Weathering, erosion and sediment transport: physical weathering (freeze-thaw and exfoliation), chemical weathering (hydrolysis of feldspar to clay, carbonation of limestone, oxidation) and biological weathering; the distinction between weathering and erosion; transport by traction, saltation, suspension and solution, and how transport rounds and sorts grains to determine the maturity of a sediment.

What this dot point is asking

Eduqas wants you to describe physical weathering (freeze-thaw, exfoliation), chemical weathering (hydrolysis, carbonation, oxidation) and biological weathering, to draw a precise distinction between weathering and erosion, to describe the modes of transport (traction, saltation, suspension, solution), and to interpret the rounding, sorting and maturity of a sediment as a record of its transport history. These surface processes supply the raw material for every sedimentary rock.

The answer

Weathering versus erosion

These two terms are routinely confused, so the examiner rewards a sharp distinction.

In short, weathering loosens the rock where it sits; erosion carries the debris away. A cliff face crumbling under frost is weathering; the river that sweeps the fallen fragments downstream is eroding.

Physical (mechanical) weathering

Physical weathering breaks rock into smaller pieces without changing its chemistry, which increases the surface area available for later chemical attack:

• Freeze-thaw (frost shattering). Water in joints freezes and expands by about $9\%$, prising the rock apart; repeated cycles shatter it, so it is most effective where the temperature crosses $0\,^{\circ}\mathrm{C}$ frequently.
• Exfoliation (pressure release). When deeply buried rock such as granite is unloaded by erosion above, the confining pressure falls and the rock expands, so curved outer shells peel away (onion-skin weathering).

Chemical weathering

Chemical weathering alters the minerals into new substances, and is fastest in warm, wet climates because it depends on water:

• Hydrolysis. Feldspar and other silicates react with slightly acidic water; soluble cations are removed in solution and the residue becomes clay minerals plus released silica. This is the main way feldspar-rich rocks rot to clay.
• Carbonation. Rainwater dissolves carbon dioxide to weak carbonic acid, which reacts with the calcium carbonate of limestone to form soluble calcium bicarbonate that is washed away, creating karst features such as caves and grikes.
• Oxidation. Iron-bearing minerals react with oxygen to form iron oxides, giving a red or brown stain and weakening the rock.

Biological weathering

Living things assist both kinds of breakdown: tree roots wedge into joints and widen them (physical), and organic acids from roots, lichens and decaying matter speed chemical reactions such as hydrolysis. Biological weathering rarely acts alone; it accelerates the physical and chemical processes above.

Transport: traction, saltation, suspension and solution

Once eroded, sediment is carried by water (or wind) in four modes set by grain size and the energy of the flow:

• Traction. The largest grains (gravel, boulders) roll and slide along the bed.
• Saltation. Sand-sized grains bounce along the bed in a series of short hops.
• Suspension. Fine silt and clay are held up within the moving water and carried in the body of the flow.
• Solution. Dissolved material (for example the calcium bicarbonate from carbonation) is carried invisibly in true solution.

Rounding, sorting and maturity

The journey progressively changes the grains, and the resulting texture is the evidence you read:

• 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 and gives good sorting; rapid dumping by ice or a debris flow gives poor sorting.
• Maturity combines these with composition: a mature sediment is well-rounded, well-sorted and dominated by stable quartz, because unstable feldspar and olivine weather away over the long journey (mirroring Bowen's series).

Examples in context

Example 1. Quartz sandstone as a mature sediment. Long transport weathers away unstable feldspar and mica by hydrolysis and abrades the grains, leaving a well-rounded, well-sorted, quartz-rich sand: the surface mirror of Bowen's reaction series.

Example 2. Karst from carbonation. In a limestone upland, carbonic acid in rainwater dissolves the calcium carbonate along joints, opening up caves, sinkholes and limestone pavements from one chemical weathering process.

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 away from the source.

Q2. Name and describe one process of physical weathering and one of chemical weathering. [3 marks]

• Cue. Physical: freeze-thaw, where water in cracks freezes, expands by about $9\%$ and prises the rock apart. Chemical: hydrolysis, where slightly acidic water removes soluble cations from feldspar, leaving clay.

Q3. Name the four modes of transport and state which carries the finest grains. [3 marks]

• Cue. Traction (rolling), saltation (hopping), suspension (in the flow) and solution (dissolved); suspension carries the finest silt and clay.