The three classes of rock (igneous, sedimentary and metamorphic), how each forms, and the textural and mineralogical features used to recognise each class in hand specimen.

What this dot point is asking

WJEC wants you to define the three rock classes by the process that forms them and then recognise each in hand specimen from its texture and mineralogy. The three classes are the framework for the whole subject, and Component 1 expects fast, confident classification of specimens and photographs, so the diagnostic features must be automatic.

The answer

The three classes by process

Rocks are grouped by how they form. Igneous rocks solidify from a molten silicate liquid (magma below ground or lava at the surface). Sedimentary rocks form at or near the surface from weathered fragments, chemical precipitates or the remains of organisms that are deposited, buried and cemented. Metamorphic rocks form in the solid state when a pre-existing rock is altered by heat, pressure or chemically active fluids without melting.

Recognising igneous rocks

An igneous rock is made of interlocking crystals that grew as the melt cooled, fitting together with no pore space and no cement. There is no bedding, no rounding and no fossils. Crystal size records cooling rate: slow cooling at depth gives large, visible crystals (coarse-grained, as in granite), while fast cooling at the surface gives small crystals (fine-grained, as in basalt) or, with quenching, a glassy texture (as in obsidian).

Recognising sedimentary rocks

A clastic sedimentary rock is made of separate grains (fragments of older rock) that were transported, deposited and then bound by a cement, so individual grains sit in a matrix. Non-clastic sedimentary rocks form by chemical precipitation (such as rock salt) or from organic remains (such as fossil-rich limestone or coal). Diagnostic features are bedding (layering), rounding and sorting of grains, fossils, and structures such as ripple marks. Many limestones fizz with dilute acid.

Recognising metamorphic rocks

A metamorphic rock forms in the solid state and shows either a foliated texture (parallel alignment of platy minerals from directed pressure, seen as slaty cleavage in slate, schistosity in schist or banding in gneiss) or a non-foliated crystalline texture from contact metamorphism (such as marble or quartzite, which have interlocking recrystallised grains but no alignment). New minerals such as garnet may grow.

Examples in context

The same minerals, three rocks. Quartz sand cemented into sandstone, then metamorphosed to quartzite, then melted into a granite shows how one mineral threads through all three classes while the texture records the process. Building stone choice. Slate roofs exploit the foliation of a metamorphic rock that splits into thin sheets, while granite kerbstones exploit the interlocking, poreless igneous texture that resists weathering. Reading Earth history. A sequence of bedded fossil-rich limestone, cut by a coarse igneous intrusion and folded into a foliated schist, lets a geologist reconstruct deposition, intrusion and mountain building in order from the textures alone.

Try this

Q1. State the process that forms each rock class. [3 marks]

• Cue. Igneous from solidifying melt; sedimentary from deposition and cementation at the surface; metamorphic from solid-state alteration by heat and pressure.

Q2. A specimen is made of large, interlocking, randomly oriented crystals. Which class is it, and was it intrusive or extrusive? [2 marks]

• Cue. Igneous; the large crystals show slow cooling at depth, so it is intrusive.

Q3. Give one feature that distinguishes a non-foliated metamorphic rock from an igneous rock. [1 mark]

• Cue. A non-foliated metamorphic rock such as marble or quartzite formed from a pre-existing rock in the solid state and may retain ghosts of original structures, and is found in metamorphic field settings rather than as an intrusion or lava flow.