How do rocks break down, and how does transport sort and shape the sediment produced?
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.
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What this dot point is asking
OCR wants you to describe the processes of mechanical weathering (freeze-thaw, exfoliation, abrasion) and chemical weathering (solution, hydrolysis, oxidation), to distinguish weathering from erosion, to describe transport by water, wind and ice, and to interpret the rounding, sorting and maturity of a sediment as a record of its transport history.
The answer
Weathering versus erosion
These are often confused, so OCR rewards the precise distinction.
Mechanical (physical) weathering
Mechanical weathering breaks rock into smaller pieces without changing its chemistry, increasing the surface area for later chemical attack:
- Freeze-thaw (frost shattering). Water enters cracks, freezes and expands by about , prising the rock apart; repeated cycles shatter it. Most effective where temperatures cross often.
- Exfoliation (pressure release). When overlying rock is removed by erosion, the confining pressure on a deep rock (for example granite) falls, so it expands and outer layers peel off in sheets.
- Abrasion. Rock surfaces are worn by the impact of transported particles (in rivers, wind or ice).
Chemical weathering
Chemical weathering alters the minerals into new substances, usually faster in warm, wet climates:
- Solution. Soluble minerals (for example halite) and rocks such as limestone dissolve, especially in slightly acidic rainwater (carbonic acid) for limestone.
- Hydrolysis. Silicate minerals such as feldspar react with weak acids; soluble ions are removed and insoluble clay minerals are left behind. This is the main way feldspar-rich rocks rot to clay.
- Oxidation. Iron-bearing minerals react with oxygen to form iron oxides, giving a red or brown stain and weakening the rock.
Transport, rounding and sorting
Once eroded, sediment is transported by water, wind or ice, and the journey changes the grains:
- Rounding increases with transport distance, because repeated abrasion knocks off corners. Angular grains are immature (short transport); well-rounded grains are mature (long transport).
- Sorting is how uniform the grain sizes are. A consistent medium (a river, the wind) separates grains by size, giving good sorting; ice or a debris flow dumps everything together, giving poor sorting.
- Maturity combines these: a mature sediment is well-rounded, well-sorted and dominated by stable minerals such as quartz (because unstable minerals weather away during the long journey).
Examples in context
Example 1. Quartz sandstone as a mature sediment. Long transport weathers away unstable feldspar and mica and abrades the grains, leaving a well-rounded, well-sorted, quartz-rich sand: a mature sediment recording a long journey.
Example 2. Glacial till as an immature deposit. Ice dumps an unsorted mix of clay, sand and striated boulders with little rounding: a texturally immature deposit that identifies a glacial origin.
Try this
Q1. State the difference between weathering and erosion. [2 marks]
- Cue. Weathering is the breakdown of rock in place; erosion is the removal and transport of the weathered material.
Q2. Name and describe one process of mechanical weathering. [2 marks]
- Cue. Freeze-thaw: water in cracks freezes, expands by about and prises the rock apart over repeated cycles.
Q3. Explain what well-rounded, well-sorted grains tell you about a sediment's transport history. [2 marks]
- Cue. Well-rounded grains have travelled far (abrasion smooths them); good sorting shows a consistent transport medium has separated the grains by size, so the transport history was long and energetic.
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 20194 marksDistinguish between mechanical and chemical weathering, giving one named process of each, and explain how chemical weathering of feldspar produces clay.Show worked answer →
Define each type, then work through the feldspar reaction qualitatively.
Mechanical weathering breaks rock into smaller fragments without changing its chemical composition. A named process is freeze-thaw (water in cracks freezes, expands by about and prises the rock apart) or exfoliation (release of pressure as overlying rock is removed, so outer layers peel off).
Chemical weathering alters the minerals chemically into new substances. A named process is hydrolysis, solution or oxidation.
Feldspar to clay. Hydrolysis: feldspar reacts with slightly acidic rainwater (carbonic acid), and the soluble ions (for example potassium, sodium) are removed in solution, leaving behind insoluble clay minerals. The rock therefore weakens and produces clay plus dissolved ions.
Markers reward a clear physical-versus-chemical distinction, a correct named process of each, and hydrolysis removing soluble ions to leave clay.
OCR H414/03 20214 marksTwo sediment samples are examined. Sample A is well-sorted with well-rounded grains; Sample B is poorly sorted with angular grains. Suggest, with reasons, how the transport history of the two samples differed.Show worked answer →
Read rounding and sorting as a record of transport distance and energy.
Sample A (well-sorted, well-rounded). Well-rounded grains have been transported a long distance, because repeated abrasion during transport knocks off corners and smooths the grains. Good sorting means the grains are a similar size, which happens when a single, consistent transport medium (for example a river or wind) has separated grains by size over a long journey. So Sample A has had a long, energetic transport history.
Sample B (poorly sorted, angular). Angular grains have been transported only a short distance, so there has been little abrasion. Poor sorting (a wide range of grain sizes together) suggests rapid deposition with little separation, for example by ice (glacial till) or a debris flow. So Sample B has had a short transport history.
Markers reward linking rounding to transport distance (abrasion) and sorting to the transport medium and energy.
Related dot points
- 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.
- 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.
- Igneous processes: Bowen's reaction series as the order in which silicate minerals crystallise from a cooling magma; the discontinuous (olivine to biotite) and continuous (calcium-rich to sodium-rich plagioclase) branches; the use of the series to explain fractional crystallisation, magma differentiation and the resistance of minerals to weathering.
A focused answer to the OCR H414 dot point on Bowen's reaction series. Covers the discontinuous and continuous branches, the crystallisation order of silicate minerals, how fractional crystallisation drives magma differentiation from basic to acid compositions, and how the series predicts weathering resistance.
- Sedimentary environments: the concept of facies as a body of rock reflecting a particular depositional environment; sedimentary structures (bedding, cross-bedding, graded bedding, ripple marks and desiccation cracks) and their interpretation; the characteristics of the main environments (fluvial, deltaic, shallow marine, deep marine and desert); the construction and interpretation of sedimentary logs to reconstruct environmental change.
A focused answer to the OCR H414 dot point on sedimentary environments. Covers facies, sedimentary structures (bedding, cross-bedding, graded bedding, ripple marks, desiccation cracks) and their interpretation, the fluvial, deltaic, shallow-marine, deep-marine and desert environments, and how sedimentary logs reconstruct environmental change.
- Groundwater: porosity and permeability and how they differ between rock types; aquifers, aquitards and the water table; confined and unconfined aquifers; the calculation of porosity from pore and total volumes; the use of a simple form of Darcy's law to relate groundwater discharge to hydraulic conductivity, hydraulic gradient and area; the issues of over-abstraction and contamination.
A focused answer to the OCR H414 dot point on groundwater. Covers porosity and permeability and how they vary between rock types, aquifers, aquitards and the water table, confined and unconfined aquifers, calculating porosity, using a simple form of Darcy's law for groundwater flow, and the issues of over-abstraction and contamination.
Sources & how we know this
- OCR A Level Geology (H414) Specification — OCR (2017)