How are the three rock classes linked by the rock cycle?
The rock cycle as the set of processes (weathering, erosion, transport, deposition, burial, lithification, metamorphism, melting and crystallisation) that recycle material between the three rock classes, driven by internal heat and surface energy.
A focused answer to WJEC and Eduqas A-Level Geology F1 on the rock cycle, covering how weathering, erosion, transport, deposition, lithification, metamorphism, melting and crystallisation recycle material between igneous, sedimentary and metamorphic rocks, and the internal and external energy sources that drive it.
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What this dot point is asking
WJEC wants you to describe the rock cycle as a connected set of processes that move and transform material between the three rock classes, and to identify the energy sources that drive it. This ties F1 together: it shows why the same atoms can pass through igneous, sedimentary and metamorphic states many times over geological history, and it frames the surface and internal processes studied later in F2 and F4.
The answer
The rock cycle as a system
The rock cycle is the continuous set of processes that creates, destroys and recycles rock, linking the three classes. There is no fixed route around it: a rock may move from any class to any other depending on where it ends up in the Earth. The cycle is best understood as a balance between processes that break rock down at the surface and processes that build and transform it at depth.
The processes, step by step
Starting at the surface and following one common path:
Weathering breaks rock down in place, physically and chemically, into fragments and dissolved ions. Erosion loosens and removes that material. Transport by water, wind, ice or gravity carries it, rounding and sorting the grains. Deposition drops the load where energy falls, in a basin such as a lake, delta or sea floor. Burial under later sediment, then lithification (compaction and cementation), turns the sediment into sedimentary rock.
Continuing into the Earth: metamorphism alters buried or tectonically stressed rock in the solid state by heat and pressure, producing metamorphic rock. If temperatures rise high enough, melting produces magma, and crystallisation of that magma (below ground or as erupted lava) produces new igneous rock. Uplift by tectonic forces then returns rock to the surface, where weathering begins again.
What drives it
Two energy sources power the cycle. Internal heat (from radioactive decay plus residual heat of formation) drives mantle convection and plate tectonics, causing melting, metamorphism and uplift. External solar energy drives the atmosphere and the water cycle, powering weathering, erosion, transport and deposition, with gravity moving material downhill. Plate tectonics is the master process that connects the two: it carries crust into the deep Earth (subduction) and builds mountains, exposing rock to both engines.
Examples in context
Recycled crust at subduction zones. Oceanic sediment carried down a subduction zone is metamorphosed and partly melted, feeding volcanoes that build new igneous rock, a working example of the cycle at a plate margin. The Welsh slate belt. Mudstones deposited in a sea were buried, lithified, then regionally metamorphosed to slate during mountain building, then uplifted and quarried, several rock-cycle stages preserved in one region. Time and the cycle. Because the cycle has run throughout Earth history, very old crustal atoms have passed through many rocks, which is why dating individual minerals can give older ages than the rock that now hosts them.
Try this
Q1. Define lithification and name its two processes. [2 marks]
- Cue. The conversion of sediment into rock by compaction (packing under load) and cementation (precipitation of mineral cement between grains).
Q2. Name the process that converts magma into igneous rock and the process that converts a buried rock into a metamorphic rock. [2 marks]
- Cue. Crystallisation (cooling of magma) makes igneous rock; metamorphism (solid-state alteration by heat and pressure) makes metamorphic rock.
Q3. State the two energy sources that drive the rock cycle and what each powers. [2 marks]
- Cue. Internal heat drives tectonics, melting and metamorphism; solar energy (with gravity) drives weathering, erosion, transport and deposition.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC Eduqas 20185 marksDescribe the sequence of processes by which an igneous rock could be converted into a sedimentary rock and then into a metamorphic rock.Show worked answer →
Give the processes in order, naming each, because the marks track the steps of the rock cycle.
The igneous rock is first broken down at the surface by weathering and then loosened and removed by erosion, producing sediment.
The sediment is transported by water, wind or ice and deposited in a basin such as the sea floor.
Burial under later sediment compacts the grains and cement precipitates between them, so the loose sediment is lithified into a sedimentary rock.
Deeper burial or proximity to an intrusion or a plate margin then subjects the rock to raised heat and pressure, recrystallising it in the solid state into a metamorphic rock.
Markers reward the ordered sequence weathering, erosion, transport, deposition, lithification, then metamorphism, with each process correctly named.
WJEC Eduqas 20223 marksExplain the energy sources that drive the rock cycle.Show worked answer →
Two energy sources drive the cycle. Internal heat from radioactive decay in the Earth's interior, together with residual heat of formation, powers mantle convection and plate tectonics, which causes uplift, melting, metamorphism and the recycling of crust at plate margins.
External (solar) energy drives the atmosphere and the water cycle, powering weathering, erosion, transport and deposition at the surface, with gravity moving material downhill and down-section.
Together the internal engine raises and reworks rock at depth while the external engine breaks it down and redistributes it at the surface.
Markers reward naming internal (radioactive and residual) heat driving tectonics and solar energy plus gravity driving surface processes.
Related dot points
- 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.
A focused answer to WJEC and Eduqas A-Level Geology F1 on the three rock classes, covering how igneous, sedimentary and metamorphic rocks form and the diagnostic textures (interlocking crystals, grains and cement, foliation) and mineralogy used to recognise each in hand specimen.
- The mechanisms of physical and chemical weathering, the distinction between weathering, erosion and transport, and how transport agents round and sort sediment to record transport history.
A focused answer to WJEC and Eduqas A-Level Geology F2 on surface processes, covering physical and chemical weathering mechanisms, the difference between weathering, erosion and transport, and how rounding and sorting of sediment by water, wind and ice record transport distance and energy.
- The sources of the Earth's internal heat, the geothermal gradient and heat flow, and how heat drives mantle convection, melting and metamorphism as the internal limb of the rock cycle.
A focused answer to WJEC and Eduqas A-Level Geology F2 on internal processes, covering the sources of the Earth's internal heat (radioactive decay and residual heat), the geothermal gradient and heat flow, and how heat drives mantle convection, partial melting and metamorphism as the deep limb of the rock cycle.
- The development of plate tectonic theory from continental drift, and the evidence for it (continental fit, matching geology and fossils, palaeoclimate, sea-floor spreading and palaeomagnetic stripes).
A focused answer to WJEC and Eduqas A-Level Geology F4 on plate tectonic theory, covering the development from continental drift, and the evidence (continental fit, matching geology and fossils, palaeoclimate, sea-floor spreading and the symmetry of palaeomagnetic stripes) that confirmed it.
- The definition of a mineral, and the diagnostic physical properties (hardness, cleavage, fracture, lustre, colour, streak, density and crystal habit) used to identify common rock-forming minerals in hand specimen.
A focused answer to WJEC and Eduqas A-Level Geology F1, covering what defines a mineral and how hardness, cleavage, fracture, lustre, colour, streak, density and crystal habit are used to identify common rock-forming minerals such as quartz, feldspar, mica, calcite and the ferromagnesian minerals in hand specimen.
Sources & how we know this
- WJEC Eduqas A-level Geology specification — WJEC Eduqas (2017)