Physical processes and landscapes practice questions.

In a hot desert, rock surfaces break apart along shallow layers parallel to the surface without any chemical change to the minerals. Which process is mainly responsible?

• Thermal fracture driven by repeated heating and cooling
• Carbonation of calcium carbonate by weak carbonic acid
• Hydrolysis of feldspars into clay minerals
• Abrasion by sediment carried in a river

Answer: Thermal fracture driven by repeated heating and cooling

Large diurnal temperature ranges expand and contract the outer few millimetres of rock at a different rate from the interior, and the resulting stress detaches flakes. Chemical weathering needs water, which is exactly what a desert lacks, and abrasion is a transport process rather than weathering.

A crescent-shaped dune has a gentle windward slope, a steep slip face and horns pointing downwind. Which dune type is this, and what does its shape indicate?

• A barchan, indicating a limited sand supply and one dominant wind direction
• A seif, indicating two winds meeting at an acute angle
• A star dune, indicating winds from many directions
• A transverse ridge, indicating an unlimited sand supply

Answer: A barchan, indicating a limited sand supply and one dominant wind direction

Barchans form where sand is too scarce to cover the ground completely, so a single dune migrates across a firm surface and its thinner edges travel faster, producing the horns. Dune shape is a record of wind regime and sediment supply, which is why it is worth reading rather than just naming.

Desert landscapes contain deep dry valleys and large fans of poorly sorted sediment, yet rainfall is very rare. How is this best explained?

• Rare intense storms produce flash floods on surfaces with little vegetation
• Wind erosion carves valleys and deposits fans over long periods
• The landforms are inherited from a period when the area was glaciated
• Groundwater seepage slowly dissolves the bedrock into valleys

Answer: Rare intense storms produce flash floods on surfaces with little vegetation

In deserts water is the dominant erosional agent despite its rarity: baked or crusted surfaces have low infiltration, sparse vegetation offers no protection, and a single storm generates a high-energy flow that moves large volumes in hours. Wind sorts and moves sand, but it does not cut wadis.

A glaciated upland shows a steep-sided hollow with a rock lip, opening into a valley with a flat floor and truncated spurs. Which sequence of processes produced this?

• Snow accumulation and rotational flow, then valley erosion by a glacier
• Wind deflation, then deposition of moraine across the valley
• River meandering, then uplift and incision of the channel
• Frost shattering alone, without any movement of ice

Answer: Snow accumulation and rotational flow, then valley erosion by a glacier

The hollow is a cirque, deepened by rotational movement of ice with plucking and abrasion, and the rock lip marks where that rotation lifted rather than cut. Downstream the trunk glacier widened and straightened the valley, removing the interlocking spurs a river had left.

On a flat periglacial surface, stones are arranged in polygons with fine material in the centre and coarse clasts around the edges. Which sequence of processes best explains this pattern?

• Repeated freezing lifts coarse clasts upwards, and they then roll outwards from the doming centre.
• Wind removes fine material from the centre and piles coarse stones around the outside.
• Meltwater washes the fines outwards, leaving the coarse material stranded in the centre.
• Solifluction carries coarse debris downslope and sorts it into polygons on the flat.

Answer: Repeated freezing lifts coarse clasts upwards, and they then roll outwards from the doming centre.

Sorted patterned ground needs frost heave plus gravity: ice growth raises the larger clasts, the centre domes slightly, and the clasts creep outwards to the margins. Solifluction needs a slope, and wind cannot lift stones at all.

In a desert, salt crystals growing in the pores of a sandstone block break it into fragments where it stands; months later a flash flood carries those fragments two kilometres down a wadi. Which pair of terms describes the two stages, in the right order?

• Erosion, then weathering
• Erosion, then transport
• Weathering, then transport
• Mass movement, then weathering

Answer: Weathering, then transport

Weathering is breakdown in place with no movement of material, so salt crystal growth cannot be erosion; only once a moving agent carries the debris away does transport begin. Getting this order wrong is one of the most costly terminology errors in the subject.