Quick questions on Weathering, erosion and transport - WJEC A-Level Geology

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 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.

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).

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.

Distinguish between weathering and erosion. [2 marks]

Name the chemical weathering process that dissolves limestone and the acid responsible. [2 marks]

A till deposit is angular and poorly sorted. Identify the transport agent and justify your answer. [2 marks]