3.2 Igneous petrology (Bowen's series, classification)

Igneous Rocks: Formation and Classification

Igneous rocks crystallize from cooling magma (molten rock at depth) or lava (molten rock erupted at the surface). Classification rests on two independent variables: composition (chemistry and mineralogy) and texture (crystal size, governed by cooling rate). The rock cycle continually converts igneous rock to sediment and metamorphic rock and back, but igneous processes are the ultimate source of new crustal material.

Cooling Rate and Texture

• Intrusive (plutonic) rocks cool slowly at depth, growing large crystals, producing a coarse-grained phaneritic texture (crystals visible to the unaided eye) — for example, granite and gabbro.
• Extrusive (volcanic) rocks cool rapidly at or near the surface, giving a fine-grained aphanitic texture (crystals microscopic) — for example, rhyolite and basalt.
• Porphyritic texture (large phenocrysts set in a finer groundmass) records two-stage cooling: slow at depth, then fast after eruption.
• Glassy (obsidian) reflects instantaneous quenching with no crystals; vesicular (pumice, scoria) preserves gas-bubble holes; pyroclastic/fragmental (tuff) is welded volcanic ash.

Compositional Classification

Rocks are grouped by silica (SiO2) content and mineralogy:

"Felsic" comes from feldspar plus silica, rich in Si, Al, Na, and K. "Mafic" comes from magnesium plus ferric iron, rich in Fe, Mg, and Ca. Felsic magmas are cooler, more viscous, and gas-rich (explosive eruptions); mafic magmas are hotter and fluid (effusive lava flows).

A quick field proxy for composition is the color index — the volume percentage of dark ferromagnesian minerals. Felsic rocks are light-colored (low color index), mafic rocks are dark and dense, and ultramafic rocks consist almost entirely of dark minerals. Because extrusive rocks are too fine-grained to identify minerals by eye, geologists rely on color, density, and chemistry rather than mineral counting. Additional textures include pegmatitic (exceptionally coarse, centimeter-scale crystals grown from volatile-rich residual melts) and equigranular versus inequigranular fabrics, while intrusive bodies range from thin dikes and sills to enormous batholiths, with slower, deeper cooling generally yielding coarser crystals.

Bowen's Reaction Series

N. L. Bowen's 1920s melting experiments explain the order in which minerals crystallize from a cooling mafic magma and why minerals occur in predictable associations. The series has two simultaneous branches:

• Discontinuous branch — ferromagnesian minerals crystallize in discrete steps, each with an abruptly different silicate structure: olivine (isolated tetrahedra) then pyroxene (single chain) then amphibole (double chain) then biotite (sheet). Early crystals react with the remaining melt to convert to the next mineral down. This olivine-to-pyroxene conversion is a reaction (peritectic) relationship: under slow, equilibrium cooling early olivine is entirely resorbed, but rapid cooling can preserve olivine cores rimmed by pyroxene (reaction rims). This is also why olivine and quartz almost never coexist in one rock — they sit at opposite ends of the series.
• Continuous branch — plagioclase feldspar changes composition continuously through solid solution, from calcium-rich anorthite at high temperature to sodium-rich albite at low temperature, without changing its framework structure.
• Both branches converge at the base to potassium feldspar, muscovite, and finally quartz, which crystallizes last, at the lowest temperature.

A key exam link: minerals that crystallize early (high temperature) weather most readily at the surface, while quartz, forming last, is the most weathering-resistant. The surface weathering order (Goldich series) is essentially the reverse of Bowen's series, which is why quartz dominates mature sediments.

Magma Differentiation

A single parent magma can yield diverse rock compositions through magmatic differentiation:

• Fractional crystallization — early-formed crystals such as olivine settle out (crystal settling) or are otherwise removed, so the residual melt becomes progressively more felsic (SiO2-enriched). This is the principal mechanism pushing a mafic magma toward granitic compositions.
• Partial melting — rocks melt incrementally, and the first melt is enriched in the low-melting, felsic components; partial melting of mantle peridotite therefore yields basaltic (relatively more felsic) magma, which is why mid-ocean-ridge and hotspot volcanism is dominantly basaltic while subduction zones produce more andesitic and granitic magmas.
• Assimilation (magma dissolving surrounding wall rock) and magma mixing further modify composition.

The final volatile-rich residual liquids, enriched in incompatible elements, crystallize as pegmatites, which host many rare-element and gem minerals. Bowen's series thus ties together crystallization order, mineral compatibility, magmatic differentiation, and weathering stability into one framework that the FG and PG exams test repeatedly.