Weathering and Erosion Questions and Answers

Answer: A

Weathering prepares the material and erosion carries it away, so the two act together but are quite distinct processes. A rock face can be deeply weathered yet unmoved, and fresh rock can be eroded without weathering at all.

Answer: B

Correct answer: weathering, erosion, transport, deposition

Rock is first broken down in place by weathering; the loosened material is then detached and picked up by erosion; it is carried away by an agent - water, wind, ice; and it settles where the agent loses the energy to carry it. The order matters because each stage supplies the material for the next, and it is the whole of the denudation cycle by which continents are worn down and sedimentary rocks made.

Answer: C

That expansion in a confined joint generates pressures well beyond the tensile strength of most rocks. It requires repeated crossing of the freezing point rather than merely cold conditions, so it is most active in temperate uplands.

Answer: D

Correct answer: frequent crossings of the freezing point

The damage is done by the alternation, not by the cold: each freezing expands the water in a crack by about nine per cent and prises it wider, and each thaw lets more water in to be frozen next time. Permanently frozen ground gives no cycles, and a permanently warm climate gives none either. The most severe conditions are therefore high mountains and periglacial regions where the temperature crosses zero daily.

Answer: A

Correct answer: release of confining pressure as overburden is removed

Granite crystallised at depth is under great confining pressure, and as erosion strips the rock above it the mass expands upward. The tension that results opens fractures parallel to the ground surface, closely spaced near the top and further apart with depth, and the resulting sheets spall away to leave the rounded dome. It is a mechanical process driven by unloading, not by frost, salt or chemistry.

Answer: B

Correct answer: arid and coastal environments

Salt crystallisation needs a supply of salt and repeated wetting and drying. Both are found in deserts, where groundwater rises and evaporates at the surface, and on coasts, where spray supplies the salt. Crystals grow in the pores and exert pressures great enough to disintegrate the stone, which is why it is such a destructive agent on coastal buildings and desert monuments.

Answer: C

Different minerals expand by different amounts and the surface heats more than the interior, setting up stresses at each daily cycle. Its effectiveness alone is debated, but it certainly assists other processes in deserts.

Answer: D

Correct answer: clay minerals

Hydrolysis is the reaction of feldspar with slightly acid water, in which the potassium, sodium or calcium is taken into solution and the aluminosilicate framework is reorganised into a clay mineral - kaolinite where leaching is intense. It is the most important chemical weathering reaction of all, since feldspar is the commonest mineral in the crust, and it is where the world’s clay comes from.

Answer: A

Correct answer: a weak carbonic acid

Rainwater dissolves carbon dioxide from the atmosphere and from the soil, forming carbonic acid, and that weak acid dissolves calcium carbonate to soluble calcium bicarbonate which is carried away. It is a slow reaction but a relentless one, and over geological time it produces the whole karst landscape - the enlarged joints, sinkholes and caverns that make limestone so troublesome an engineering material.

Answer: B

Correct answer: iron-bearing minerals

Oxidation attacks the ferrous iron in olivine, pyroxene, amphibole and biotite, converting it to ferric iron that forms the red and brown oxides staining any weathered outcrop. The reaction also disrupts the mineral lattice, so it weakens the rock as well as colouring it. Quartz and calcite contain no iron and are not affected in this way.