Notes · GeologyUK · A-Levels

Surface and internal processes of the rock cycle

This chapter develops the rock cycle as the organising model of geology: the stores of rock, sediment and magma and the processes that move material between them. It treats the surface processes of weathering, erosion, transport and deposition in detail, then the internal-energy processes of burial, lithification, metamorphism and melting that complete the loop, and the external and internal energy sources that drive the whole system.

4 sections~15 min reading time3 competenciesLevel Foundation 1 · Standard 3

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Exam profile

AO1 · Describe weathering, erosion, transport, deposition and the internal processes of the rock cycleAO2 · Apply the grain-size and transport relationships and interpret sorting and roundingAO3 · Analyse how surface and internal processes and their energy sources link the rock cycle

Operators:describeexplaininterpretdeduceanalyse

basic level

AS-Level expects you to outline the rock cycle, describe the types of weathering, and the sequence of erosion, transport and deposition.

higher level

The full A-Level requires you to explain the processes mechanistically, use grain size and sorting to deduce transport history, and connect surface and internal processes through the cycle's energy sources.

Depth

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Contents · 4 sections

Surface and internal processes of the rock cycle

The rock cycle: stores and processes#

FoundationWJEC/Eduqas A level Geology (Component 2) — Surface and internal processes of the rock cycle

The rock cycle

Fig. 1The rock cycle as stores (rock classes, magma, sediment) linked by processes; external energy drives the surface path and internal energy the deep path.

The rock cycle is a model that shows how the three rock classes are continually transformed into one another over geological time, with no beginning or end. Its stores are the reservoirs where material sits — igneous rock, sedimentary rock, metamorphic rock, and the transient stores of magma and of loose sediment — and its processes are the pathways that move material between them: weathering and erosion, transport and deposition, burial and lithification, metamorphism, melting and crystallisation, and uplift. Any given atom may pass round the cycle many times, spending millions of years in one store and moments in a process.

The power of the model is that it links every other topic in the course. Surface processes (weathering, erosion, transport, deposition) are driven by external energy from the Sun and gravity and break rock down and move it to where it is deposited; internal processes (metamorphism, melting, crystallisation, uplift) are driven by the Earth's internal heat and by plate tectonics and rebuild rock at depth and raise it back to the surface. The cycle therefore couples the atmosphere and oceans to the deep Earth, and plate tectonics is the engine that keeps it turning.

The pathways are not fixed: material can take shortcuts. A sedimentary rock may be uplifted and weathered straight back to sediment without ever being buried deeply; an igneous rock may be metamorphosed without first becoming sediment; and a metamorphic rock may melt to magma and crystallise as a new igneous rock. Being able to trace a valid route round the cycle, and to explain each step, is a standard examination demand, and it rewards seeing the cycle as a network of processes rather than a fixed circle.

The cycle also makes sense of the balance of the Earth's surface. Uplift and weathering would, unopposed, wear the continents flat, while deposition would fill the ocean basins; it is the continual renewal by plate tectonics — building mountains, creating and destroying oceanic crust — that maintains relief and keeps the cycle running. Recognising the rock cycle as a dynamic steady state driven by tectonics, rather than a one-way process of decay, is the mature understanding the specification is looking for.

Worked example

Tracing a route round the cycle

A basalt lava is exposed at the surface. Describe a route by which it could become a sedimentary rock and then a metamorphic rock, naming the process at each stage.

Break it down
At the surface the basalt is attacked by weathering, then eroded and the fragments transported by rivers, ice or wind.
Build a sedimentary rock
The transported grains are deposited, then buried, compacted and cemented (lithification) into a clastic sedimentary rock such as a sandstone or mudstone.
Metamorphose it
Deep burial or nearby intrusion subjects the sedimentary rock to heat and directed pressure, recrystallising it into a metamorphic rock (mudstone to slate, for example).

Result: Basalt to sediment (weathering, erosion, transport) to sedimentary rock (deposition, lithification) to metamorphic rock (heat and pressure).

Typical mistakes

Treating the rock cycle as a fixed one-way circle; material can take shortcuts (for example sedimentary rock uplifted and weathered straight back to sediment).
Forgetting that plate tectonics is the engine that renews relief and keeps the cycle turning.

Active revision

Describe how an atom in a granite could end up in a schist, naming each process it passes through on the way.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — the rock cycle (British Geological Survey)

Weathering#

StandardWJEC/Eduqas A level Geology (Component 2) — Surface and internal processes of the rock cycle

Types of weathering

Fig. 2Weathering acts in place by mechanical, chemical and biological processes; the processes reinforce one another and are tuned by climate.

Weathering is the breakdown and alteration of rock in place, at or near the surface, by physical, chemical and biological agents; crucially it involves no transport, which distinguishes it from erosion. Weathering weakens and disintegrates rock, preparing it for erosion, and it produces both the loose fragments that become clastic sediment and the dissolved ions that become chemical sediment. The rate and type of weathering depend strongly on climate, so the weathering products of a rock record the environment in which it broke down.

Mechanical (physical) weathering breaks rock into smaller pieces without changing its chemistry, increasing the surface area available for chemical attack. Freeze-thaw (frost) weathering, important in cold climates, works because water expands by about 9% on freezing, so water trapped in cracks that repeatedly freezes and thaws prises the rock apart. Pressure release (exfoliation or unloading) occurs when erosion removes overlying rock so that a deeply formed rock such as granite expands and sheets of it spall off. Thermal expansion from strong daily temperature changes and salt crystallisation in pores contribute in hot deserts.

Chemical weathering alters the minerals themselves, forming new, more stable minerals and releasing ions into solution. The most important process is hydrolysis, in which slightly acidic rainwater reacts with feldspar (the commonest crustal mineral) to form clay minerals, releasing silica and metal ions; this is why so much of the sediment on Earth is clay. Carbonation is the solution of limestone by carbonic acid in rainwater, forming soluble calcium bicarbonate, which produces limestone (karst) scenery. Oxidation attacks iron-bearing minerals, producing the rusty-red iron oxides seen on weathered surfaces. Chemical weathering is fastest in warm, wet climates.

Biological weathering is the breakdown of rock by living things and spans both mechanisms: tree roots growing into cracks wedge rock apart mechanically, while organic acids from plants, lichens and decaying matter, and the carbon dioxide respired by soil organisms, accelerate chemical attack. In reality the processes act together and reinforce one another — mechanical weathering increases the surface area for chemical weathering, and chemical weathering weakens rock for mechanical breakdown — so a weathered outcrop usually records the combined effect of several processes tuned by its climate.

Worked example

Matching weathering to environment

Granite in a high mountain area is seen to have angular blocks split along joints and rounded, clay-rich weathered rims. Identify the weathering processes and the conditions each indicates.

The angular splitting
Angular blocks split along joints indicate mechanical freeze-thaw weathering, consistent with a cold, high-altitude climate where water repeatedly freezes and thaws in cracks.
The clay-rich rims
Clay-rich weathered rims indicate chemical hydrolysis of the feldspar in the granite to clay minerals, which requires water and is favoured when the rock is wetted.
Combine
The outcrop records both mechanical (freeze-thaw) and chemical (hydrolysis) weathering acting together, the mechanical breakdown increasing the surface area for the chemical attack.

Result: Freeze-thaw split the granite mechanically while hydrolysis altered its feldspar to clay; the two processes reinforced one another.

Typical mistakes

Confusing weathering with erosion; weathering breaks rock down in place, erosion carries the products away.
Saying water 'expands when it freezes' without the mechanism; the roughly 9% expansion in confined cracks is what prises the rock apart.

Active revision

Explain why chemical weathering is more rapid in a warm, wet tropical climate than in a cold, dry polar one, referring to specific processes.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — weathering (British Geological Survey)

Erosion, transport and deposition#

StandardWJEC/Eduqas A level Geology (Component 2) — Surface and internal processes of the rock cycle

Settling velocity and grain size

Fig. 3Relative settling velocity by grain class (illustrative): because settling velocity rises steeply with grain size, currents sort sediment, depositing the coarsest first.

Erosion is the removal of weathered material by a moving agent — water, ice, wind or the sea — and it both wears the land down and shapes it. The eroded material is then transported, and the way it is carried depends on its size and on the energy of the medium: large clasts roll and slide along the bed (traction), sand-sized grains bounce along in a series of hops (saltation), fine silt and clay are held up in the flow (suspension), and dissolved ions travel in solution. As the energy of the transporting medium falls, it can no longer carry its largest load, and deposition begins with the coarsest material first.

The competence of a river or current is the largest particle it can move, and it rises steeply with velocity, so a fast flow can move boulders while a slow one can carry only mud; the capacity is the total load it can carry. Because settling velocity increases sharply with grain size, sediment is sorted during transport and deposition: as a current slows, gravel drops out first, then sand, then silt, and finally clay in the stillest water. This size-selective deposition is why we find well-defined beds of gravel, sand and mud in different settings, and it is the physical basis for reading past environments from grain size.

Transport also changes the shape and sorting of grains, and these carry information. Rounding increases with distance and time of transport as grains are chipped and abraded, so angular grains indicate a short journey and well-rounded grains a long one. Sorting — the range of grain sizes present — improves the more the sediment is worked by a consistent medium, so a well-sorted sand of uniform size indicates prolonged reworking (for example by waves or wind), whereas a poorly sorted deposit of mixed sizes indicates rapid deposition with little sorting, as by ice or a debris flow. Grain size, sorting and rounding together are the 'maturity' of a sediment.

The relationship between grain size and settling is captured, for small grains in still water, by Stokes' law, in which settling velocity is proportional to the square of the grain diameter (for a given density and fluid). This is why a quadrupling of grain diameter multiplies the settling velocity roughly sixteenfold, and why clay, once suspended, stays up almost indefinitely and is deposited only in the quietest water such as a deep sea or a lake. The strong dependence of settling on size is the quantitative heart of sorting and of environmental interpretation.

v \propto d^{2}

Settling velocity (Stokes' law, small grains)

For small grains settling in still water at a fixed density, the settling velocity v is proportional to the square of the grain diameter d, so larger grains settle far faster and are deposited first.

Worked example

Comparing settling velocities

Using Stokes' law (v proportional to d squared), compare the settling velocities of a fine sand grain of diameter 0.2 mm and a silt grain of diameter 0.05 mm, and comment on what this means for their deposition.

Find the diameter ratio
The sand is $0.2 / 0.05 = 4$ times the diameter of the silt.
Apply the square law
Since v is proportional to d squared, the velocity ratio is $4^{2} = 16$ .
$\frac{v_{\text{sand}}}{v_{\text{silt}}} = \left(\frac{0.2}{0.05}\right)^{2} = 16$
Interpret
The sand settles about sixteen times faster than the silt, so as a current slows the sand is deposited first while the silt stays suspended and travels on to quieter water, producing sorting.

Result: The sand settles about 16 times faster than the silt, so it is deposited first; this size-selective settling is what sorts sediment.

Typical mistakes

Confusing sorting (range of grain sizes) with rounding (grain shape); a sediment can be well rounded but poorly sorted, or vice versa.
Assuming the largest grains travel furthest; in fact the finest material is carried furthest and deposited last, in the quietest water.

Active revision

A sandstone is well sorted and well rounded. Deduce its likely transport history and a possible depositional environment, justifying your answer.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — sediments (British Geological Survey)

Internal processes and the energy that drives the cycle#

StandardWJEC/Eduqas A level Geology (Component 2) — Surface and internal processes of the rock cycle

From sediment to magma: the internal path

Fig. 4The internal-energy path completing the cycle: burial and lithification, then metamorphism and melting, driven by the Earth's internal heat and plate tectonics.

Once sediment is deposited, a chain of internal-energy processes completes the rock cycle. Continued deposition buries the sediment, and the weight of the overburden compacts it, squeezing out water and reducing pore space; cementation then binds the grains as minerals precipitate from pore fluids in the spaces between them. Together compaction and cementation are lithification, the turning of loose sediment into solid sedimentary rock. Deeper burial raises the temperature and pressure further, and where these exceed the limits of the sediment's minerals the rock is metamorphosed in the solid state, and deeper still it may melt to magma.

These internal processes are driven by the Earth's internal heat and by the movements of plate tectonics, in contrast to the surface processes driven by solar energy and gravity. Burial, metamorphism and melting occur because temperature and pressure rise with depth; uplift, which returns deep rocks to the surface to be weathered again, is driven by tectonic forces that build mountains and by the buoyant rise of the crust as overlying rock is removed (isostasy). Without this internal energy the cycle would run down: the continents would be worn flat and the ocean basins filled, and the loop would stop.

The energy that powers the internal half of the cycle comes chiefly from two sources: the heat left over from the Earth's formation and gravitational differentiation, and the continuing heat produced by the decay of radioactive isotopes (mainly uranium, thorium and potassium) in the mantle and crust. This internal heat drives mantle convection, which in turn drives plate tectonics, and so ultimately powers metamorphism, melting, mountain-building and the creation and destruction of oceanic crust. The surface half of the cycle, by contrast, is powered by the Sun, which drives the water cycle and the winds, and by gravity, which moves water, ice and sediment downhill.

Seeing the two energy systems side by side explains the whole cycle as a coupling of the Earth's interior to its surface. External solar and gravitational energy break rock down and spread its products across the surface; internal radiogenic and primordial heat rebuild rock at depth and lift it back up. Plate tectonics is the crucial link, using internal heat to renew the surface faster than weathering can wear it away, and so the rock cycle is best understood not as slow decay but as a tectonically maintained steady state that has kept the Earth geologically alive for billions of years.

Worked example

Explaining lithification

A sand deposited on a beach is buried under later sediment and, millions of years later, is a solid sandstone. Explain the two processes that turned the loose sand into rock.

Compaction
The weight of the overlying sediment pressed the sand grains together, reducing the pore space and squeezing out much of the pore water.
Cementation
Minerals such as silica or calcite precipitated from the pore fluids into the remaining spaces, binding the grains together.
Conclude
Compaction and cementation together (lithification) turned the loose sand into a coherent sandstone.

Result: Compaction reduced the pore space and cementation glued the grains, together lithifying the sand into sandstone.

Typical mistakes

Confusing compaction (squeezing out water under the weight of overburden) with cementation (precipitation of minerals binding grains); lithification is both together.
Attributing all of the cycle to the Sun; the deep half is powered by the Earth's internal heat, largely from radioactive decay.

Active revision

Explain why the rock cycle would eventually stop without the Earth's internal heat, referring to the processes that internal energy drives.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — how rocks form (British Geological Survey)

References & sources

Sources

WJEC / Eduqas

WJEC/Eduqas A level Geology specification

British Geological Survey

British Geological Survey — the rock cycle

Surface and internal processes of the rock cycle

4 sections~15 min reading time3 competenciesLevel Foundation 1 · Standard 3

Surface and internal processes of the rock cycle

Key points

Tracing a route round the cycle

Key points

Matching weathering to environment

Key points

Comparing settling velocities

Key points

Explaining lithification

Sources

Surface and internal processes of the rock cycle

Key points

Tracing a route round the cycle

Key points

Matching weathering to environment

Key points

Comparing settling velocities

Key points

Explaining lithification

Sources