How do weathering, erosion and transport break down and move rock at the surface?
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.
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What this dot point is asking
WJEC wants you to describe how rock is broken down by physical and chemical weathering, to distinguish weathering from erosion and transport, and to interpret the rounding and sorting of sediment as a record of how far and how energetically it travelled. These surface processes supply the raw material for clastic sedimentary rocks and are the starting point of the rock cycle.
The answer
Weathering, erosion and transport are different things
These three terms are often muddled, so define them sharply. Weathering is the breakdown of rock in place, with little or no movement. Erosion is the loosening and removal of the weathered products by an agent (water, wind, ice or gravity). Transport is the carrying of that material away from the source.
Physical (mechanical) weathering
Physical weathering breaks rock into smaller pieces without changing its chemistry, increasing the surface area for chemical attack. The main mechanisms are freeze-thaw (water in cracks expands by about 9 percent on freezing, prising the rock apart, important in cold and upland climates), exfoliation or pressure release (removal of overlying rock lets a body expand and shed outer shells), thermal expansion (repeated heating and cooling stresses mineral grains, important in deserts) and biological action (roots wedging into cracks).
Chemical weathering
Chemical weathering decomposes minerals by reaction, usually with water, oxygen and dissolved carbon dioxide. The main reactions are hydrolysis (feldspar reacts with weak carbonic acid to give clay minerals plus soluble ions, the dominant breakdown of silicates), carbonation (limestone dissolves as calcite reacts with carbonic acid to form soluble calcium bicarbonate, producing karst landscapes), oxidation (iron-bearing minerals react with oxygen to give iron oxides, staining rocks red-brown) and dissolution (soluble minerals such as halite simply dissolve). Chemical weathering is fastest in warm, wet climates.
Transport: rounding and sorting record history
As sediment is transported, collisions wear the grains. Rounding is the wearing away of sharp edges and corners by abrasion: angular grains have been transported a short distance, well-rounded grains a long distance. Sorting is the degree to which grains are of a similar size: well-sorted sediment was carried by a single agent of fairly constant energy (wind or a sustained river), poorly sorted sediment was dumped quickly without size separation (as in glacial till or a debris flow).
Examples in context
Karst scenery of the limestone uplands. Carbonation slowly dissolves limestone along joints, opening the caves, sinkholes and limestone pavements seen in the Yorkshire Dales and the Brecon Beacons. Red beds and oxidation. Desert sandstones are stained red because iron in the grains has been oxidised, a chemical-weathering signature preserved in the rock. Glacial till versus beach sand. Unsorted, angular till dropped by ice contrasts sharply with well-sorted, well-rounded beach sand, letting a geologist read the transport agent straight from the sediment.
Try this
Q1. Distinguish between weathering and erosion. [2 marks]
- Cue. Weathering is the breakdown of rock in place; erosion is the loosening and removal of the weathered products by a moving agent.
Q2. Name the chemical weathering process that dissolves limestone and the acid responsible. [2 marks]
- Cue. Carbonation; weak carbonic acid (from carbon dioxide dissolved in rainwater).
Q3. A till deposit is angular and poorly sorted. Identify the transport agent and justify your answer. [2 marks]
- Cue. Ice (a glacier); ice carries all grain sizes together and dumps them without abrasion or size sorting, so the till is angular and poorly sorted.
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 20194 marksExplain how chemical weathering of feldspar produces clay minerals and contributes to the formation of sedimentary rocks.Show worked answer →
Start with the chemical process, because the marks reward the mechanism, not just naming weathering.
Feldspar is attacked by hydrolysis: slightly acidic rainwater (carbonic acid from dissolved carbon dioxide) reacts with the feldspar, breaking the silicate framework.
The reaction removes soluble cations such as potassium and sodium into solution and rearranges the remaining aluminium, silicon and oxygen into clay minerals (such as kaolinite), which are stable at the surface.
The clay is then eroded and transported and is deposited as mud, which is buried and lithified into mudstone or shale, so chemical weathering supplies the raw material for clastic sedimentary rocks.
Markers reward naming hydrolysis, the loss of soluble cations, the production of clay, and the link to the deposition and lithification of mud.
WJEC Eduqas 20213 marksA river sediment is well rounded and well sorted. Explain what this indicates about its transport history.Show worked answer →
Well-rounded grains have had their sharp edges and corners worn off by repeated collisions during transport, so the sediment has travelled a long distance, giving abrasion time to act.
Well-sorted sediment has grains of a similar size, which indicates sustained transport by a single agent of fairly constant energy that separated the grains by size, again consistent with long transport in a river.
Together, good rounding and good sorting indicate prolonged transport by water that has had time to abrade and to size-sort the grains.
Markers reward linking rounding to abrasion over distance and sorting to sustained transport at a consistent energy, and concluding a long transport history.
Related dot points
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Sources & how we know this
- WJEC Eduqas A-level Geology specification — WJEC Eduqas (2017)