OCR A-Level Geology: sedimentary and metamorphic processes overview

What this topic actually demands

Sedimentary and metamorphic processes complete the rock-forming story begun with igneous rocks. The topic runs from how rocks break down at the surface, through how the resulting sediment becomes rock, to how heat and pressure transform existing rocks, and finally ties the three families together in the rock cycle. The examiners test two linked skills: describing the processes precisely, and reading the texture and mineralogy of a rock as a record of how it formed.

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

Weathering, erosion and transport

The first step is the breakdown of rock. Weathering happens in place: mechanical weathering (freeze-thaw, exfoliation, abrasion) breaks rock without changing its chemistry, while chemical weathering (solution, hydrolysis, oxidation) alters minerals, for example hydrolysing feldspar to clay. Erosion then removes and transports the material. Keep the two distinct, because OCR marks it.

Transport by water, wind or ice changes the sediment in two ways the exam asks about. Rounding increases with distance, because abrasion knocks off corners, so rounding records transport distance. Sorting records the medium: a river or the wind separates grains by size (well-sorted), while ice or a debris flow dumps everything together (poorly sorted). A texturally mature sediment is well-rounded, well-sorted and quartz-rich; an immature one is angular and poorly sorted.

Sedimentary rock formation and classification

Loose sediment becomes rock by lithification: compaction (overburden squeezes grains together and expels pore water) followed by cementation (minerals such as silica, calcite or iron oxide precipitated from pore water bind the grains). Sands rely heavily on cementation; muds lithify mostly by compaction.

Sedimentary rocks fall into three classes by origin. Clastic rocks are fragments of older rock, named by grain size (conglomerate or breccia, sandstone, siltstone, mudstone). Chemical rocks are precipitates (evaporites such as halite and gypsum). Biogenic (biochemical) rocks form from organisms (limestone from shells, coal from plants). Describe clastic texture with grain size (energy), sorting (medium) and roundness (distance), and remember to classify limestone and evaporites by origin, not by clast size.

Metamorphic processes and grade

Metamorphism transforms an existing rock in the solid state (no melting) using three agents: heat, pressure (confining or directed) and chemically active fluids. There are three types. Regional metamorphism (heat plus directed pressure over a large area at convergent margins) produces foliated rocks; contact metamorphism (mainly heat from an intrusion, in a local aureole) produces non-foliated marble and hornfels; dynamic metamorphism (directed stress in fault zones) produces mylonite.

Foliation is the alignment of platy minerals by directed pressure, so it is the signature of regional metamorphism and absent in contact metamorphism. Grade (intensity, mainly temperature) rises through the mudstone prograde sequence slate, phyllite, schist, gneiss, and index minerals (chlorite low, garnet medium, kyanite and sillimanite high) let geologists map metamorphic zones.

The rock cycle

The rock cycle ties everything together: material moves continuously between the three families. Crystallisation makes igneous rock; weathering, erosion, transport, deposition and lithification make sedimentary rock; metamorphism makes metamorphic rock; melting restarts the cycle; and uplift returns deep rocks to the surface. Plate tectonics is the engine: subduction and melting make new igneous rock, collision drives metamorphism, and mountain building uplifts rocks for weathering. Crucially, the cycle has shortcuts, so any rock type can become any other.

How this topic is examined

A typical OCR profile for sedimentary and metamorphic processes:

• Process and definition questions (Paper 1). Distinguishing weathering from erosion, describing lithification, comparing regional and contact metamorphism.
• Specimen and interpretation questions (Paper 3). Classifying a sedimentary rock from grain size and origin, interpreting sorting and roundness as a transport history, identifying a metamorphic rock and its grade.
• Map and zone questions (Paper 3). Reading index minerals to map how metamorphic grade changes across a region.
• Level-of-response extended answers (Papers 1 and 2). The comparison of metamorphism types, and tracing material through the rock cycle with named processes and tectonic drivers, are predictable six-mark questions.

Check your knowledge

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

1. Distinguish between weathering and erosion. (2 marks)
2. Name and describe one mechanical and one chemical weathering process. (4 marks)
3. Explain how compaction and cementation lithify a sand into sandstone. (3 marks)
4. Classify limestone, sandstone and halite by origin. (3 marks)
5. Compare regional and contact metamorphism, referring to the agents and the textures produced. (4 marks)
6. Place slate, gneiss, phyllite and schist in order of increasing metamorphic grade. (2 marks)
7. Explain how index minerals are used to map metamorphic grade across a region. (3 marks)
8. Describe how an igneous rock could be converted into a sedimentary rock, naming the processes. (4 marks)