Definition and Nature

Magma is subsurface molten or partially molten silicate material consisting of liquid melt with suspended crystals and dissolved volatiles; when erupted, it is termed lava. Because magma is generally less dense than surrounding rock, buoyancy and internal gas pressure drive its ascent through the crust. The principal chemical elements in typical magmas include oxygen, silicon, aluminum, iron, magnesium, calcium, sodium, potassium, titanium, and manganese. According to the U.S. Geological Survey, these components crystallize upon cooling to form Igneous rock, with the magma-to-lava distinction based solely on whether the melt is below or above the surface. USGS. (usgs.gov)

Composition, Types, and Volatiles

Most terrestrial magmas are silicate melts spanning roughly 45–75 wt% SiO₂ and are commonly grouped as basaltic (or mafic), andesitic (intermediate), and rhyolitic (felsic). Basaltic magmas have lower silica (about 45–55 wt%) and higher Fe–Mg contents; rhyolitic magmas have higher silica (about 65–75 wt%) and higher K–Na; andesitic magmas are intermediate. Dissolved gases—primarily H₂O with CO₂ and minor S, Cl, and F—are ubiquitous and strongly influence eruptive behavior as pressure decreases during ascent. These compositional ranges and volatile assemblages are summarized in widely used igneous petrology texts and reference works. Britannica; Tulane University. (britannica.com)

Physical Properties

Temperature varies systematically with composition: basaltic magmas commonly erupt at about 1000–1200 °C, andesitic at ~800–1000 °C, and rhyolitic at ~650–800 °C. Field measurements at Hawaiian volcanoes show summit magma storage near 1200 °C, with mantle-derived inputs hotter at depth. Viscosity increases with higher silica and lower temperature by several orders of magnitude, helping to explain the contrast between fluid basaltic lavas and viscous rhyolitic domes. Basaltic magmas at ~1100 °C can be >10⁵ times more viscous than water, while rhyolitic magmas at ~800 °C can exceed 10⁷ times water viscosity. USGS; Britannica; Tulane University. (usgs.gov)

Generation of Magma

Magma forms chiefly by partial melting driven by three endmember processes connected to Plate tectonics:

• –Decompression melting: Upwelling mantle crosses its solidus as pressure drops, typical beneath a Mid-ocean ridge. This generates basaltic magmas that build new oceanic crust. NOAA PMEL. (pmel.noaa.gov)
• –Flux melting: Volatile addition (especially H₂O) from subducting slabs lowers the solidus of the overlying mantle wedge in a Subduction zone, producing hydrous, calc-alkaline magmas. Britannica. (britannica.com)
• –Heat transfer (thermal) melting: Hot, mantle-derived magma intrudes and heats crustal rocks, inducing partial melting that can yield intermediate to felsic magmas; this is common in thickened continental crust above long-lived arcs and hotspots. [The Encyclopedia of Volcanoes](book://H. Sigurdsson|The Encyclopedia of Volcanoes|Academic Press|2015); [Philpotts & Ague](book://A. R. Philpotts and J. J. Ague|Principles of Igneous and Metamorphic Petrology|Cambridge University Press|2009).

Global production is dominated by ridge settings: oceanographic syntheses estimate that the great majority of volcanic magma is generated beneath the seafloor where decompression melting supplies basaltic melt that constructs a ~6–7 km-thick basaltic crustal layer. Woods Hole Oceanographic Institution. (whoi.edu)

Differentiation and Evolution

Once generated, magmas evolve through fractional crystallization, assimilation of crustal rocks, and magma mixing. Removal of early-formed mafic minerals drives residual melts toward higher silica; assimilation can introduce crustal signatures; and mingling between distinct batches produces hybrid compositions. Studies at ridges show that, while most lavas are MORB-like basalts, andesite–dacite can form via extensive fractionation and assimilation of altered oceanic crust. Journal of Petrology (USGS Publications Warehouse); [Winter](book://J. D. Winter|An Introduction to Igneous and Metamorphic Petrology|Pearson|2010). (usgs.gov)

Storage and Transport in the Crust

Magma resides in crustal reservoirs that range from crystal-rich mush zones to more melt-rich bodies often termed magma chambers. Geophysical imaging at Yellowstone reveals two stacked reservoirs: a shallow rhyolitic body from ~5–17 km depth with ~5–15% melt and a deeper basaltic reservoir from ~20–50 km with ~2% melt, illustrating that much of a volcano’s plumbing can be predominantly crystalline with interstitial melt. Reservoirs can be laterally offset from vents by several kilometers, with offset broadly scaling with depth. USGS Yellowstone Volcano Observatory; USGS. (usgs.gov)

Magma migrates through dikes and sills when overpressure overcomes rock strength; gas exsolution during ascent affects temperature, viscosity, and fragmentation potential. Thermodynamic modeling shows decompression can cool magma–gas mixtures substantially, influencing conduit flow and eruption dynamics. USGS Publications Warehouse. (usgs.gov)

Eruptive Expression and Hazards

Upon reaching the surface, magma becomes lava and may erupt effusively as flows or explosively as tephra, depending largely on viscosity and gas content. Basaltic magmas, typically hotter and less viscous, favor fluid lava flows; rhyolitic magmas, cooler and silica-rich, commonly generate explosive activity and pyroclastic deposits. These contrasts underpin the differing morphologies and hazards associated with shield volcanoes versus dome-building eruptions. Britannica; USGS. (britannica.com)

Tectonic Settings

• –Divergent boundaries: Beneath Mid-ocean ridge systems, decompression melting of upwelling mantle generates MORB magmas that feed axial magma lenses and construct oceanic crust; much melt crystallizes within the crust without erupting. NOAA PMEL. (pmel.noaa.gov)
• –Convergent margins: In Subduction zone arcs, volatile-rich melts form in the mantle wedge, producing calc-alkaline magmas and stratovolcanoes. Britannica. (britannica.com)
• –Intraplate settings: Hotspots associated with buoyant upwelling of deep mantle material, or Mantle plume activity, generate basaltic magmas beneath ocean islands and continental flood provinces. [Philpotts & Ague](book://A. R. Philpotts and J. J. Ague|Principles of Igneous and Metamorphic Petrology|Cambridge University Press|2009); USGS—HVO. (usgs.gov)

Temperature Benchmarks and Monitoring Examples

At Kīlauea (Hawai‘i), magma ascends from mantle sources near ~1500 °C and cools to ~1200 °C in the summit storage system before eruption when directly tapped, illustrating the thermal pathway from deep input to shallow reservoir. These values are constrained by sampling of lava lakes and petrologic studies. USGS. (usgs.gov)

Internal Links

First mentions above link to related entries: Igneous rock, Plate tectonics, Mid-ocean ridge, Subduction zone, Mantle plume, Volcano.