MAGMAS

Magmas are naturally occurring melts that form within the Earth’s interior. They consist of a silicate liquid phase, suspended crystals, and dissolved or exsolved volatiles such as water vapor, carbon dioxide, and sulfur dioxide. Temperatures typically range from about 700 to 1300 degrees Celsius, depending on depth and composition. Because magmas are usually less dense than surrounding rock, they tend to accumulate in magma chambers and migrate through crustal conduits toward the surface.

Chemically, magmas are categorized by silica content and major element composition. Felsic magmas are high in silica and light-colored minerals such as quartz and feldspar; intermediate magmas sit between felsic and mafic; mafic magmas are lower in silica and richer in dark minerals like pyroxene and olivine; ultramafic magmas are even richer in heavy minerals and poorer in silica. The rocks that crystallize from these magmas include rhyolite and granite (felsic), andesite and diorite (intermediate), basalt (mafic), and ultramafic counterparts. Viscosity generally increases with silica content, so felsic magmas are more viscous than mafic magmas.

Magmas form primarily by partial melting of existing rocks in the mantle or crust, and they evolve through processes such as fractional crystallization, magma mixing, and assimilation of surrounding material. These processes modify chemistry and mineralogy, producing a spectrum of magmas along differentiation trends.

Volatile content strongly influences behavior at ascent and eruption. As magma ascends and pressure drops, dissolved gases exsolve, increasing internal pressure and potentially driving explosive eruptions in highly viscous magmas, or effusive lava flows for less viscous varieties.

Most magmatism is linked to plate tectonics, with divergent boundaries producing basaltic magmas, subduction zones yielding andesitic to rhyolitic magmas, and intraplate settings host to alkaline magmas and hotspots.