WJEC A-Level Geology G1 Rock Forming Processes: a deep dive on igneous, metamorphic and sedimentary processes at A2

What G1 actually demands

G1 is where geology becomes interpretive. The AS units named the rocks; G1 asks why they have the textures and compositions they do, and what those tell you about the conditions of formation. Examiners want process reasoning: link cooling rate to crystal size, directed pressure to foliation, current behaviour to sedimentary structures, and use that reasoning to read a hand specimen or a field relationship.

Each of the three rock classes has its own dot-point page with worked questions; this overview ties them together.

Igneous processes

Magma forms by partial melting, which concentrates silica in the liquid, so melting mantle peridotite gives basalt. Melting is triggered by added heat, decompression at ridges, or water flux at subduction zones. A magma then evolves by fractional crystallisation along Bowen's reaction series: removing the early silica-poor crystals drives the residual liquid from basalt through andesite to rhyolite. Texture records cooling history, from glassy quench through fine and coarse grains to two-stage porphyritic textures, and intrusive forms (dykes, sills, batholiths, laccoliths) are read by geometry and contact relationships such as chilled margins and baked aureoles.

Metamorphic processes

Metamorphism alters rock in the solid state by heat, pressure and fluids. Contact metamorphism is heat-dominated and gives non-foliated hornfels; regional metamorphism adds high directed pressure and gives the foliated sequence slate, schist, gneiss as grade rises. Index minerals (chlorite to sillimanite) and metamorphic facies record the pressure and temperature, so the rock becomes a record of its tectonic history.

Sedimentary processes

Sediments are clastic, biogenic or chemical, and become rock through diagenesis (compaction, cementation, recrystallisation). Sedimentary structures preserve the conditions: cross-bedding gives current direction, graded bedding records a decelerating turbidity current and the way up, ripples and desiccation cracks record shallow or exposed settings. Turbidity currents show how an infrequent process is reconstructed through models.

Check your knowledge

Attempt these under timed conditions, then check against the solutions.

1. Explain why partial melting of peridotite gives a basaltic magma. (3 marks)
2. State the crystallisation order of olivine, quartz, pyroxene and potassium feldspar. (2 marks)
3. Describe the cooling history recorded by a porphyritic texture. (2 marks)
4. Place chlorite, garnet and sillimanite in order of increasing grade. (2 marks)
5. Explain why contact metamorphism gives non-foliated hornfels. (2 marks)
6. State how a graded bed shows the way up of strata. (2 marks)