27.02.02 · earth-science / minerals-rocks

Rock cycle and igneous processes: crystallization, Bowen's reaction series

stub3 tiersLean: nonepending prereqs

Anchor (Master): Bowen, N. L. — The Evolution of the Igneous Rocks (1928)

Intuition Beginner

There are three main rock types on Earth. Igneous rocks form when magma cools and solidifies. Granite forms underground from slowly cooling magma, growing large visible crystals. Basalt erupts from volcanoes as lava and cools fast, producing fine-grained or glassy rock. Sedimentary rocks form when pieces of other rocks, minerals, or organic material are compressed and cemented together over time. Sandstone, limestone, and shale are common examples.

Metamorphic rocks form when existing rocks are changed by heat and pressure without fully melting. Marble forms from limestone, and slate forms from shale. The original rock transforms in place, recrystallizing into new minerals while staying solid.

These three types cycle into each other endlessly. Igneous rock erodes into sediment, which becomes sedimentary rock. Sedimentary rock is buried and metamorphosed. Metamorphic rock melts back into magma, forming new igneous rock. Any rock type can transform into any other through the right geologic processes.

Bowen's reaction series explains why different minerals crystallize from magma at different temperatures. As magma cools, olivine and pyroxene crystallize first at high temperatures. Feldspar and quartz crystallize last at lower temperatures. This orderly sequence governs which minerals appear together in igneous rocks.

Visual Beginner

Rock type	How it forms	Examples	Key feature
Igneous (intrusive)	Magma cools slowly underground	Granite, diorite, gabbro	Large visible crystals
Igneous (extrusive)	Lava cools rapidly at surface	Basalt, andesite, rhyolite	Fine-grained or glassy
Sedimentary	Sediment compacted and cemented	Sandstone, limestone, shale	Layers, may hold fossils
Metamorphic	Heat and pressure alter existing rock	Marble, slate, schist	Harder, often foliated

Bowen's branch	Mineral sequence	Temperature
Discontinuous	Olivine → Pyroxene → Amphibole → Biotite	High → Low
Continuous	Ca-plagioclase → Na-plagioclase	High → Low
Converges to	K-feldspar, muscovite, quartz	Lowest

Worked example Beginner

A geologist collects a light-colored rock with large pink feldspar crystals, clear quartz grains, and thin black biotite flakes. The crystals are several millimeters across. What is it, and where did it form?

The large crystal size means the rock cooled slowly underground. This is a phaneritic texture, characteristic of intrusive igneous rocks. Extrusive rocks cool too fast for large crystals to grow.

The mineral composition gives the rock's identity. Pink potassium feldspar, quartz, and biotite are felsic minerals, rich in silica. This combination identifies the rock as granite, the most common intrusive rock in continental crust.

Granite forms when silica-rich magma intrudes deep in the crust and cools over thousands to millions of years. The slow cooling allows each mineral to grow large, visible crystals. According to Bowen's reaction series, quartz and potassium feldspar are among the last minerals to crystallize from a cooling magma, consistent with the felsic composition.

If the same magma had erupted at the surface, it would have cooled rapidly to form rhyolite, a fine-grained volcanic rock with the same chemistry as granite but a different texture.

Check your understanding Beginner

Formal definition Intermediate+

Magma generation

Magma is generated by partial melting of solid rock in the upper mantle or lower crust. Three principal mechanisms produce the necessary conditions for melting.

Decompression melting occurs when hot mantle rock rises adiabatically, crossing its solidus because pressure decreases faster than temperature along the ascent path. This is the dominant process at mid-ocean ridges, where divergent plate motion drives mantle upwelling. The degree of partial melting, typically 10-20 percent, determines the volume and composition of the basaltic melt produced.

Flux melting occurs at subduction zones where water and other volatiles released from the descending slab lower the solidus temperature of the overlying mantle wedge. The added water disrupts silicate bonds in mantle minerals, allowing melting at temperatures hundreds of degrees below the dry solidus. This process generates the calc-alkaline magmas characteristic of volcanic arcs.

Heat-transfer melting occurs when hot magma or mantle plume material intrudes into cooler crustal rock, raising its temperature above the solidus. This mechanism operates at hot spots and in continental rift zones where basaltic magma ponds at the base of the crust.

Magma composition

Magma composition is classified by silica content. Mafic magmas contain 45-52 percent SiO $_{2}$ and are rich in magnesium and iron. They produce basalt (extrusive) and gabbro (intrusive). Intermediate magmas contain 52-65 percent SiO $_{2}$ , yielding andesite and diorite. Felsic magmas contain greater than 65 percent SiO $_{2}$ , producing rhyolite and granite. Ultramafic magmas, with less than 45 percent SiO $_{2}$ , produce komatiite (rare, mostly Archean) and peridotite.

Silica content controls magma viscosity. Felsic magmas are viscous and tend to trap gases, producing explosive eruptions. Mafic magmas are fluid and degas readily, producing effusive eruptions of lava flows.

Intrusive versus extrusive igneous rocks

Cooling rate governs crystal size and therefore igneous texture. Intrusive (plutonic) rocks cool slowly at depth, producing phaneritic texture with crystals visible to the unaided eye (greater than 1 mm). Extrusive (volcanic) rocks cool rapidly at the surface, producing aphanitic texture with crystals too fine to see without magnification.

Porphyritic texture, featuring large crystals (phenocrysts) embedded in a finer matrix (groundmass), records a two-stage cooling history: slow initial cooling followed by rapid eruption. Glassy texture (obsidian) forms when lava is quenched so rapidly that no crystals have time to grow.

Bowen's reaction series

Norman Levi Bowen demonstrated in the early 20th century that magma crystallization follows a predictable sequence. The series has two branches.

The discontinuous branch tracks mafic mineral crystallization: olivine crystallizes first at the highest temperature, then reacts with the melt to form pyroxene, which reacts to form amphibole, which reacts to form biotite. Each successive mineral has a different crystal structure and lower formation temperature.

The continuous branch tracks plagioclase feldspar crystallization. Calcium-rich plagioclase (anorthite) forms first at high temperature. As cooling proceeds and calcium is progressively removed from the melt, increasingly sodium-rich plagioclase crystallizes, ending with albite at lower temperatures.

Both branches converge at the lowest temperatures, where potassium feldspar, muscovite, and quartz crystallize from the residual melt. These minerals characterize felsic rocks.

Fractional crystallization and magma differentiation

Fractional crystallization occurs when early-formed crystals are physically separated from the remaining melt, preventing re-equilibration. As crystals settle or are removed, the residual melt evolves in composition, becoming progressively enriched in silica, sodium, and potassium. A single parent magma can produce a range of rock compositions through fractional crystallization.

Mass balance governs this process. If a magma of initial composition $C_{0}$ crystallizes a solid of composition $C_{s}$ while the coexisting melt has composition $C_{l}$ , and $F$ is the mass fraction of melt remaining:

C_{0} = F \cdot C_{l} + (1 - F) \cdot C_{s}

Rayleigh fractionation describes continuous crystal removal:

C_{l} = C_{0} \cdot F^{(D - 1)}

where $D$ is the bulk distribution coefficient. For elements preferentially excluded from the crystallizing minerals ( $D < 1$ ), concentration in the melt increases as fractionation proceeds.

Magma chamber processes

Magma chambers are not simple containers of uniform liquid. Assimilation occurs when hot magma melts and incorporates surrounding wall rock, modifying its composition. Magma mixing occurs when two magmas of different composition encounter each other, producing a hybrid composition. These processes, combined with fractional crystallization, account for much of the compositional diversity observed in igneous rocks.

Plutonic bodies

Intrusive igneous bodies are classified by size and geometry. Batholiths are the largest, with surface exposures exceeding 100 km $^{2}$ , and are typically composed of multiple plutons of granitic to dioritic composition. Stocks are smaller exposures (less than 100 km $^{2}$ ). Dikes are tabular intrusions that cut across bedding or foliation. Sills are tabular intrusions that are concordant with (parallel to) existing layering. Laccoliths are dome-shaped intrusions that arch overlying strata upward.

Volcanic rocks

Volcanic rocks are classified by composition and texture. Basalt (mafic, aphanitic) is the most abundant volcanic rock on Earth and forms the ocean floor. Andesite (intermediate) dominates volcanic arcs above subduction zones. Rhyolite (felsic) forms from highly viscous, gas-rich magmas that tend to produce explosive eruptions and welded tuffs. Komatiite (ultramafic) is rare in the Phanerozoic but was abundant in the Archean, indicating higher mantle temperatures in early Earth history.

Key result: Bowen's reaction series and magma evolution Intermediate+

Bowen's reaction series provides the conceptual framework for understanding how a single parent magma can generate the full spectrum of igneous rock compositions. The series predicts that early-crystallizing minerals (olivine, calcium-rich plagioclase) are mafic and dense, while late-crystallizing minerals (quartz, potassium feldspar) are felsic and less dense.

The lever rule quantifies the proportions of liquid and solid at any temperature in a binary system. For a system with end-members A and B, if the bulk composition is $X_{0}$ and the system sits at a temperature where liquid has composition $X_{l}$ and solid has composition $X_{s}$ , the mass fraction of liquid is:

F_{l} = \frac{X _{0} - X _{s}}{X _{l} - X _{s}}

For natural multi-component systems, this principle extends to trace element modeling. The concentration of a trace element in the melt during equilibrium batch melting is:

C_{l} = \frac{C _{0}}{D + F ( 1 - D )}

where $C_{0}$ is the source concentration, $D$ is the bulk distribution coefficient, and $F$ is the melt fraction. For fractional melting, the expression becomes:

C_{l} = \frac{C _{0}}{D} \cdot (\frac{D}{D + F ( 1 - D )})^{1/ (1 - D)}

These equations allow petrologists to model the chemical evolution of magmas and compare model predictions with observed rock compositions. The agreement between trace element models and natural rock suites provides strong evidence that fractional crystallization and partial melting are the dominant processes generating igneous diversity.

Exercises Intermediate+

Exercise 4 (hard, short answer).

Explain how Bowen's reaction series accounts for the observation that olivine-rich rocks weather faster than quartz-rich rocks at Earth's surface.

Hint

Consider the temperature at which each mineral forms relative to surface conditions, and what this means for thermodynamic stability at the surface.

Answer

Olivine crystallizes at the highest temperature in Bowen's series (roughly 1200-1400 degrees Celsius), far from surface conditions. At the low temperatures and in the presence of water at Earth's surface, olivine is far from its equilibrium conditions and reacts readily with water and acids, breaking down quickly. Quartz crystallizes at the lowest temperature in Bowen's series, close to surface conditions, making it the most stable silicate mineral at the surface. Minerals that crystallize at temperatures furthest from surface conditions are thermodynamically least stable there and weather fastest. This pattern, known as the Goldich dissolution series, mirrors Bowen's reaction series in reverse.

Advanced results Master

Phase equilibria in silicate systems

The quantitative foundation of igneous petrology rests on experimental phase equilibria. Binary and ternary phase diagrams map the stability fields of liquid, solid, and liquid-plus-solid as functions of temperature and composition. The simplest model system capturing essential igneous behavior is the forsterite-silica (Mg $_{2}$ SiO $_{4}$ -SiO $_{2}$ ) binary, which demonstrates incongruent melting: enstatite (MgSiO $_{3}$ ) melts to produce forsterite plus liquid rather than melting congruently.

Eutectic crystallization occurs when a liquid of specific composition solidifies at the lowest temperature in the system, producing two solid phases simultaneously. Most natural magmas crystallize near eutectic points in multi-component systems, which constrains the range of possible magma compositions.

Solid solution occurs when two end-members share a crystal structure and mix freely across a range of compositions. The plagioclase series (albite-anorthite) is the classic example. Exsolution occurs when a homogeneous solid solution becomes unstable at lower temperatures and unmixes into two distinct phases. Perthite, the intergrowth of sodium-rich and potassium-rich feldspar, records exsolution during slow cooling of granite.

Experimental petrology

Experimental petrology reproduces the pressure-temperature conditions of Earth's interior in the laboratory. Piston-cylinder apparatus reaches pressures equivalent to upper mantle depths (up to roughly 3 GPa). Multi-anvil presses extend the range to transition zone conditions. Diamond-anvil cells reach lower mantle and core pressures.

Norman Bowen's experiments at the Carnegie Institution's Geophysical Laboratory (1910s-1930s) established the foundational phase equilibria for the major silicate systems. Modern experiments incorporate controlled oxygen fugacity, volatile species (H $_{2}$ O, CO $_{2}$ ), and precise analytical characterization of run products by electron microprobe and synchrotron X-ray diffraction.

Geothermobarometry

Geothermobarometry uses the compositions of coexisting mineral pairs or assemblages to estimate the temperature and pressure at which a rock equilibrated. Exchange thermometers exploit temperature-sensitive cation partitioning between two phases: the garnet-biotite Fe-Mg exchange thermometer and the two-pyroxene thermometer are widely used. Net-transfer barometers exploit pressure-sensitive reactions: the garnet-plagioclase-aluminosilicate-quartz barometer constrains crustal pressures.

The calibration of these thermobarometers relies on experimentally determined phase equilibria and thermodynamic mixing models. When applied to granulite-facies or eclogite-facies assemblages, they yield P-T estimates that constrain the thermal structure of the crust and mantle during orogenesis.

REE patterns and trace element modeling

Rare earth elements (REE) provide powerful tracers of magmatic processes because they are largely immobile during metamorphism and their systematic variation in ionic radius across the lanthanide series produces diagnostic fractionation patterns. Chondrite-normalized REE diagrams reveal the character of the source region and the degree of partial melting or fractional crystallization.

Batch melting and fractional crystallization equations predict the trace element concentrations in melts and residues. Light REE enrichment (steep negative slope on chondrite-normalized diagrams) indicates small degrees of partial melting of garnet-bearing mantle. Flat or heavy REE-enriched patterns indicate melting in the spinel stability field or large degrees of melting. Negative europium anomalies indicate plagioclase fractionation, because Eu $^{2 +}$ substitutes for Ca $^{2 +}$ in plagioclase.

Magma ocean crystallization

The early Earth and Moon both experienced magma ocean stages: periods when a substantial fraction of the body was molten. The Hadean Earth (greater than 4.4 Ga) may have had a magma ocean hundreds of kilometers deep following the Moon-forming impact. Crystallization of such a magma ocean proceeds from the bottom up: dense minerals (olivine, pyroxene) crystallize first and settle, while incompatible elements are concentrated in the residual melt near the surface.

The lunar magma ocean model explains the compositional stratification of the Moon. Cumulate olivine and pyroxene form the mantle. Plagioclase flotation produces the anorthositic highlands crust. The final dregs of residual melt, enriched in potassium, rare earth elements, and phosphorus (KREEP), form a geochemical signature concentrated near the Procellarum region.

Layered mafic intrusions

Layered mafic intrusions record fractional crystallization on a colossal scale. The Skaergaard intrusion (Greenland) crystallized from a single magma chamber, producing a stratigraphic sequence from olivine-rich cumulates at the base through pyroxene and plagioclase layers to granophyre at the top. The Bushveld Complex (South Africa) is the world's largest layered intrusion, hosting the bulk of the planet's platinum-group element resources in distinct sulfide-rich layers.

These intrusions preserve a complete record of magma differentiation. Modal layering, graded bedding, and cryptic layering (systematic changes in mineral composition through the stratigraphy) document the progressive evolution of the magma chamber during crystallization.

Crustal differentiation and TTG suites

The Archean continental crust is dominated by tonalite-trondhjemite-granodiorite (TTG) suites, which differ from modern granitic rocks in their high sodium content, steep REE patterns, and absence of negative Eu anomalies. Experimental studies show that TTG magmas form by partial melting of hydrated basalt (amphibolite or eclogite) at pressures greater than 1.5 GPa, corresponding to crustal depths greater than 50 km.

The transition from TTG-dominated Archean crust to the more potassic granitic crust of the Proterozoic and Phanerozoic reflects changes in the style of crustal differentiation, possibly linked to the onset of modern-style plate tectonics and subduction.

Zircon as the oldest mineral

Zircon (ZrSiO $_{4}$ ) is the oldest known terrestrial mineral. Detrital zircons from the Jack Hills conglomerate in Western Australia yield U-Pb ages up to 4.4 Ga, within 150 million years of Earth's formation. Zircon survives weathering, metamorphism, and erosion because of its extreme chemical and physical durability.

Jack Hills zircons carry oxygen isotope signatures ( $δ^{18}$ O values higher than mantle-equilibrium) that suggest the parent magmas incorporated material that had interacted with surface water at low temperatures. This evidence points to the existence of liquid water on Earth's surface as early as 4.3 Ga, with implications for the thermal and atmospheric conditions of the Hadean Earth.

Connections Master

Connections to plate tectonics

The three principal magma generation mechanisms correspond directly to the three types of plate boundaries. Decompression melting at divergent boundaries produces the basaltic ocean floor. Flux melting at convergent boundaries generates the calc-alkaline magmas of volcanic arcs. Hot spots (intraplate volcanism) produce ocean island basalts by melting beneath plates far from any boundary.

The composition of volcanic rocks tracks plate tectonic setting. Tholeiitic basalt dominates mid-ocean ridges. Calc-alkaline andesite dominates subduction zones. Alkali basalt characterizes intraplate hot spots. This compositional fingerprint allows geologists to infer ancient tectonic settings from the geochemistry of preserved igneous rocks.

Connections to Earth history

Igneous rocks record the thermal evolution of the Earth. Komatiites, ultramafic volcanic rocks abundant in the Archean but rare afterward, indicate that the Archean mantle was roughly 200-300 degrees Celsius hotter than today. The secular cooling of the mantle is recorded in the declining magnesium content of komatiites through geologic time.

The transition from TTG-dominated crust in the Archean to the more diverse granitic crust of later eons reflects the maturation of plate tectonic processes. Zircon geochemistry provides a continuous record of crustal extraction events, revealing pulses of continental growth that correspond to supercontinent assembly.

Connections to mineral resources

Igneous processes concentrate many economically important metals. Chromite and platinum-group elements accumulate in layered mafic intrusions by crystal settling. Nickel sulfides form when sulfide liquid separates from silicate melt and scavenges chalcophile elements. Pegmatites, the last crystallization products of granitic magma, host lithium, beryllium, tantalum, and rare earth elements.

Porphyry copper deposits, the world's primary source of copper and molybdenum, form from hydrothermal fluids exsolved from cooling intermediate-composition plutons above subduction zones. Understanding the crystallization history and volatile evolution of these plutons is essential for mineral exploration models.

Connections to planetary science

Magma ocean crystallization operated on all terrestrial planets and large moons. The lunar anorthositic crust formed by plagioclase flotation in the lunar magma ocean. Mars shows evidence of early differentiation producing a basaltic crust with distinct geochemical provinces. Vesta, the differentiated asteroid, preserves a basaltic crust overlying an olivine mantle and iron core, a miniature magma ocean product.

Comparative planetology reveals how initial conditions (size, composition, distance from the Sun) control the style and duration of igneous activity. Small bodies cool quickly and produce only early igneous rocks. Large bodies sustain mantle convection and volcanism for billions of years.

Connections to the rock cycle (Unit 27.02.01)

This unit extends the rock cycle framework from Unit 27.02.01 by treating igneous processes in depth. The crystallization pathways described by Bowen's reaction series determine which minerals are available for weathering at the surface. Minerals that crystallize at high temperatures (olivine, calcium plagioclase) weather rapidly, contributing ions to surface water and sediment. Minerals that crystallize at low temperatures (quartz) are resistant and accumulate as detrital sediment. This weathering selectivity controls the composition of sedimentary rocks downstream in the rock cycle.

Historical and philosophical context Master

Norman Bowen and the experimental tradition

Norman Levi Bowen (1887-1956) transformed petrology from a descriptive catalog of rock types into a quantitative experimental science. Working at the Carnegie Institution's Geophysical Laboratory in Washington, D.C., Bowen conducted systematic experiments on silicate melts under controlled temperature and composition conditions between 1910 and 1935. His 1928 book The Evolution of the Igneous Rocks synthesized these experiments into a unified theory of igneous petrogenesis centered on fractional crystallization.

Bowen's reaction series emerged from his observation that the minerals in natural igneous rocks appear in a predictable crystallization order. He demonstrated experimentally that olivine reacts with melt to form pyroxene, pyroxene reacts to form amphibole, and so on, as temperature decreases. The continuous branch of plagioclase composition change was confirmed by controlled cooling experiments.

Bowen's emphasis on fractional crystallization as the dominant mechanism of magma differentiation placed him in opposition to proponents of multiple primary magmas (the "Granite Debate" of the 1940s). Subsequent research has shown that both processes operate: fractional crystallization drives differentiation within magma chambers, while partial melting of different source materials generates the primary diversity of magma compositions.

The development of experimental petrology

Bowen's laboratory methods were limited to one-atmosphere experiments and simple binary systems. The development of high-pressure apparatus in the 1950s and 1960s by H.S. Yoder, A.L. Boettcher, and others extended experimental petrology to the conditions of the upper mantle. These experiments revealed the role of water in lowering solidus temperatures and demonstrated that basaltic magma is generated by partial melting of peridotite at mantle pressures.

Modern experimental petrology employs piston-cylinder, multi-anvil, and diamond-anvil cells to cover the full range of Earth's interior conditions. Combined with advances in analytical instrumentation (electron microprobe, ion microprobe, synchrotron X-ray diffraction), these experiments provide the thermodynamic data that underpin quantitative models of magma generation and differentiation.

The granite controversy

The origin of granite was one of the most contentious debates in geology during the 19th and early 20th centuries. The "magmatists," following Bowen, argued that granite crystallized from silicate melt. The "transformists," led by the French petrographer Auguste Michel-Levy and later Hans Ramberg, argued that granite formed by solid-state replacement (metasomatism) of pre-existing rock. Field evidence from cross-cutting relationships, contact metamorphic aureoles, and geochemical data eventually settled the debate in favor of a magmatic origin for most granites, although the distinction between magmatic and metasomatic processes remains relevant for specific rock types such as charnockites and some migmatites.

Bibliography Master

Tarbuck, F. K. and Lutgens, E. J., Earth Science, 15th ed. (Pearson, 2018), Ch. 3–4.
Best, M. G., Igneous and Metamorphic Petrology, 2nd ed. (Blackwell, 2003), Ch. 1–4.
Bowen, N. L., The Evolution of the Igneous Rocks (Princeton University Press, 1928), Reaction series.
Yoder, H. S., Generation of Basaltic Magma (National Academy of Sciences, 1976).
Philpotts, A. R. and Ague, J. J., Principles of Igneous and Metamorphic Petrology, 2nd ed. (Cambridge University Press, 2009).
Winter, J. D., An Introduction to Igneous and Metamorphic Petrology (Pearson, 2001).
Rollinson, H. R., Using Geochemical Data: Evaluation, Presentation, Interpretation (Longman, 1993).
Wilde, S. A., Valley, J. W., Peck, W. H. and Graham, C. M., "Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago," Nature 409 (2001) 175–178.
Elkins-Tanton, L. T., "Continental magmatism caused by lithospheric delamination," in Archean Geodynamics and Environments (AGU Geophysical Monograph 164, 2006) 279–292.
Naslund, H. R. and McBirney, A. R., "Mechanisms of formation of igneous layering," in Layered Intrusions (Elsevier, 1996) 1–43.