Petrology
Petrology is the study of rocks — their origin, composition, texture and classification. The three rock families — igneous, sedimentary and metamorphic — form a dynamic rock cycle, with each type transforming into another through geological time.
A naturally occurring aggregate of one or more minerals (or, occasionally, of mineraloid or organic matter). Rocks are typically classified by their mode of origin: igneous (from cooling magma/lava), sedimentary (from accumulated sediment) and metamorphic (from existing rocks altered by heat/pressure).
The rock cycle
Magma cools to form igneous rock. Weathering produces sediment, which compacts and cements into sedimentary rock. Burial subjects either to high P-T conditions, producing metamorphic rock. Further heat re-melts material into magma — completing the cycle. The cycle is driven by plate tectonics, gravity, and solar heat.
Igneous rocks
Form from solidification of magma (below the surface) or lava (at the surface). Two main categories by depth of crystallisation:
- Intrusive (plutonic): cooled slowly at depth; coarse-grained (phaneritic). Examples: granite, gabbro, diorite.
- Extrusive (volcanic): cooled rapidly at or near the surface; fine-grained (aphanitic) or glassy. Examples: basalt, rhyolite, obsidian, pumice.
Bowen's reaction series
N. L. Bowen (1922) showed that minerals crystallise from a cooling silicate melt in a predictable order:
Discontinuous (mafic) series: olivine → pyroxene → amphibole → biotite. Continuous (felsic) series: Ca-rich plagioclase → Na-rich plagioclase. Both converge to K-feldspar → muscovite → quartz at the bottom.
Earlier-crystallising minerals (olivine, Ca-plagioclase) are dense and Mg/Ca-rich; later ones (quartz, K-feldspar) are silica-rich. Differential crystallisation explains the diversity of igneous rocks.
IUGS classification
The QAPF diagram (Quartz–Alkali feldspar–Plagioclase–Feldspathoid) classifies plutonic and volcanic rocks by mineral content. Major end-members:
| Rock | Coarse | Fine | Composition |
|---|---|---|---|
| Granitic / felsic | Granite | Rhyolite | Quartz + K-feldspar dominant |
| Intermediate | Diorite | Andesite | Plagioclase + amphibole |
| Mafic | Gabbro | Basalt | Pyroxene + plagioclase |
| Ultramafic | Peridotite | Komatiite | Olivine + pyroxene |
Silica content decreases mafic → ultramafic; ferromagnesian content increases.
- Texture records cooling history: glassy → quenched; porphyritic → two cooling stages; vesicular → gas bubbles.
- Basalt is the most abundant volcanic rock; granite is the most abundant intrusive in continental crust.
- Magma viscosity increases with silica content: rhyolitic magma is much more viscous than basaltic, producing explosive eruptions (Vesuvius) vs effusive flows (Hawaii).
Sedimentary rocks
Form at or near the surface from deposition and lithification of sediment. Cover ~75 % of land area but make up only ~5 % of crustal volume. Three major types:
Clastic (detrital)
Built from particles eroded from older rocks. Classified by grain size:
| Grain size (mm) | Sediment | Rock |
|---|---|---|
| > 256 | Boulder | Conglomerate / breccia |
| 2 – 256 | Pebble / cobble | Conglomerate (rounded) or breccia (angular) |
| 0.0625 – 2 | Sand | Sandstone |
| 0.004 – 0.0625 | Silt | Siltstone |
| < 0.004 | Clay | Shale / mudstone |
Chemical
Precipitated from solution. Includes:
- Limestone (CaCO₃) — by far the most common chemical sedimentary rock; often biogenic in origin.
- Dolostone (CaMg(CO₃)₂) — formed by post-depositional alteration of limestone.
- Chert / flint — microcrystalline SiO₂.
- Evaporites — gypsum, anhydrite, halite formed from evaporating brines.
Organic
Accumulation of plant or animal remains: coal (from compacted plant matter), oil shale, certain limestones (chalk = coccolith ooze).
Sedimentary structures
Beds, cross-bedding, ripple marks, mud cracks, graded bedding, fossils. These structures record depositional environment (river, beach, dune, marine, glacial) and way-up in deformed rocks.
Metamorphic rocks
Form from solid-state recrystallisation of existing rocks (protoliths) under elevated temperature, pressure or chemically active fluids. No melting (otherwise igneous).
Types of metamorphism
- Regional: large-scale, accompanies mountain building. Produces foliated rocks (slate, phyllite, schist, gneiss).
- Contact (thermal): caused by heat from a nearby intrusion. Produces non-foliated rocks (hornfels, marble, quartzite).
- Dynamic (cataclastic): shearing in fault zones; produces mylonites.
- Hydrothermal: chemical alteration by hot fluids; produces serpentinite, soapstone.
- Burial / very low grade: deep sedimentary basins; produces zeolites and prehnite-pumpellyite mineral assemblages.
- Shock: meteorite impacts; produces high-pressure phases (coesite, stishovite).
Metamorphic grade and index minerals
Increasing grade in pelites (originally clay-rich sediments):
shale → slate → phyllite → schist → gneiss → migmatite (partial melting)
Diagnostic index minerals at increasing grade: chlorite → biotite → garnet → staurolite → kyanite → sillimanite.
| Foliated rock | Grain | Typical protolith |
|---|---|---|
| Slate | Very fine | Shale |
| Phyllite | Fine, silky | Shale |
| Schist | Medium-coarse | Shale, basalt |
| Gneiss | Coarse, banded | Granite, sedimentary |
| Non-foliated rock | Protolith |
|---|---|
| Marble | Limestone |
| Quartzite | Sandstone |
| Hornfels | Various, contact-metamorphosed |
| Anthracite | Coal |
Metamorphic facies
A facies is a set of mineral assemblages that develop together under specific P-T conditions. Major facies in order of increasing T at moderate P: zeolite → prehnite-pumpellyite → greenschist → amphibolite → granulite. High-P facies: blueschist → eclogite, characteristic of subduction zones.
A useful exam mnemonic: clay → slate → schist → gneiss → migmatite shows the foliated metamorphic sequence with increasing grade. Limestone → marble, sandstone → quartzite, coal → anthracite show common protolith–product pairs.
The rock cycle in tectonic context
Plate tectonics drives the rock cycle:
- Divergent boundaries (mid-ocean ridges): basaltic magma forms new oceanic crust.
- Convergent boundaries (subduction): andesitic / granitic magma forms volcanic arcs; metamorphism produces blueschist–eclogite belts.
- Continental collision: regional metamorphism and granitic intrusion (Himalayan example).
- Transform boundaries: shear-zone (mylonitic) metamorphism.
Within continents, sedimentary basins form by extension and subsidence; once buried, sediments become rock and may be uplifted and exposed again to the surface.
Petrology thereby connects the chemistry of minerals (mineralogy) with the engine of plate tectonics, making rocks the readable archive of Earth's history.