How are the three rock families linked, and how does material cycle between them?
The rock cycle: the continuous transformation between igneous, sedimentary and metamorphic rocks; the processes that link them (crystallisation, weathering, erosion, transport, deposition, lithification, metamorphism, melting, uplift and exposure); the role of plate tectonics in driving the cycle; recognising that any rock type can be converted into any other.
A focused answer to the OCR H414 dot point on the rock cycle. Covers the continuous transformation between igneous, sedimentary and metamorphic rocks, the processes that link them (crystallisation, weathering, transport, lithification, metamorphism, melting and uplift), the role of plate tectonics in driving the cycle, and how any rock type can become any other.
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What this dot point is asking
OCR wants you to describe the rock cycle as the continuous transformation between igneous, sedimentary and metamorphic rocks, name the processes that link them, explain how plate tectonics drives the cycle, and recognise that any rock type can be converted into any other.
The answer
What the rock cycle is
The rock cycle is the idea that the three rock families are not permanent: material moves continuously between them over geological time, driven by processes at and below the surface. No rock is a final product; each can be destroyed and remade.
The processes that link the families
Each arrow in the cycle is a named process you must be able to use:
- Crystallisation (cooling magma) forms igneous rock.
- Weathering, erosion, transport and deposition break down any exposed rock and move the sediment.
- Lithification (compaction and cementation) turns sediment into sedimentary rock.
- Metamorphism (heat and pressure, solid state) turns any rock into metamorphic rock.
- Melting of any rock at depth produces magma, restarting the cycle.
- Uplift and exposure bring deep rocks back to the surface to be weathered again.
Plate tectonics as the engine
Plate tectonics provides the energy and the settings that drive the cycle:
- At destructive margins, subduction and melting generate new igneous rock; burial and collision drive metamorphism.
- Mountain building uplifts rocks, exposing them to weathering and feeding the sedimentary part of the cycle.
- Sedimentary basins (in subsiding regions) collect and bury sediment, allowing lithification and, if buried deeply, metamorphism.
Any rock can become any other
Crucially, the cycle is not a fixed loop: there are shortcuts. An igneous rock can be metamorphosed without first becoming sediment; a metamorphic rock can be weathered straight to sediment; a sedimentary rock can melt. The defining idea is that any of the three rock types can be converted into any other, given the right processes.
Examples in context
Example 1. The Andes as a rock-cycle engine. Subduction generates andesitic magma (new igneous rock), collision and burial metamorphose older rocks, and uplift of the mountains feeds vast volumes of sediment into adjacent basins: all three rock-cycle pathways operating at one margin.
Example 2. Quartzite from sandstone. A quartz sandstone metamorphosed by heat and pressure becomes quartzite, showing a direct sedimentary-to-metamorphic step without any return to magma.
Try this
Q1. Name the process that converts loose sediment into sedimentary rock. [1 mark]
- Cue. Lithification (compaction and cementation).
Q2. Explain how a metamorphic rock can be converted into a sedimentary rock. [3 marks]
- Cue. Uplift and exposure bring it to the surface; it is weathered into fragments and ions, eroded and transported, deposited as sediment, and lithified by compaction and cementation.
Q3. State one way plate tectonics drives the rock cycle. [1 mark]
- Cue. For example, subduction at a destructive margin causes melting that forms new igneous rock (or collision drives metamorphism, or mountain building uplifts rocks for weathering).
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H414/01 20186 marksDescribe how an igneous rock could be converted, over geological time, into a metamorphic rock and then into a sedimentary rock, naming the processes involved at each stage.Show worked answer →
A level-of-response answer; give an ordered chain of named processes.
- Start: an igneous rock
- For example a granite, formed by crystallisation of magma at depth.
- Igneous to metamorphic
- If the granite is buried deeply and subjected to heat and directed pressure (for example at a convergent margin during mountain building), it is metamorphosed in the solid state into a metamorphic rock such as gneiss. The process is regional metamorphism.
- Metamorphic to sedimentary
- Uplift and erosion expose the gneiss at the surface. It is then weathered (mechanically and chemically) into fragments and dissolved ions, eroded and transported by water, wind or ice, and deposited as sediment. Burial then compacts and cements the sediment (lithification) into a sedimentary rock such as sandstone.
- The cycle
- Each step is a named process, and the sequence shows how plate tectonics, uplift and surface processes move material between the rock families.
Top-band answers give the correct order with named processes at each transition (crystallisation, metamorphism, uplift, weathering, erosion, transport, deposition, lithification).
OCR H414/02 20214 marksExplain the role of plate tectonics in driving the rock cycle, giving two specific examples of tectonic processes that convert one rock type into another.Show worked answer →
Link tectonic processes to specific rock-cycle transformations.
Plate tectonics provides the energy and the settings that move material between the rock families.
Example 1, subduction and melting. At a destructive margin, oceanic crust is subducted; it and the overlying mantle partially melt, generating magma that crystallises into new igneous rock (for example andesite). This converts old crust into new igneous rock.
Example 2, collision and metamorphism (or uplift). At a collision zone, rocks are buried deeply and subjected to heat and directed pressure, metamorphosing sedimentary and igneous rocks into metamorphic rocks; the same collision uplifts rocks so they are exposed and weathered, feeding the sedimentary part of the cycle.
Markers reward two correct tectonic processes each tied to a specific rock-cycle transformation (melting, metamorphism or uplift and erosion).
Related dot points
- Igneous rocks: classification by silica content (acid, intermediate, basic and ultrabasic) and by grain size (coarse-grained intrusive, fine-grained extrusive); the relationship between cooling rate and crystal size; igneous textures (phaneritic, aphanitic, porphyritic, glassy and vesicular) and what they show about the cooling history; naming common igneous rocks such as granite, gabbro, basalt and rhyolite.
A focused answer to the OCR H414 dot point on igneous rock classification. Covers acid, intermediate, basic and ultrabasic compositions, coarse versus fine grain size, the link between cooling rate and crystal size, the main textures (phaneritic, aphanitic, porphyritic, glassy, vesicular), and naming granite, gabbro, basalt and rhyolite.
- Surface processes: mechanical weathering (freeze-thaw, exfoliation and abrasion) and chemical weathering (solution, hydrolysis and oxidation); the difference between weathering and erosion; transport by water, wind and ice and its effect on the rounding and sorting of sediment; how the maturity and texture of a sediment record its transport history.
A focused answer to the OCR H414 dot point on surface processes. Covers mechanical weathering (freeze-thaw, exfoliation, abrasion) and chemical weathering (solution, hydrolysis, oxidation), the difference between weathering and erosion, transport by water, wind and ice, and how rounding, sorting and maturity record a sediment's transport history.
- Sedimentary rocks: the stages from sediment to rock (deposition, compaction and cementation as lithification); the classification of sedimentary rocks into clastic (by grain size, from conglomerate to mudstone), chemical (precipitates such as evaporites) and biogenic or biochemical (limestone and coal); the description of clastic texture using grain size, sorting and roundness.
A focused answer to the OCR H414 dot point on sedimentary rocks. Covers lithification (deposition, compaction and cementation), the clastic, chemical and biogenic or biochemical classes, the grain-size scale from conglomerate to mudstone, and how clastic texture is described using grain size, sorting and roundness.
- Metamorphic rocks: the agents of metamorphism (heat, pressure and chemically active fluids); the types of metamorphism (regional, contact and dynamic) and their settings; the development of foliation under directed pressure; metamorphic grade and the prograde sequence from mudstone (slate, phyllite, schist, gneiss); the use of index minerals (chlorite, garnet, kyanite, sillimanite) to indicate grade.
A focused answer to the OCR H414 dot point on metamorphism. Covers the agents (heat, pressure and fluids), regional, contact and dynamic metamorphism, the development of foliation, metamorphic grade and the mudstone prograde sequence (slate, phyllite, schist, gneiss), and the use of index minerals to indicate grade.
- Plate tectonics: the development of the theory from continental drift (Wegener's evidence) through sea-floor spreading to plate tectonics; the evidence of palaeomagnetism and symmetrical magnetic striping at mid-ocean ridges; the increasing age of oceanic crust away from ridges; the driving mechanisms of mantle convection, ridge push and slab pull.
A focused answer to the OCR H414 dot point on the development of plate tectonics. Covers Wegener's continental drift evidence, sea-floor spreading, palaeomagnetism and symmetrical magnetic striping, the increasing age of oceanic crust away from ridges, and the driving mechanisms of mantle convection, ridge push and slab pull.
Sources & how we know this
- OCR A Level Geology (H414) Specification — OCR (2017)