ERB222 Chap.5 Magma Chamber Processes and Crystallisation
Magma Chamber Processes and Crystallisation
Where rising magma stops
Most magma never reaches the surface. Week 4 follows it from the source to storage, where it stalls at traps such as the Moho or the brittle to ductile transition, at neutral buoyancy, or where flow turns sideways into sills.
Beneath volcanoes such as Mount St Helens, storage zones sit at 5 to 15 km, and the crust makes room through floor sagging, doming, stoping, faulting and incremental sheet stacking.
How a stored magma solidifies
Crystallisation runs over roughly 200°C, so a chamber freezes from its walls inward through a solidification front: a suspension zone at the liquidus, a capture front below 25% crystals, a mush of 25 to 50% and a rigid crust above 50%.
Mineral densities decide whether crystals settle or, as plagioclase can, float.
Reading crystallisation on phase diagrams
Binary eutectics and the An-Di-Fo ternary show how the liquid moves away from each crystallising mineral, along cotectic valleys, to a eutectic where temperature stays fixed until a phase is used up.
Bowen's reaction series generalises this into continuous and discontinuous branches, but the lecture is clear about its limits as a recipe for granite.
What this chapter covers
- 01
Magma ascent and stalling
- 02
Storage depth and space making
- 03
Solidification fronts and mush
- 04
Crystal settling and flotation
- 05
Binary eutectic crystallisation
- 06
An-Di-Fo ternary paths
- 07
Continuous and discontinuous series
- 08
Limits of Bowen's reaction series
Worked example · free
Lever rule on the B-rich side of a model eutectic
- 1Liquidus on the B side: T = 1180 + 140(0.40 − 0.25)/0.40 = 1232.5°C, where B starts to crystallise.
- 1Liquid at 1215°C: X = 0.40 − 0.40(1215 − 1180)/140 = 0.30 A.
- 1Lever with crystals of pure B (0% A): fraction B = (0.30 − 0.25)/(0.30 − 0) = 0.167, about 17%.
- 1Just above the eutectic: fraction B = (0.40 − 0.25)/0.40 = 0.375, so 37.5% is solid and 62.5% liquid freezes at 1180°C.
Key terms
- Solidification Front
- The zone between liquidus and solidus at a cooling chamber margin, where melt progressively turns to crystals.
- Mush Zone
- The part of a solidification front holding roughly 25 to 50% crystals, between the capture front and the rigid crust.
- Cotectic
- A boundary between two phase fields on a phase diagram, a temperature valley along which two minerals crystallise together.
- Continuous Reaction Series
- The branch of Bowen's series in which one mineral, plagioclase, keeps crystallising while its composition shifts from Ca-rich toward Na and K.
Magma Chamber Processes and Crystallisation FAQ
How deep are magma chambers under volcanoes?
Seismic and petrological studies beneath Mount St Helens and Long Valley place crustal storage zones at about 5 to 15 km. Storage concentrates at traps such as the Moho and the brittle to ductile transition, yet pressures recorded by pyroxene and amphibole show crystallisation can happen at many levels.
What is the difference between continuous and discontinuous crystallisation?
Continuous crystallisation belongs to solid-solution minerals such as olivine, pyroxene and plagioclase, whose compositions change gradually with the liquid. Discontinuous crystallisation involves minerals without solid solution that appear abruptly at a cotectic or eutectic, so the assemblage changes in discrete steps as temperature falls.
Why does temperature stay constant at a eutectic?
At the eutectic two solids and a liquid coexist, and it is the lowest temperature at which liquid can exist in that system. Cooling cannot continue until one of the three phases is used up, which in crystallisation is the last liquid, so the system holds its temperature while it freezes.
Exam move
Sketch the four-zone solidification front with its 25% and 50% crystal boundaries until you can label it from memory. Then practise lever-rule questions on a hand-drawn binary, writing the liquid composition before every lever, and trace two An-Di-Fo paths, one starting in each mineral field. Close by listing the five problems with Bowen's series without notes.
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