Magmatism and metamorphism
Domain 3 - Magmatism and metamorphism. Expect questions that link magma generation to tectonic setting, then use texture, composition and mineral assemblages to interpret cooling history and pressure–temperature conditions.
Magma generation and differentiation
- Melting mechanisms: decompression at ridges and plumes, addition of water at subduction zones (flux melting lowers the solidus), and crustal heating during collision.
- Fractional crystallisation follows Bowen's series: discontinuous branch olivine → pyroxene → amphibole → biotite; continuous branch Ca-plagioclase → Na-plagioclase; then orthoclase, muscovite, quartz.
- Silica controls viscosity: basalt (~50 % SiO2) is fluid and hot; rhyolite (~72 %) is viscous and explosive. Dissolved volatiles add explosivity.
- Eruption styles: Hawaiian, Strombolian, Vulcanian, Plinian, Pelean; products include lava flows, scoria, pumice, ignimbrite, ash fall and lahars.
- Intrusive bodies: batholith, stock, laccolith, sill, dyke; textures record cooling rate - phaneritic (slow), porphyritic (two stages), glassy and vesicular (very fast).
Metamorphism
- Agents: heat, lithostatic and directed pressure, and fluids. Types: contact (thermal aureole, hornfels, non-foliated), regional (directed pressure, foliation), dynamic, burial and shock.
- Facies and index minerals: zeolite, greenschist (chlorite, biotite), amphibolite (garnet, staurolite), granulite, blueschist (glaucophane, low T, high P) and eclogite.
- Protoliths: limestone → marble, sandstone → quartzite, shale → slate → phyllite → schist → gneiss, basalt → greenschist or amphibolite. Anatexis produces migmatites.
Worked example
A foliated rock with garnet, kyanite and biotite, cut by an undeformed granite: the assemblage indicates regional metamorphism at roughly 600 °C and 6–8 kbar (kyanite is the high-pressure Al2SiO5 polymorph), so it records a collisional, medium-P setting; the granite post-dates the fabric because it truncates the foliation.
Traps
- Saying granite forms by cooling basalt at the surface - that gives basalt or obsidian.
- Confusing foliation with sedimentary bedding.
- Forgetting that metamorphism is solid-state; only anatexis produces melt.
- Equating high grade with high pressure: granulite is hot and relatively shallow.