BEST IELTS Academic Reading Test 491

BEST IELTS Academic Reading Test 491

IELTS Academic Reading Test

Sand dunes

A dune is generally classified as a type of landform, which most commonly consists of sand and which is formed by grains of sand blown by the wind. These kinds of land formations are found in deserted environments and by beaches in coastal areas, but they can also be found inland and, perhaps unexpectedly, in some underwater locations, such as riverbeds.

They can also be artificially made. The oldest dunes are over 9,000 years old, whereas others are only a few years old. Natural dune formation occurs through what is known as an Aeolian process; that is when the wind has the ability to change the shape of the Earth’s surface. A dune begins its formation when the wind blows sand into a sheltered area behind some sort of obstacle, and from here, the dune grows.

IELTS Academic Reading Test

As the dune grows, there are usually two features that develop; the windward side of the dune, which is where the wind blows the sand grains and pushes them in an upward direction, and the slipface, which is the opposite side and faces away from the wind and appears much smoother than the windward side. However, there are five major categories of dunes based on their shape and the wind direction.

Crescentic sand dunes are the most common and the fastest moving of all known dunes. These are characterised by a letter ‘C’ shape, with the windward side being the widest area. Similar to crescentic dunes are parabolic dunes, which have a similar ‘U’ shape. In this type of formation, alternatively known as a blowout, the centre of the dune has been hollowed out by the wind.

IELTS Academic Reading Test

The dunes in the Sahara Desert, though, are examples of star dunes. These have pointed peaks and at least three slipfaces because the wind attacks the dune from many different directions. The rarest dune shape is known as the dome; this is circular in shape and, unlike other dunes, has no slipface – this dune is formed by the wind coming from any direction. The linear dune mostly consists of a fairly straight line of a continuous peak, but can include regular curves along its length. These are formed from the wind coming in two directions at an approximately equal speed.

In order for sand dunes to form, there are various conditions which need to be met. Firstly, there needs to be a large quantity of loose sand available. There should be little or no vegetation in the sand, as this helps to stabilise the sand, making it harder to move. Secondly, the wind is an essential factor for the formation of dunes. Finally, some kind of obstacle or object must be present in the direction of the wind, such as a tree or a rock. This is needed to force the momentum of the sand to stop and begin to accumulate, therefore forming a dune.

IELTS Academic Reading Test

Being an important element in the formation of dunes, the wind can move the sand in three different ways. The most common way for sand grains to be moved is by the wind bouncing them along the surface until they settle next to an object; this is called saltation and occurs 95% of the time in dune formation. If the grains of sand collide into each other and force movement, this is known as creeping and occurs in only 4% of formations.

The least common method for sand grains to move is called suspension, and this is where the grains are blown high up in the air and fall down to settle, to account for the remaining 1% of occurrences. During formation, the sand grains keep moving until they are halted by an obstacle. The heavier grains settle against the object, whereas the lighter grains fall on the other side of the object. As the dune grows, lighter grains are blown up the windward side of the dune, eventually reaching the top, or the crest, of the dune.

IELTS Academic Reading Test

These lighter grains then fall down the slipface. Occasionally, the crest crumbles under pressure and this is how it is considered that dunes move. Dunes are constantly moving due to the pressure of the wind as the crest tumbles and then is built up again. In fact, in some areas, this dune migration threatens agriculture and towns because of the continual shifting of dune crests, encroaching on human habitats.

Even though dunes shift, there are some animal and plant species that can survive in this otherwise hostile environment. There is no soil, so grasses with shallow roots do well in dunes. In some dune areas, a lizard called a sandfish thrives in the sand and rare species like the yuz, or Asiatic cheetah have adapted to living in a dune environment. These species are highly specialised and often endangered, so conserving dunes is extremely important for their survival.

IELTS Academic Reading Test

Dunes are threatened by human recreation and land development as well as by climate change. They often cannot migrate further inland because of redeveloped land, and this proves a problem when sea levels rise, leaving the dunes with nowhere to migrate. Beachgoers and visitors also contribute to dune destruction by trampling over the dunes causing an interruption to the natural process.

Additionally, strong sea defences prevent the availability of new sand for the replenishment of dunes. However, some countries, such as the USA and UK, have dedicated programmes of protection. Measures taken to help protect dunes include the planting of vegetation to help stabilise and decelerate growth in order to conserve some of the rarest animal and plant species on Earth.

IELTS Academic Reading Test

Choose ONE WORD ONLY for each answer.

14. Where might you be surprised to find dunes?

15. What part of a dune sometimes collapses causing the dune to migrate?

16. What is the name given to the side of the dune where air forces the sand upwards?

IELTS Academic Reading Test

Choose NO MORE THAN TWO WORDS AND/OR A NUMBER for each answer.

IELTS Academic Reading Test

IELTS Academic Reading Test

Do the following statements agree with the information given in Reading Passage 2? In boxes 22-26 on your answer sheet, write

TRUE – if the statement agrees with the information

FALSE – if the statement contradicts the information

NOT GIVEN – if there is no information on this

22. For sand dunes to form, plant life should be virtually non-existent.

23. The process known as saltation happens more frequently in coastal areas than inland.

24. Lighter sand grains generally accumulate on the windier side of an obstacle.

25. Vegetation with deep roots thrives in sand dunes.

26. Climbing over sand dunes can cause damage to them.

IELTS Academic Reading Test

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BEST IELTS Academic Reading Test 491

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14. RIVERBEDS

15. CREST

16. WINDWARD

17. WIND

18. BLOWOUT

19. STAR

20. CENTRE

21. 3/THREE SLIPFACES

22. TRUE

23. NOT GIVEN

24. FALSE

25. FALSE

26. TRUE

IELTS Academic Reading Test

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