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As magma cools, minerals crystallize in a predictable sequence (Bowen's series) determined by thermodynamic stability. Fractional crystallization—where crystals separate from liquid—changes the liquid's composition over time, explaining why a single magma can produce rocks of different compositions.
Track how melt composition evolves as minerals remove elements during crystallization. Compare computed trends with natural rock compositions.
From your study of magma generation, you know that melting and crystallization depend on temperature, pressure, and composition. Bowen's reaction series takes this a step further by showing that as a magma cools, minerals do not all appear at once — they crystallize in a predictable sequence. The minerals that form at the highest temperatures (olivine, calcium-rich plagioclase) appear first, and those stable at lower temperatures (quartz, potassium feldspar, muscovite) form last. This sequence was established experimentally by N.L. Bowen in the early twentieth century and remains one of the most powerful organizing frameworks in igneous petrology.
The series has two branches that operate simultaneously. The discontinuous branch on the left side describes ferromagnesian minerals that change abruptly in crystal structure as temperature drops: olivine gives way to pyroxene, then amphibole, then biotite. Each transition involves a reaction between the existing crystals and the remaining liquid — the early crystal becomes unstable and is replaced by a new mineral with a different structure. The continuous branch on the right describes plagioclase feldspar, which changes composition smoothly from calcium-rich (anorthite) at high temperatures to sodium-rich (albite) at low temperatures, as calcium and sodium continuously exchange between crystal and melt. Both branches converge at the bottom of the series where potassium feldspar, muscovite, and quartz crystallize from the most silica-rich residual liquid.
Fractional crystallization is the process that makes this sequence consequential for rock diversity. If early-formed crystals stay in contact with the melt, they react with it and the final rock is compositionally uniform. But if crystals are physically separated from the liquid — by sinking due to their higher density, by being filtered out as magma migrates, or by being left behind on chamber walls — the remaining melt changes composition. Each mineral that separates removes specific elements: olivine strips out magnesium and iron, plagioclase removes calcium and aluminum. The residual liquid becomes progressively enriched in silica, sodium, and potassium. This is why a single batch of basaltic magma can ultimately produce rocks ranging from gabbro to granite — not by adding new material, but by subtracting crystals at each stage.
Think of it like making maple syrup by boiling sap: as water evaporates (analogous to crystals removing elements), the remaining liquid becomes increasingly concentrated in sugar. In magma, "removing" early minerals concentrates the components that form later minerals. This process — called magmatic differentiation — explains much of the compositional diversity observed in igneous rock suites. A layered intrusion like the Bushveld Complex in South Africa preserves this process frozen in stone: dense olivine-rich cumulates at the base, progressively more felsic rocks toward the top, recording the evolutionary path of a cooling and differentiating magma chamber over millions of years.