Reading: Sentence Completion: Question 9

Karst is the name given to a distinctive type of landscape that develops where a thick layer of soluble rock, most commonly limestone, lies close to the surface and is gradually dissolved by slightly acidic rainwater. Rain absorbs a small amount of carbon dioxide as it falls through the atmosphere and picks up more as it passes through soil, forming a weak solution of carbonic acid that reacts chemically with the calcium carbonate in limestone, slowly widening cracks in the rock over thousands of years.

As this process, known as chemical weathering, continues, surface streams that once flowed across the rock may disappear entirely, sinking underground through widened cracks called sinkholes and continuing their course as subterranean rivers. Over time, the enlarged cracks can join to form extensive networks of underground caves and passages, some large enough to be explored on foot, along which the dissolved water continues to carve new channels or deposit dissolved minerals as stalactites and stalagmites where it drips from a cave roof.

On the surface, karst regions are often recognisable from a distinctive lack of visible rivers and lakes, since most drainage has migrated underground, combined with a landscape pitted with numerous sinkholes and, in some regions, taller residual towers of unweathered rock left standing after the surrounding limestone has been dissolved away. Farming on karst terrain can be difficult, since soils are often thin and rainwater drains away too quickly to be retained near the surface for crops, though the underground cave networks themselves can support unusual, often highly localised ecosystems adapted to permanent darkness.

Karst landscapes also present distinctive hazards. Because so much of the rock beneath the surface has been dissolved away, the ground above an underground cavity can collapse suddenly if it becomes too weak to support its own weight, a hazard that engineers must account for carefully before approving construction in karst regions. Water flowing rapidly through underground channels is also difficult to filter naturally, so pollutants introduced at the surface can travel long distances underground and contaminate drinking water drawn from distant wells with little warning.

Complete the sentences below. Choose NO MORE THAN TWO WORDS from the passage for each answer.

1. Karst landscapes form where a layer of soluble rock, usually ______, lies close to the surface.
2. As it falls and passes through soil, rainwater becomes a weak solution of ______.
3. Surface streams may disappear underground through widened cracks called ______.
4. Dissolved minerals may be deposited as stalactites and ______ where water drips from a cave roof.
5. Farming on karst terrain is difficult partly because soils are thin and ______ drains away too quickly.
6. The ground above an underground cavity can collapse if it becomes too ______ to support its own weight.