Weathering and Erosion Questions and Answers

Q: Comparing a residual with a transported soil for foundation purposes, the residual soil

Correct answer: can be anticipated from the rock beneath itA residual soil is the decomposition product of the rock below, so knowing that rock allows its character to be predicted - a granite gives a clayey sand, a basalt a heavy clay - and the profile grades continuously into fresh rock. A transported soil has no such relation to what lies beneath and must be investigated on its own. Residual soils certainly contain clay, and are not automatically stronger.

Q: None of the following is a form of mass movement except

Correct answer: soil creep on a hillsideMass movement is the downslope movement of material under gravity without a transporting medium, and creep is its slowest form. The other three all involve an agent doing the work: a river abrading its bed, water dissolving limestone, and wind removing fine material. Mass movement is distinguished precisely by gravity acting alone on the mass.

51.

A cutting in fresh rock stands at 70 degrees, but the same rock weathered fails at 40 degrees. The explanation is that weathering has

A.increased the rock's density and weight
B.cemented the joints closed
C.lowered the friction along relict joint surfaces
D.removed the joints from the rock mass

Answer: C

Correct answer: lowered the friction along relict joint surfaces

Weathering leaves the original joints in place as relict discontinuities and coats them with clay produced by the decomposition of the minerals along them. Those clay-coated surfaces have a far lower friction angle than the fresh rock joints, so the slope fails on them at an angle at which the fresh rock would stand. The rock looks similar and behaves quite differently, which is why weathering grade must be logged in a slope investigation.

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52.

Assertion: quartz sand is abundant at the earth's surface. Reason: quartz is the most resistant of the common rock-forming minerals to weathering.

A.both the assertion and the reason are true but the reason does not explain the assertion
B.the assertion is true but the reason given for it is false
C.the assertion is false although the reason offered is a true statement in itself
D.both the assertion and the reason are true and the reason explains the assertion

Answer: D

Correct answer: both the assertion and the reason are true and the reason explains the assertion

Both are true and the reason accounts for the assertion. Quartz has no cleavage, is chemically inert to hydrolysis and oxidation, and stands at the stable end of the Goldich series - so when a granite decomposes the feldspar becomes clay and is carried away in suspension while the quartz survives as sand grains. Every beach and desert dune is the residue of that selective survival.

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53.

Assertion: freeze-thaw weathering is most severe in permanently frozen ground. Reason: water expands by about nine per cent when it freezes.

A.the assertion is false although the reason offered is a true statement in itself
B.both the assertion and the reason are true and the reason explains the assertion
C.both the assertion and the reason are true but the reason does not explain the assertion
D.the assertion is true but the reason given for it is false

Answer: A

Correct answer: the assertion is false although the reason offered is a true statement in itself

The reason is correct - water does expand by about nine per cent on freezing - but the assertion inverts the condition required. Damage is done by the cycle: each freeze prises the crack wider and each thaw admits more water. Permanently frozen ground gives no cycles, so freeze-thaw is most severe where the temperature crosses zero frequently, as in high mountains and periglacial regions.

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54.

Comparing a residual with a transported soil for foundation purposes, the residual soil

A.contains no clay minerals
B.can be anticipated from the rock beneath it
C.bears no relation to the underlying rock
D.is always stronger than a transported soil

Answer: B

Correct answer: can be anticipated from the rock beneath it

A residual soil is the decomposition product of the rock below, so knowing that rock allows its character to be predicted - a granite gives a clayey sand, a basalt a heavy clay - and the profile grades continuously into fresh rock. A transported soil has no such relation to what lies beneath and must be investigated on its own. Residual soils certainly contain clay, and are not automatically stronger.

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55.

None of the following is a form of mass movement except

A.abrasion of a river bed
B.solution of limestone along joints
C.soil creep on a hillside
D.deflation of a desert surface

Answer: C

Correct answer: soil creep on a hillside

Mass movement is the downslope movement of material under gravity without a transporting medium, and creep is its slowest form. The other three all involve an agent doing the work: a river abrading its bed, water dissolving limestone, and wind removing fine material. Mass movement is distinguished precisely by gravity acting alone on the mass.

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