The Core Idea
From Mineral Sequence to Rock Family
Bowen's Reaction Series, first covered under Minerals, explains the order in which minerals crystallize from cooling magma — but its real power for rock classification is explaining why certain minerals consistently occur together in the same igneous rock and never in others. Because olivine and pyroxene crystallize at the highest temperatures, they dominate mafic and ultramafic rocks (basalt, gabbro, peridotite). Because quartz and potassium feldspar crystallize last at the lowest temperatures, they dominate felsic rocks (granite, rhyolite). Rocks with intermediate composition (andesite, diorite) contain amphibole and intermediate plagioclase, reflecting the middle portion of the series.
This is why you'll never find quartz growing alongside olivine in the same igneous rock under normal conditions — their crystallization temperatures are so far apart that by the time the melt cools enough for quartz to form, all the magnesium and iron needed for olivine has already been used up by earlier-forming minerals.
💡 Memory Trick
'Old Pigs Always Bring Kids Milk, Quietly' — O-P-A-B-K-M-Q: Olivine, Pyroxene, Amphibole, Biotite, K-feldspar, Muscovite, Quartz — the full discontinuous-plus-converged crystallization order, high to low temperature. Picture old pigs on a farm delivering milk to kids, getting quieter (cooler) with every stop — by the time they reach Quartz, the farm has gone completely silent, just like the melt has gone completely cool. The two branches (discontinuous: olivine→pyroxene→amphibole→biotite; continuous: plagioclase shifting Ca-rich to Na-rich) converge right before K-feldspar.
Applying the Series
Predicting Rock Composition from Crystallization Order
1
Early/High-Temperature Minerals → Mafic/Ultramafic Rocks
Olivine and calcium-rich plagioclase crystallize first, at the highest temperatures, and dominate mafic and ultramafic rocks.
Example: peridotite is almost entirely olivine and pyroxene — the earliest-forming minerals in the series.
2
Mid-Series Minerals → Intermediate Rocks
Amphibole and intermediate (calcium-sodium) plagioclase crystallize in the middle temperature range, producing intermediate composition rocks.
Example: diorite/andesite show a characteristic mix of dark amphibole and lighter intermediate plagioclase.
3
Late/Low-Temperature Minerals → Felsic Rocks
Biotite mica, potassium feldspar, muscovite mica, and quartz crystallize last, at the lowest temperatures, producing felsic rocks.
Example: granite is dominated by potassium feldspar and quartz, the very last minerals to crystallize.
Real-World Consequence
Why Magma Composition Changes As It Cools
As early-forming minerals like olivine crystallize and settle out of a magma chamber, they physically remove iron, magnesium, and calcium from the remaining liquid — a process called fractional crystallization. This means the leftover magma becomes progressively more silica-rich and enriched in the elements needed for later minerals, even without any new material being added. This single process explains how a single starting magma body can eventually produce a whole range of rock compositions, from mafic rock crystallizing early to felsic rock crystallizing from the leftover, evolved magma much later.
🖥️ Applied Scenario
A petrology student is asked to explain why a single magma chamber produced both gabbro at its base and granite at its top.
1
The student recognizes that olivine and calcium-rich plagioclase crystallized first, at the chamber's base, as the magma began cooling — consistent with early Bowen's Series minerals sinking due to their higher density.
2
As these minerals crystallized and settled out, they removed iron, magnesium, and calcium from the remaining liquid magma, leaving it progressively more enriched in silica, sodium, and potassium.
3
Conclusion: by the time the remaining magma finally cooled at the top of the chamber, its chemistry had evolved enough to crystallize quartz and potassium feldspar instead — producing granite from the very same original magma body that produced gabbro at its base, entirely through fractional crystallization.
📌 Exam Application
Exams frequently ask you to predict which minerals would be found together in a given rock, or to explain fractional crystallization as the mechanism behind compositional variation within a single magma body — be ready to connect Bowen's Series directly to rock classification, not just mineral identification.
⚠️ Most Common Bowen's Reaction Series Mistakes
Don't treat Bowen's Reaction Series as only a mineral-identification topic — its main geologic importance is explaining why specific mineral assemblages define specific rock types, and why a single magma source can produce multiple different rock compositions over time. Also remember the discontinuous branch involves minerals reacting and changing identity, while the continuous branch (plagioclase) involves one mineral gradually shifting composition without changing identity — these are two fundamentally different kinds of change.
✓ Quick Self-Test
1) Which minerals dominate mafic rocks according to Bowen's Series, and which dominate felsic rocks? 2) What is fractional crystallization, and how does it change remaining magma composition? 3) Explain the difference between the discontinuous and continuous branches of the series.
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Granite and Granitic Rocks
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