2,650–2,800 kg/m³; the surrounding mantle peridotite is ~3,300 kg/m³.

How Magma Forms

Contrary to popular belief, the Earth’s mantle is not a liquid ocean of magma—it is solid rock that flows very slowly over geological time. Magma only forms under three specific conditions:

1. Decompression Melting: Occurs at divergent boundaries (mid-ocean ridges) and above mantle plumes. As mantle rock rises toward the surface, the pressure drops faster than the temperature decreases. Melting occurs not because the rock gets hotter, but because the melting point of rock decreases with falling pressure—and the rock’s actual temperature exceeds this lower melting point. This process generates basaltic magma at mid-ocean ridges worldwide.
2. Flux Melting: Occurs at subduction zones. Water and other volatiles released from a sinking tectonic plate rise into the hot mantle wedge above. This water lowers the melting point of the mantle rock dramatically—by 100°C or more—much like salt lowers the freezing point of ice on a road. The mantle rock begins to melt not because it has become hotter, but because its melting point has dropped below its actual temperature.
3. Heat Transfer (Conductive Melting): Very hot basaltic magma rising from the mantle can pool at the base of the continental crust, transferring heat to the rock above it. This melts silica-rich crustal material, generating new, light-colored, felsic magmas that can produce the explosive eruptions characteristic of continental volcanic arcs.

Magma Evolution: From Primitive to Evolved

Magma rarely stays the same composition from source to surface. It changes through a suite of processes collectively called Magmatic Differentiation:

• Fractional Crystallization: As magma slowly cools in a magma chamber, high-temperature minerals crystallize first. Olivine and pyroxene crystallize early; as they sink or attach to chamber walls, they remove iron and magnesium from the melt. The remaining liquid becomes progressively more silica-rich. This process can, over tens of thousands of years, transform a basaltic magma into an andesitic or even rhyolitic one.
• Assimilation: Magma can melt and incorporate surrounding country rock as it rises. Assimilating silica-rich continental crust shifts the magma toward more felsic (and more explosive) compositions. The ratio of assimilation to fractional crystallization is summarized in the “AFC” (Assimilation and Fractional Crystallization) model widely used in igneous petrology.
• Mixing: Two different magma bodies may meet and mix in a chamber, creating a hybrid composition. Magma mixing is thought to be an important trigger for eruptions, as the injection of hot, primitive basalt into a cooler silicic magma chamber rapidly increases volatile content and temperature, destabilizing the system.

Chemical Classification

Magma is classified primarily by its silica (SiO₂) content:

• Mafic (Basaltic): ~45–52% silica. Hot, fluid, and dark-colored. Examples include the lavas of Hawaii and Iceland. Dominates oceanic crust formation.
• Intermediate (Andesitic/Dacitic): ~52–63% silica. Moderately explosive and common in subduction-zone stratovolcanoes like Mount St. Helens and Galunggung.
• Felsic (Rhyolitic): >63% silica. Cool, sticky, and light-colored. These produce domes, ignimbrites, and supervolcanic calderas.

Less common types include alkalic magmas (enriched in sodium and potassium), carbonatitic magmas (dominated by carbonate minerals rather than silicates—erupted at Ol Doinyo Lengai in Tanzania), and ancient komatiitic lavas (ultra-mafic, high-temperature flows now found only in Archean terranes).

Planetary Perspective

Magmatism is not unique to Earth. The dark “seas” (maria) on the Moon are vast plains of ancient basaltic magma erupted 3–4 billion years ago. Io, a moon of Jupiter, is the most volcanically active body in the solar system, erupting ultra-hot silicate magmas driven by tidal heating from Jupiter’s immense gravity. Venus appears to have very recent or ongoing volcanic activity, with radar images showing fresh lava flows and possible surface changes detected by the Magellan spacecraft. Conversely, icy moons like Enceladus (Saturn) may produce “cryomagma”—slushy water and ammonia—demonstrating that magma, broadly defined as internally generated melt, is a fundamental planetary process wherever sufficient heat is present.

Related Terms

Lava is magma that has erupted at the surface. Magma chamber is the subsurface reservoir where magma is stored and evolves. Fractional crystallization describes the chemical evolution of magma as it cools. Volatile refers to the dissolved gases (H₂O, CO₂, SO₂) that drive explosive eruptions when they exsolve from rising magma.