ERB222 Chap.10 The Subduction Factory and Calc-Alkaline Magmas
The Subduction Factory and Calc-Alkaline Magmas
The third way to make magma
Week 9 completes the unit's account of magma generation. Oceanic crust altered on the seafloor is carried down a subduction zone, where hydrous minerals such as chlorite and amphibole break down with increasing pressure. The water they release rises into the mantle wedge above the slab and lowers its melting point, so arc volcanoes line up roughly 100 to 110 km above the slab.
Over geological time this subduction factory builds continental crust, which explains why bulk crust is andesitic.
Island arcs, continental arcs and two chemical trends
Island arcs grow on oceanic crust and erupt mostly low-K tholeiitic basalt and basaltic andesite. Continental arcs such as the Andes sit on thick crust and erupt andesite with a calc-alkaline trend.
On the AFM diagram the tholeiitic series bends like a boomerang through early iron enrichment, while the calc-alkaline series runs straight because magnetite crystallises early in wetter, higher-pressure magma.
Andesites in thin section
The Week 9 practical describes crystal-rich andesites full of disequilibrium textures: sieve-textured and zoned plagioclase, glomerocrysts, and hornblende with opacite rims.
Many crystals are antecrysts or xenocrysts collected from older mush, and mixing between mafic and felsic magma is central to making andesite. Expect to describe one of these lavas in the practical exam and to explain what its textures say about ascent and recharge.
What this chapter covers
- 01
Slab dehydration and flux melting
- 02
Mantle wedge and the subduction factory
- 03
Arc geometry and slab dip
- 04
Island arcs versus continental arcs
- 05
Tholeiitic versus calc-alkaline trends on the AFM diagram
- 06
Effects of water and pressure on crystallisation
- 07
Andesite petrography and disequilibrium textures
Worked example · free
Calculate AFM coordinates for an arc lava
- 1Add the alkalis: Na2O + K2O = 3.4 + 1.2 = 4.6 wt%.
- 1Form the AFM total: 4.6 + 6.4 + 3.6 = 14.6 wt%.
- 1Divide each part by 14.6: A = 4.6/14.6 = 0.315, F = 6.4/14.6 = 0.438, M = 3.6/14.6 = 0.247.
- 1Check that A + F + M = 1.000, then plot the point with the rest of its suite before calling a trend.
Key terms
- Mantle Wedge
- The wedge of asthenosphere above the slab and beneath the arc, fluxed by slab fluids and acting as the source region for arc magmas.
- Subduction Factory
- The set of thermal, chemical and mechanical processes that carry material down with the slab and return part of it to the surface through arcs, with continental crust as its long-term product.
- Benioff Zone
- The inclined zone of earthquakes that tracks the descending slab, lying about 100 to 110 km beneath most arc volcanoes.
- Opacite Rim
- A dark reaction rim of anhydrous phases around hornblende that forms as the magma ascends slowly and dehydrates.
- Sieve Texture
- A spongy pattern of melt inclusions in a crystal, usually plagioclase, produced when the crystal becomes unstable and is partly resorbed.
- Antecryst
- A crystal that grew in an earlier magma or mush of the same system and was later recycled into the erupting magma.
The Subduction Factory and Calc-Alkaline Magmas FAQ
Why do arc volcanoes form at a fixed depth above the subducting plate?
The controlling dehydration reactions depend on pressure. Hydrous minerals such as chlorite give up their water near 100 km, and that water triggers melting in the overlying wedge. A steeper slab reaches this depth closer to the trench, while a slab flatter than about 20 degrees can shut volcanism off.
What is the difference between tholeiitic and calc-alkaline magma series?
They are two liquid lines of descent. Tholeiitic magmas crystallise shallow with little water, enrich in iron first and deplete it once magnetite appears. Calc-alkaline magmas carry more water at higher pressure, so magnetite and amphibole appear early and the MgO to FeO* ratio barely changes.
Why are andesites so crystal-rich and texturally messy?
Many of their crystals did not grow from the erupted liquid. Basalt from the wedge mixes with felsic mush, picks up older crystals and recharges shallow reservoirs, leaving sieve textures, reverse zoning and mixed crystal populations. Small, short-lived batches then erupt before the mixture can equilibrate.
Exam move
Sketch the subduction cross-section from memory, labelling where amphibole and chlorite break down and why volcanoes sit 100 to 110 km above the slab. Build a two-column card contrasting the tholeiitic and calc-alkaline series, then practise AFM coordinates on a few invented analyses. For the practical, rehearse a full andesite description and train yourself to explain every opacite rim and sieve texture you report.
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