Succeed in IELTS Volume 9
Reading Practice Test 2
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READING PASSAGE 1
You should spend about 20 minutes on Questions 1-13,
Questions 1-13, which are based on Reading Passage
One.
page 1
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WEATHERING IN THE DESERT
A
In the deserts, as elsewhere, rocks at the earth's surface are changed by weathering, which
may be de ned as the disintegration of rocks where they lie. Weathering processes are either
chemical, when alteration of some of the constituent particles is involved; or mechanical, when
there is merely the physical breaking apart and fragmentation of rocks.
Which process will
dominate depends primarily on the mineralogy and texture of the rock and the local climate, but
several individual processes usually work together to the common end of rock disintegration.
B
The great daily changes in temperature of deserts have long been supposed to be responsible
for the disintegration of rocks, either by the differential heating of the various rock-forming
minerals or by differential heating between the outer and inner parts of rock masses. However,
both eld observations and laboratory experiments have led to a reassessment of the
importance of ’ exposure to the sun's rays in desert weathering.
Almost half a century
ago Barton remarked that the buried parts of some of the ancient monuments in Egypt were
more weathered than were those parts fully exposed to the sun's rays, and attributed this to
the effects of water absorption below the ground surface. Laboratory experiments have shown
that rocks subjected to many cycles of large temperature oscillations (larger than those
experienced in nature) display no evidence of ssuring or fragmentation, as a result. However,
when marked uctuations of temperature occur in moist conditions small rock
fragments quickly form.
C
The expansive action of crystallising salts is often alleged
to exert suf cient force to
disintegrate rocks. Few would dispute that this mechanism is capable of disrupting ssile or
well-cleaved rocks or rocks already weakened by other weathering agencies; wood
is splintered, terracotta tiles disintegrated and clays
disturbed by the mechanism, but its
importance when acting upon fresh and cohesive crystalline rocks remains uncertain.
D
Weathering achieves more than the disintegration of rocks, though this is its most important
geomorphic effect. It causes speci c landforms to develop. Many boulders possess a
super cial hard layer of iron oxide and/or silica, substances which have migrated in solution
from the inside of the block towards the surface. Not only is the exterior thus case-hardened
but the depleted interior disintegrates easily. When weathering penetrates the shell the inside
is rapidly attacked and only the hard outer layer remains to give hollowed or 'tortoiseshell'
rocks.
E
page 2
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Another
super cial layer, the precise nature of which is little understood, is the well-known
desert varnish or patina, a shiny coat on the surface of rocks and pebbles and characteristic of
arid environments. Some varnishes are colourless,
others light brown, yet others so dark a
brown as to be virtually black. It's origin is unknown but is signi cant, for it has been
suggested that the varnish grows darker
with the passage of time; obviously before such a
criterion could be used with con dence as a chronological tool its origin must be known with
precision. Its formation is so slow that in Egypt, for example, it has been estimated that a light
brown coating requires between 2,000 and 5,000 years to develop, a fully formed blackish
veneer between 20,000 and 50,000 years.
F
The development of relatively impermeable soil horizons that are subsequently exposed at the
surface because of erosion of once overlying, easily eroded materials, and which thus become
surface crusts, is widespread in arid regions, although it is also known outside the deserts, and
indeed many of the examples in arid lands probably originated
in former periods of
humid climate. The crusts prevent the waters of occasional torrential downpours
from penetrating deeply into the soil, and thus they contribute to the rapid run-off associated
with desert storms. Also, after erosion has cut through the crust and exposed underlying soil
layers, the hard layer forms a resistant capping (duricrust) on plateaux and mesas, such as are
common in many parts of arid and semi-arid Australia.
G
Some duricrust layers have been used as time markers for landforms and geological formations.
The necessary conditions for this are that the crust forms fairly rapidly, and that it is suf ciently
distinct in appearance to preclude the possibility of confusion with other crusts formed at other
times. The Barrilaco calcrete of Mexico for instance is believed to date from about 7,000 B.C.
The main silcrete of the northern districts of South Australia is believed to date from the Lower
Miocene, the laterite of northern Australia to be of the Lower or Middle Miocene age.
