Magma originates from partial melting of rocks inside the Earth, primarily in the mantle and, to a lesser extent, the crust, and it then rises through the crust to feed volcanic activity or crystallize underground as plutons. The exact setting and mechanism depend on tectonic context and rock composition. Key sources of magma formation

• Mantle melting at divergent plate boundaries and hotspots: Upwelling mantle material undergoes partial melting as pressure drops or heat increases, producing mafic magmas (basaltic compositions) common at mid-ocean ridges and large basaltic shield volcanoes.
• Subduction zones: The descent of oceanic crust introduces water into the mantle, lowering melting temperatures and promoting melting that yields more intermediate to felsic magmas (andesite, dacite, rhyolite) often associated with explosive eruptions.
• Continental rifts and crustal melting: Thinning crust and localized heating can generate magmas with varied compositions, including felsic varieties.

What happens after magma forms

• Migration and storage: Melt-rich zones in the mantle or crust migrate upward, accumulating in magma chambers or trans-crustal mush zones.
• Differentiation processes: Fractional crystallization, crustal contamination, magma mixing, and degassing modify the evolving melt’s composition before eruption or solidification.
• Surface expressions: If magma reaches the surface, it erupts as lava; if it stalls underground, it forms intrusive bodies such as dikes, sills, laccoliths, plutons, or batholiths.

Additional context

• Magma is the parent material for igneous rocks and often contains dissolved gases; gas exsolution during ascent can drive explosive volcanism, especially in higher-viscosity magmas.
• The term “lava” refers to magma once it erupts onto or near the surface, while “magma” is used for molten or semi-molten rock beneath the surface.
• While most Earth magma originates from mantle or crustal melting, magma-like melts have been studied in various planetary settings, where heating and pressure conditions differ.

If you’d like, I can tailor this to a particular tectonic setting (e.g., subduction zones vs. mid-ocean ridges) or provide a concise comparative table of magma types and their typical tectonic environments.