Igneous Petrology: Magma, Classification and Textures
What the domain covers
Igneous Petrology: Magma, Classification and Textures tests how magma forms and evolves, how igneous rocks are classified, and how cooling history is read from texture. Expect questions on viscosity, Bowen reaction series, silica content, colour index and plutonic versus volcanic rocks.
Core rules
- Magma forms by decompression melting, addition of volatiles, or heating. Basaltic magma is low in silica, hot and fluid; rhyolitic magma is high in silica, cooler and viscous.
- Bowen reaction series shows that mafic minerals crystallize early and felsic minerals later; this explains fractional crystallization and compositional change.
- Classification uses silica percentage, colour index and mineralogy. Felsic rocks are light and silica-rich; mafic rocks are dark and silica-poor. Plutonic rocks are coarse-grained; volcanic rocks are fine-grained.
- Textures record cooling. Phaneritic means visible crystals from slow cooling; aphanitic means fine crystals from rapid cooling; porphyritic means large phenocrysts in a finer groundmass; glassy means no crystals; vesicular means gas cavities; pyroclastic means fragmental.
Worked example
A hand specimen is light coloured, coarse grained, and contains quartz, alkali feldspar, plagioclase and biotite. Coarse grain size indicates slow cooling at depth, so it is plutonic. Quartz plus feldspar indicates high silica and felsic composition. The rock is granite. If the same magma cooled rapidly at the surface, the fine-grained equivalent would be rhyolite.
Traps
Porphyritic texture does not automatically mean volcanic; it records two cooling stages. Glassy rocks such as obsidian have no crystals, so do not describe them as coarse-grained. Pegmatite is exceptionally coarse-grained, not aphanitic.