How do magmas form, evolve and produce the range of igneous rocks?
Partial melting of the mantle and crust, the controls on melting (temperature, pressure and water), magma series, and the evolution of magma by fractional crystallisation interpreted through Bowen's reaction series.
A focused WJEC and Eduqas A-Level Geology G1 answer on how magmas form by partial melting, the roles of temperature, pressure and water, the basalt-andesite-rhyolite series, and how fractional crystallisation governed by Bowen's reaction series evolves a magma and produces the range of igneous rocks.
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What this dot point is asking
G1 lifts the AS picture of igneous rocks to a process level. WJEC wants you to explain why magmas form where they do, what controls melting, and how a magma changes composition as it cools, using Bowen's reaction series as the organising model. This is the foundation for igneous textures, for the geohazards theme, and for resource topics later in the course.
The answer
How magma forms: partial melting
Rocks are mixtures of minerals with different melting points, so they melt over a range of temperature rather than all at once. Partial melting produces a liquid enriched in the lower-melting-point, more silica-rich components, leaving a refractory solid residue behind.
Three things trigger melting, and each maps to a tectonic setting:
- Raising temperature, for example where a mantle plume delivers extra heat (hot spots such as Hawaii or Iceland).
- Lowering pressure (decompression melting), where mantle rises and the confining pressure drops at constructive margins and rifts, so the solidus is crossed without adding heat.
- Adding water (flux melting), where water driven off a subducting slab lowers the mantle solidus above destructive margins.
Magma series and composition
Magmas are classified by silica content, which controls viscosity and eruptive style. Basaltic magma (about 50 percent silica) is hot and runny; andesitic (about 60 percent) is intermediate; rhyolitic (about 70 percent) is cool, viscous and gas-rich. Higher silica means more polymerised melt, higher viscosity and more explosive eruptions.
How magma evolves: fractional crystallisation and Bowen's reaction series
A cooling magma does not crystallise all at once. Bowen's reaction series sets the order: high-temperature minerals form first and low-temperature minerals last.
Fractional crystallisation is the removal of these early-formed crystals from the melt, by gravity settling or by the liquid being squeezed away. Because the early crystals are silica-poor (olivine, calcium plagioclase), removing them leaves a residual liquid that is progressively richer in silica, sodium and potassium. Repeated fractionation can therefore evolve a basaltic parent through andesite to rhyolite, so a single source generates a range of rock compositions.
Examples in context
Mid-ocean ridges melt rising mantle by decompression, producing vast volumes of basalt that build new oceanic crust. Subduction zones such as the Andes flux-melt the mantle wedge with slab water, and contamination by thick continental crust pushes magmas to andesitic and rhyolitic compositions, giving explosive volcanoes. Layered intrusions such as the Bushveld preserve fractional crystallisation directly, with dense early minerals concentrated in lower layers, which is also why some ore deposits sit at particular levels.
Try this
Q1. Explain why partial melting of mantle peridotite produces a basaltic rather than an ultramafic magma. [3 marks]
- Cue. Only the lower-melting-point minerals melt, so the liquid is enriched in silica relative to the peridotite source, giving a mafic (basaltic) composition.
Q2. State the order in which olivine, quartz, pyroxene and potassium feldspar crystallise from a cooling magma. [2 marks]
- Cue. Olivine, then pyroxene, then potassium feldspar, then quartz (high to low temperature on Bowen's series).
Q3. Explain how fractional crystallisation makes a residual magma more felsic. [3 marks]
- Cue. Early silica-poor crystals (olivine, calcium plagioclase) are removed by settling, so the remaining liquid is enriched in silica, sodium and potassium.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC Eduqas 20196 marksExplain how partial melting of the mantle and subsequent fractional crystallisation can produce a range of igneous rocks of differing composition.Show worked answer →
Begin with partial melting, because the question asks how the parent magma forms.
The mantle is peridotite, but it does not melt completely. Only the lower-melting-point minerals melt first, so the liquid produced (the partial melt) is enriched in silica relative to the source. This is why melting peridotite yields a basaltic, not an ultramafic, magma. Melting is triggered by raising temperature, lowering pressure (decompression melting at ridges) or adding water (flux melting above subduction zones).
Then explain fractional crystallisation, because that evolves the magma.
As a basaltic magma cools, minerals crystallise in the order set by Bowen's reaction series, with the highest-temperature minerals (olivine, calcium-rich plagioclase) forming first. If these early crystals are removed, by settling or by the magma moving on, the remaining liquid becomes progressively richer in silica, sodium and potassium, evolving towards intermediate and then felsic compositions.
Repeated fractionation can therefore drive a single basaltic parent towards andesite and then rhyolite, so one source produces a spectrum of rock compositions.
Markers reward partial melting concentrating silica in the melt, the trigger of heat, decompression or water, the crystallisation order from Bowen's series, and the removal of early crystals enriching the residual liquid.
WJEC Eduqas 20214 marksDescribe the effect of water on the melting of rock at a destructive plate margin.Show worked answer →
Water released from the subducting slab lowers the melting point of the overlying mantle wedge, which is the key idea.
As the oceanic slab descends and heats, hydrous minerals break down and release water into the mantle wedge above. Adding water lowers the solidus (the temperature at which melting begins), so mantle that would otherwise stay solid begins to melt. This is flux melting.
The resulting magma is hydrous and, after fractionation and contamination by continental crust, tends to be intermediate to felsic, which is why subduction-zone volcanoes are often andesitic and explosive.
Markers reward water lowering the solidus, the source of the water being the dehydrating slab, and the link to intermediate, gas-rich magmas.
Related dot points
- The control of cooling rate on crystal size and texture (glassy, fine, coarse, porphyritic), and the recognition of intrusive forms (dykes, sills, batholiths, laccoliths) and extrusive forms (lava flows, pyroclastic deposits) with their contact relationships.
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- Contact, regional and dynamic metamorphism, the controls of temperature, pressure and fluids, the increase of grade and the use of index minerals and metamorphic facies, and the textures (slate, schist, gneiss, hornfels, marble, quartzite) they produce.
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- The formation of clastic, biogenic and chemical sedimentary rocks, the processes of diagenesis (compaction, cementation, recrystallisation), and the use of sedimentary structures, including those formed by infrequent processes such as turbidity currents, as evidence of conditions.
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- The three types of plate boundary (constructive, destructive and conservative), the processes and features at each, and the driving forces of plate motion (mantle convection, ridge push and slab pull).
A focused answer to WJEC and Eduqas A-Level Geology F4 on plate boundaries, covering constructive, destructive and conservative margins, the processes and landforms at each (ridges, subduction zones, ocean trenches, volcanic arcs, fold mountains, transform faults), and the driving forces of plate motion.
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Sources & how we know this
- WJEC Eduqas A-level Geology specification — WJEC Eduqas (2017)