Why do minerals crystallise from a magma in a set order, and how does that change the magma?
Igneous processes: Bowen's reaction series as the order in which silicate minerals crystallise from a cooling magma; the discontinuous (olivine to biotite) and continuous (calcium-rich to sodium-rich plagioclase) branches; the use of the series to explain fractional crystallisation, magma differentiation and the resistance of minerals to weathering.
A focused answer to the OCR H414 dot point on Bowen's reaction series. Covers the discontinuous and continuous branches, the crystallisation order of silicate minerals, how fractional crystallisation drives magma differentiation from basic to acid compositions, and how the series predicts weathering resistance.
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What this dot point is asking
OCR wants you to describe Bowen's reaction series as the order in which silicate minerals crystallise from a cooling magma, distinguish the discontinuous and continuous branches, and use the series to explain fractional crystallisation and magma differentiation (how a magma changes composition) and to predict the relative resistance of minerals to weathering.
The answer
What Bowen's reaction series is
A magma does not freeze all at once. As it cools, different minerals crystallise at different temperatures, and Bowen's reaction series is the sequence in which the common silicate minerals form. It has two branches that operate together.
The two branches
- Discontinuous branch. A series of different minerals, each reacting with the melt to form the next as temperature falls: olivine, then pyroxene, then amphibole, then biotite. It is "discontinuous" because the crystal structure changes at each step (isolated to chain to double-chain to sheet silicates).
- Continuous branch. A single mineral, plagioclase feldspar, that continuously changes composition as it cools, from calcium-rich (anorthite) at high temperature to sodium-rich (albite) at lower temperature.
Below where the branches meet, the lowest-temperature minerals crystallise in order: potassium feldspar, then muscovite, then quartz. So olivine and calcium-rich plagioclase form first (high temperature, silica-poor), and quartz forms last (low temperature, silica-rich).
Fractional crystallisation and differentiation
If early-formed crystals stayed and re-reacted, the magma would simply yield its original composition. But often they are removed from the melt, by crystal settling (dense early crystals sink) or by the residual liquid being filtered or squeezed away. Once removed, those crystals can no longer react.
Because the first crystals (olivine, pyroxene, calcium plagioclase) are rich in iron, magnesium and calcium but poor in silica, removing them leaves a residual melt enriched in silica, sodium and potassium. Repeated, this magma differentiation can evolve a single basic (basaltic) parent magma towards intermediate (andesitic) and ultimately acid (rhyolitic) compositions. It explains why a single magma chamber can produce a range of igneous rocks, and why layered intrusions show mafic minerals concentrated near the base.
Examples in context
Example 1. Layered mafic intrusions. In bodies such as large basic intrusions, dense early crystals (olivine and pyroxene) settle to form mafic layers at the base, while later, more felsic minerals form higher up: direct field evidence for fractional crystallisation.
Example 2. Andesite at subduction zones. Fractional crystallisation of a basaltic parent magma, combined with assimilation of continental crust, is one way intermediate (andesitic) magmas are generated at destructive margins.
Try this
Q1. Name the four minerals of the discontinuous branch of Bowen's reaction series, in order of crystallisation. [2 marks]
- Cue. Olivine, pyroxene, amphibole, biotite.
Q2. Explain what is meant by fractional crystallisation. [2 marks]
- Cue. The removal of early-formed crystals from a magma (for example by settling) so they no longer react with the melt, changing the composition of the remaining liquid.
Q3. State how the silica content of a residual magma changes as early mafic minerals are removed, and why. [2 marks]
- Cue. The silica content increases, because the removed early crystals (olivine, pyroxene) are silica-poor, leaving the residual melt enriched in silica.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H414/01 20206 marksExplain how fractional crystallisation can change a basic magma into a more acid (silica-rich) residual magma. Refer to Bowen's reaction series in your answer.Show worked answer →
This is a level-of-response style answer; build a linked chain of reasoning.
- Crystallisation order
- As a magma cools, minerals crystallise in the order set by Bowen's reaction series. The first to form are high-temperature, silica-poor minerals: olivine (discontinuous branch) and calcium-rich plagioclase (continuous branch).
- Removal of early crystals
- These early-formed crystals are denser than the melt and settle to the floor of the magma chamber (crystal settling), or the remaining liquid is squeezed away from them. Either way they are removed from the system, so they can no longer react with the melt.
- Change in melt composition
- Because the removed crystals are rich in iron, magnesium and calcium and poor in silica, the residual melt becomes progressively enriched in silica, sodium and potassium. Continued cooling crystallises amphibole, then biotite, then sodium-rich plagioclase, leaving a melt that finally crystallises potassium feldspar, muscovite and quartz.
- Result
- The residual magma has evolved from basic towards acid in composition, so an originally basaltic magma can yield more andesitic or even rhyolitic liquid.
Top-band answers explicitly link removal of early mafic crystals to silica enrichment of the residual melt, using the series.
OCR H414/01 20173 marksUse Bowen's reaction series to predict, with a reason, which of olivine and quartz is more resistant to chemical weathering at the Earth's surface.Show worked answer →
State the prediction and tie it to crystallisation temperature.
Prediction: quartz is more resistant than olivine.
Reason. Bowen's series shows olivine crystallises first, at the highest temperature, far from surface conditions, so it is least stable when exposed at the cool, wet surface and weathers fastest. Quartz crystallises last, at the lowest temperature, closest to surface conditions, so it is the most stable and most resistant to chemical weathering. This is why quartz dominates many sands and sandstones while olivine is rare in sediments.
Markers reward the correct order plus the link between crystallisation temperature and surface stability.
Related dot points
- Minerals and rocks: the structure of the silicate minerals based on the silica tetrahedron; the silicate groups (isolated, chain, sheet and framework silicates) and how the degree of polymerisation links to composition; the physical properties (colour, lustre, hardness, cleavage, fracture, streak, density and habit) used to identify the common rock-forming minerals quartz, feldspar, mica, olivine, pyroxene and amphibole in hand specimen.
A focused answer to the OCR H414 dot point on rock-forming minerals. Covers the silica tetrahedron, the isolated, chain, sheet and framework silicate groups, how polymerisation links to composition, and how to identify quartz, feldspar, mica, olivine, pyroxene and amphibole from physical properties in hand specimen.
- Igneous rocks: classification by silica content (acid, intermediate, basic and ultrabasic) and by grain size (coarse-grained intrusive, fine-grained extrusive); the relationship between cooling rate and crystal size; igneous textures (phaneritic, aphanitic, porphyritic, glassy and vesicular) and what they show about the cooling history; naming common igneous rocks such as granite, gabbro, basalt and rhyolite.
A focused answer to the OCR H414 dot point on igneous rock classification. Covers acid, intermediate, basic and ultrabasic compositions, coarse versus fine grain size, the link between cooling rate and crystal size, the main textures (phaneritic, aphanitic, porphyritic, glassy, vesicular), and naming granite, gabbro, basalt and rhyolite.
- Igneous bodies: the forms of intrusive igneous bodies (batholiths, dykes, sills and laccoliths) and their relationship to the country rock (concordant versus discordant); chilled margins, baked margins and contact metamorphic aureoles as evidence of intrusion; the recognition of extrusive forms (lava flows and their cross-cutting relationships) and the use of these relationships to establish relative age.
A focused answer to the OCR H414 dot point on igneous bodies. Covers batholiths, dykes, sills and laccoliths, concordant versus discordant intrusions, chilled and baked margins and contact aureoles as evidence of intrusion, and how cross-cutting relationships of dykes, sills and lava flows establish relative age.
- Surface processes: mechanical weathering (freeze-thaw, exfoliation and abrasion) and chemical weathering (solution, hydrolysis and oxidation); the difference between weathering and erosion; transport by water, wind and ice and its effect on the rounding and sorting of sediment; how the maturity and texture of a sediment record its transport history.
A focused answer to the OCR H414 dot point on surface processes. Covers mechanical weathering (freeze-thaw, exfoliation, abrasion) and chemical weathering (solution, hydrolysis, oxidation), the difference between weathering and erosion, transport by water, wind and ice, and how rounding, sorting and maturity record a sediment's transport history.
- Ore formation: the processes that concentrate metals into economic mineral deposits (hydrothermal vein and disseminated deposits, magmatic segregation, placer deposits and residual deposits); the conditions and host rocks typical of each; the distinction between ore and gangue and the idea that a deposit is economic only if the metal is concentrated well above its crustal average.
A focused answer to the OCR H414 dot point on ore formation. Covers hydrothermal vein and disseminated deposits, magmatic segregation, placer deposits and residual deposits, the conditions and host rocks of each, the distinction between ore and gangue, and the requirement that a metal be concentrated well above its crustal average to be economic.
Sources & how we know this
- OCR A Level Geology (H414) Specification — OCR (2017)