Magma

Lore & Background

Magma is produced by melting of the mantle or crust in tectonic settings including subduction zones, continental rift zones, mid-ocean ridges, and hotspots. Mantle and crustal melts migrate upward through the crust, where they are stored in magma chambers or trans-crustal crystal-rich mush zones. During storage, magma composition may be modified by fractional crystallization, contamination with crustal melts, magma mixing, and degassing. Magma may feed a volcano and be extruded as lava, or solidify underground to form intrusions such as dikes, sills, laccoliths, plutons, or batholiths.

Reader's Guide

Magma is central to igneous petrology and volcanology, as it is the parent material for all igneous rocks. Its physical and chemical properties—especially silica content—determine eruptive behavior, from explosive felsic eruptions to fluid mafic flows. The study of magma has relied on observing lava flows, but in situ encounters during geothermal drilling in Iceland and Hawaii have provided direct samples. Magma's role in forming both volcanic and plutonic rocks makes it key to understanding Earth's internal processes and the evolution of other planetary bodies. Its classification into felsic, intermediate, mafic, ultramafic, alkaline, and nonsilicate types reflects a wide range of compositions and temperatures, influencing everything from eruption style to the formation of mineral deposits.

Did You Know?

• Magma has been encountered in situ three times during geothermal drilling: twice in Iceland and once in Hawaii.
• Felsic magmas with silica over 63% can have viscosities up to 10^11 cP, while ultramafic komatiite magmas may be as fluid as light motor oil.

Origins and the Long Ascent Through the Crust

Magma is born when the Earth's mantle or crust undergoes melting, a process driven by the planet's tectonic engine. On our world, this melting occurs in several distinct geological settings: at subduction zones where one plate dives beneath another, along continental rift zones where the crust is being pulled apart, at mid-ocean ridges where new seafloor is created, and at hotspots where plumes of hot material rise from deep within the mantle. Once generated, the molten material begins its slow upward migration through the overlying crust. Geologists believe it is temporarily held in magma chambers or in vast trans-crustal zones of crystal-rich mush, where it can linger for extended periods. During this storage phase, the magma's chemical fingerprint is constantly rewritten through fractional crystallization, mixing with other magma bodies, contamination by surrounding crustal melts, and the gradual release of dissolved gases. Ultimately, the magma's journey ends in one of two ways: it breaches the surface to feed a volcanic eruption, or it cools and solidifies deep underground, taking the form of a dike, sill, laccolith, pluton, or batholith.

A Chemical Spectrum of Molten Rock

The vast majority of magma on Earth is silicate in nature, meaning it is a molten mixture dominated by oxygen and silicon—the two most abundant elements in the crust—along with significant amounts of aluminium, calcium, magnesium, iron, sodium, and potassium, plus trace quantities of many other elements. Petrologists classify these silicate magmas into four broad chemical families based on their silica content. Felsic magmas, such as rhyolite and dacite, exceed 63 percent silica. Intermediate or andesitic magmas fall between 52 and 63 percent. Mafic or basaltic magmas range from 45 to 52 percent, while ultramafic magmas—including komatiite, picritic basalt, and boninite-forming liquids—sit below 45 percent. A rare exception exists in the form of nonsilicate magma, which can arise from the local melting of non-silicate mineral deposits or from the separation of a single melt into immiscible silicate and nonsilicate liquid phases. Additionally, some silicic magmas carry elevated levels of alkali metal oxides, a feature particularly common in continental rifting regions.

Viscosity, Temperature, and the Style of Eruption

The physical behavior of magma during eruption is governed largely by its silica content, which dictates viscosity, and by its temperature. This makes explosive, pyroclastic eruptions the norm, though rhyolite can occasionally ooze out as thick domes, spines, or short coulees. Mafic basaltic magmas, erupting at 1,100 to 1,200 degrees, flow with a viscosity similar to ketchup, producing broad shield volcanoes and flood basalts, with ʻAʻā and pāhoehoe textures on land and pillow lavas underwater. Ultramafic komatiites, thought to have erupted near 1,600 degrees, were so fluid they flowed like light motor oil, though no modern examples exist because the mantle has cooled too much to generate them.

Magma Beyond Earth and the Rare Chance Encounter

Although magmatism is not exclusive to our planet—evidence of it has been identified on other terrestrial planets and on certain natural satellites—our direct knowledge of magma has historically been indirect. For most of geological science, researchers have had to study magma only after it has already erupted and become a lava flow, inferring its earlier properties from the solidified product. True in-situ encounters with still-molten magma are vanishingly rare. In the entire history of geothermal drilling, magma has been struck only three times: twice in Iceland, where such encounters have also opened the door to exploring magma's potential role in energy production, and once in Hawaii. When magma does approach the surface and the overlying pressure drops, dissolved gases exsolve and bubble out of the liquid, so that near-surface magma is a three-phase mixture of solid crystals, liquid rock, and gas bubbles. It is this near-surface, gas-rich, crystal-laden material that is sometimes colloquially—but incorrectly—called lava, a term that properly applies only to magma that has already reached the surface.