Optional theme: Extreme environments practice questions.

Northern Chile's Atacama Desert lies directly on the Pacific coast, yet many of its weather stations record no measurable rain in most years. Which combination of controls best explains this extreme aridity?

• Subsiding subtropical air, a cold offshore current and the Andean rain shadow
• Rising equatorial air, a warm offshore current and persistent onshore trade winds
• High altitude, permanent snow cover and a semi-permanent polar high
• Winter monsoon reversal, katabatic winds and a deep coastal fog layer

Answer: Subsiding subtropical air, a cold offshore current and the Andean rain shadow

Air that rises at the ITCZ subsides at about 30 degrees S, warming and drying, while the cold Humboldt Current chills the lowest air so that moisture is lost as coastal fog rather than rain. The Andes block moist air arriving from the east, so all three controls act in the same direction.

Saguaro cacti in the Sonoran Desert of Arizona, USA, keep their stomata closed during the day and open them at night. What is the main advantage of this adaptation?

• Water loss by transpiration is reduced because night air is cooler and more humid
• Light-dependent photosynthesis can continue throughout the hours of darkness
• Salts are prevented from accumulating in the water stored in the stem
• Oxygen uptake by the roots increases while the soil is at its coolest

Answer: Water loss by transpiration is reduced because night air is cooler and more humid

Gas exchange is shifted to the time of day when the saturation deficit is lowest, so carbon dioxide is taken in with the smallest possible water loss. Photosynthesis itself still needs light; only the uptake of carbon dioxide happens at night.

A glacially scoured outcrop has one side that is smooth, gently sloping and striated, while the opposite side is steep, craggy and stepped. Which landform is this?

• A drumlin of till deposited beneath moving ice
• An esker of sorted sand and gravel left by a meltwater stream
• A roche moutonnee of ice-moulded bedrock
• An arete separating two neighbouring corries

Answer: A roche moutonnee of ice-moulded bedrock

Ice abrades the up-glacier stoss slope smooth and striated, then meltwater refreezes on the lee side and plucks jointed blocks away, so the asymmetry records the direction of ice movement. A drumlin is depositional and has its blunt end facing up-glacier.

About half of the Trans-Alaska Pipeline System is carried above the ground on supports fitted with sealed heat pipes. What is the main purpose of these heat pipes?

• To warm the crude oil so that it remains fluid during winter
• To absorb ground movement generated by earthquakes on the Denali Fault
• To keep the surrounding permafrost frozen so the supports stay stable
• To let caribou herds migrate freely beneath the raised pipeline

Answer: To keep the surrounding permafrost frozen so the supports stay stable

Warm oil in a buried pipe would thaw ice-rich ground and the line would settle unevenly and rupture. The heat pipes move heat out of the ground in winter, so the permafrost stays frozen and keeps its bearing strength.

On long-irrigated fields in the lower Indus basin, Pakistan, white crusts appear at the soil surface and yields fall. Which soil process explains this?

• Leaching of bases downwards through the profile by percolating rain
• Podzolisation beneath acidic coniferous litter
• Gleying caused by permanent waterlogging and anaerobic conditions
• Salinisation as capillary rise and evaporation concentrate salts

Answer: Salinisation as capillary rise and evaporation concentrate salts

Irrigation water carries dissolved salts; where evaporation is high and drainage poor, water is drawn up through the soil by capillary action and evaporates, leaving the salts as a white surface crust. Leaching carries salts downwards rather than concentrating them, and gleying produces grey, mottled horizons rather than a salt crust.

In the Maradi and Zinder regions of southern Niger, farmers stopped clearing the shoots that sprout from surviving tree stumps and root systems in their millet fields, and instead protected and pruned them. Which management approach does this describe?

• Afforestation with fast-growing exotic plantation species
• Rotational enclosure of communal pasture from grazing
• Contour bunding to slow surface runoff on cropland
• Farmer-managed natural regeneration of on-farm trees

Answer: Farmer-managed natural regeneration of on-farm trees

Regeneration works from root systems that are already in the ground, so it needs labour and secure tree rights rather than seedlings, irrigation or capital. That is why it spread across large areas of the Sahel once farmers were confident the trees would be theirs.