Magma
Magma is molten or partly molten rock beneath Earth surface. It can contain liquid melt, crystals, dissolved gases, and bubbles, and it is the source material for igneous rocks, volcanic eruptions, lava flows, intrusions, and many mineral deposits.
What magma is
Magma is not simply liquid rock. It is usually a mixture of silicate melt, mineral crystals, dissolved gases, and sometimes gas bubbles or fragments of surrounding rock. Its temperature, chemistry, crystal content, and gas content control how it moves, how it erupts, and what rock it may become.
How magma forms
Most magma forms by partial melting, where only part of a rock melts. Melting can happen when hot mantle rises and pressure drops, when water and other volatiles lower the melting temperature, or when heat transfers into crustal rock. These processes occur in settings such as mid-ocean ridges, subduction zones, rifts, hot spots, and some thickened crust.
Composition and silica
Magma composition ranges from mafic to felsic. Mafic magmas, such as basaltic magma, tend to have less silica and more iron and magnesium. Felsic magmas, such as rhyolitic magma, contain more silica and often more dissolved gases. Composition affects minerals, viscosity, eruption behavior, and the final igneous rock.
Viscosity and gas
Viscosity is resistance to flow. Low-viscosity magma can move more easily and often feeds fluid lava flows. High-viscosity magma traps gases more effectively, which can raise pressure and contribute to explosive eruptions. Temperature, silica content, crystals, and gas bubbles all influence viscosity.
Magma chambers and intrusions
Magma may collect, stall, cool, mix, crystallize, or evolve in reservoirs beneath volcanoes and within the crust. Some magma reaches the surface. Some remains underground and solidifies as dikes, sills, plutons, batholiths, and other intrusions. These bodies can later be exposed by uplift and erosion.
From magma to lava
If magma finds a pathway to the surface through fractures, vents, or volcanic conduits, it becomes lava when it erupts. The eruption may produce flows, domes, fountains, ash, pumice, scoria, or pyroclastic material depending on magma chemistry, gas content, pressure, and the surrounding environment.
Crystallization and differentiation
As magma cools, minerals crystallize in changing sequences. Crystals may settle, magma may mix with other magma, surrounding rock may melt or be incorporated, and gases may separate. These processes can change magma composition over time, creating different igneous rocks from related magma systems.
Why it matters
Magma matters because it transfers heat and material from deep Earth toward the surface. It builds ocean crust, volcanoes, lava fields, intrusions, and parts of continents. Understanding magma also helps scientists interpret eruption hazards, volcanic gases, geothermal systems, mineral deposits, and the rock cycle.