Chapter 6: Up from the Inferno: Magma and Igneous Rocks

Magma originates deep within Earth through three primary melting mechanisms: decompression melting occurs when mantle material rises and pressure decreases, flux melting happens when volatile-rich fluids lower the melting point of rock, and heat-transfer melting results from the addition of thermal energy from rising magma or mantle plumes. Once formed, magma rises toward the surface due to its lower density relative to surrounding solid rock. The chapter emphasizes how magma composition, primarily determined by silica concentration, fundamentally controls its physical behavior. High-silica felsic magmas exhibit greater viscosity and slower flow rates, while low-silica mafic magmas flow more readily due to lower viscosity. Temperature, dissolved volatile content including water and carbon dioxide, and crystal fraction further influence magma behavior and eruptive style. The chapter explains Bowen's reaction series, which describes the sequential crystallization of minerals as cooling magma undergoes fractional crystallization, producing the range of igneous rock types from olivine-rich ultramafic compositions through intermediate andesite to silica-rich granitic rocks. Additional processes including magma mixing, assimilation of wall rock, and partial melting of existing crustal material generate further compositional diversity. Intrusive igneous bodies such as dikes, sills, batholiths, and plutons form when magma solidifies within the crust, while extrusive rocks develop from surface lava flows and pyroclastic eruptions. Igneous textures reflect cooling rates, ranging from glassy textures in rapidly cooled lava to coarse crystalline textures in slowly cooled plutonic rocks to fragmental textures in explosive volcanic deposits. The chapter connects igneous activity to plate tectonic settings including subduction zones where volatile-rich melting generates explosive volcanism, mid-ocean ridges where decompression melting produces new oceanic crust, and hot spots that create chains of volcanic islands and large igneous provinces capable of rapid continental flood basalt eruptions that reshape planetary chemistry and climate.