Eduqas A-Level Geology Rock-forming processes: igneous, sedimentary and metamorphic rocks and the surface processes that link them

What this concept actually demands

Rock-forming processes is the petrology core of Eduqas A-Level Geology: it builds on the minerals from Elements, minerals and rocks and supplies the rock knowledge the structures, tectonics, economic geology and petroleum topics all assume. The examiners test two linked skills: classifying igneous, sedimentary and metamorphic rocks from composition and texture, and interpreting the processes those textures record, from cooling rate to transport history to metamorphic grade.

This guide walks through the six clusters in a sensible build order, then sets out the exam patterns Eduqas repeats. Each cluster has a matching dot-point page with practice questions; this overview ties them together.

Igneous classification and textures

Igneous rocks are classified on two independent axes. By silica content they run ultramafic (peridotite, below $45\%$), mafic (basalt and gabbro, $45$ to $52\%$), intermediate (andesite and diorite, $52$ to $66\%$) and felsic (rhyolite and granite, over $66\%$); as silica falls the rocks darken. By grain size, which records cooling rate, they are coarse (phaneritic, slow, intrusive) or fine (aphanitic, fast, extrusive). The named textures are the evidence: glassy (a quench), aphanitic, phaneritic, porphyritic (two-stage cooling), vesicular (trapped gas) and pyroclastic (explosive fragments). Name a rock from both composition and texture: coarse and felsic is granite, fine and mafic is basalt.

Bowen's reaction series and magma differentiation

Bowen's reaction series is the order silicate minerals crystallise from a cooling magma: the discontinuous ferromagnesian branch (olivine, pyroxene, amphibole, biotite) and the continuous branch (plagioclase from calcium-rich to sodium-rich), converging into potassium feldspar, muscovite and quartz. Removing early, silica-poor crystals by fractional crystallisation enriches the residual melt in silica, so differentiation drives a mafic parent magma towards felsic compositions; partial melting does the reverse, extracting a silica-rich first melt. The same order predicts weathering resistance: quartz (last, coolest) survives best, olivine (first, hottest) weathers fastest.

Weathering, erosion and sediment transport

Surface processes supply the raw material for sedimentary rocks. Weathering is breakdown in place; erosion is removal and transport. Physical weathering (freeze-thaw, exfoliation) breaks rock without changing its chemistry; chemical weathering alters minerals (hydrolysis of feldspar to clay, carbonation dissolving limestone, oxidation of iron); biological weathering (roots, acids) speeds both. Transport carries grains by traction (rolling), saltation (hopping), suspension (fine grains) and solution (dissolved). The journey rounds grains by abrasion (recording distance) and sorts them by size (recording the medium); a mature sediment is well-rounded, well-sorted and quartz-rich.

Sedimentary rocks and depositional environments

Sedimentary rocks are clastic (by grain size: conglomerate or breccia, sandstone, siltstone, mudstone and shale, with sandstone refined into immature arkose, immature greywacke and mature orthoquartzite) or chemical and biogenic (limestones - oolitic, micritic, fossiliferous - chalk, evaporites of gypsum then rock salt, and coal). Sedimentary structures act as way-up and environment indicators: cross-bedding (currents, dunes), graded bedding (waning currents, coarse base up), ripple marks (waves symmetrical, currents asymmetrical) and desiccation cracks (exposure, taper down), fixing the environment as fluvial, deltaic, shallow marine, deep marine or desert. Diagenesis lithifies the sediment by compaction (reducing porosity) and cementation (binding grains).

Metamorphism, grade and facies

Metamorphism changes a protolith in the solid state by heat, pressure and chemically active fluids. Contact (thermal) metamorphism, mainly heat from an intrusion in a local aureole, produces non-foliated hornfels and marble; regional metamorphism, heat plus directed pressure over a large area at convergent margins, produces foliated slate, schist and gneiss. Directed pressure aligns platy minerals into foliation. Grade rises through the mudstone sequence slate, phyllite, schist, gneiss, and index minerals (chlorite, biotite, garnet, kyanite, sillimanite) mark increasing grade. Non-foliated marble (from limestone) and quartzite (from sandstone) form where minerals do not align. Metamorphic facies map mineral assemblages to pressure-temperature conditions.

Igneous intrusions and volcanic forms

Intrusions are concordant (sills, laccoliths, parallel to bedding) or discordant (dykes, batholiths, stocks, cutting bedding). Intrusion is shown by a chilled margin (finer crystals at the edge of the igneous body) and a baked margin or aureole (altered country rock), which proves the intrusion is younger than the rock it baked. A sill bakes rock above and below with chilled margins on both sides; a lava flow bakes only the rock below it, giving the way up. Cross-cutting relationships order events: any intrusion that cuts another rock is younger. Volcanic forms range from gentle basaltic shield volcanoes and lava plateaux to steep andesitic stratovolcanoes, cinder cones and collapse calderas.

How this concept is examined

A typical Eduqas profile for Rock-forming processes:

• Classification questions. Naming a rock from listed properties (a coarse dark rock of pyroxene and calcium plagioclase is gabbro; a clean quartz sandstone is an orthoquartzite; a banded high-grade rock is gneiss) is the single most common task.
• Process and explanation questions. How cooling rate controls crystal size, how fractional crystallisation drives differentiation, how hydrolysis rots feldspar to clay, and why regional metamorphism foliates but contact does not.
• Interpretation questions. Reading sedimentary structures for way up and environment, and ordering igneous events from cross-cutting and chilled or baked margins.
• Calculations. Comparing cooling rates, a rate of rounding or deposition, and porosity from pore and total volume are predictable short-calculation marks.
• Levels-of-response answers. Comparing contact and regional metamorphism, or explaining differentiation with Bowen's series, are reliable six-mark questions.

Check your knowledge

A mix of recall and application questions covering the whole concept. Attempt them under timed conditions, then check against the solutions.

1. State the four compositional classes of igneous rock in order of decreasing silica, with one named rock of each. (2 marks)
2. Explain why an intrusive rock is coarse-grained and an extrusive rock is fine-grained. (2 marks)
3. Name the four minerals of the discontinuous branch of Bowen's reaction series, in order. (2 marks)
4. Explain how fractional crystallisation makes a residual magma more silica-rich. (3 marks)
5. State the difference between weathering and erosion, and name one chemical weathering process. (2 marks)
6. Name the three textural varieties of sandstone defined by composition and state which is most mature. (2 marks)
7. Explain how graded bedding indicates both the environment and the way up. (3 marks)
8. Compare the agents and textures of contact and regional metamorphism. (3 marks)
9. State which margin (chilled or baked) lies in the country rock, and what its presence proves about relative age. (2 marks)