Weathering and Erosion Questions and Answers

Answer: C

Correct answer: inward from joint surfaces on all sides

Water enters along the joints, so decomposition begins at every joint face and works inward. The corners of a block are attacked from three directions at once and the edges from two, so they decay fastest, and the block becomes progressively more rounded until a spherical core of fresh rock is left within concentric shells of decomposed material. It is chemical weathering guided by the joint pattern.

Answer: D

Minerals that crystallised first at the highest temperatures are furthest from equilibrium at the surface and weather first, so olivine goes before quartz. It is a neat illustration of why quartz sand is so ubiquitous.

Answer: A

Correct answer: hot and humid

Chemical reactions accelerate with temperature, and water is the medium in which almost all of them proceed, so heat and abundant moisture together give the fastest decomposition - weathering profiles tens of metres deep in the humid tropics. Cold slows the reactions and aridity removes the water, which is why desert and polar rock weathers mainly by mechanical means and remains chemically fresh.

Answer: B

Correct answer: joint and fracture pattern

Weathering proceeds from surfaces, so the more surfaces the rock offers the faster it decays. A closely jointed mass presents water with access throughout its volume and weathers deeply and quickly; the same rock, massive and unfractured, may remain fresh a few metres below the surface. Mineralogy sets what will react and texture how fast, but the joints determine where the water can reach.

Answer: C

Warm temperatures and abundant water sustain the reactions over long periods, giving profiles of twenty, fifty or occasionally a hundred metres. Foundations may have to be taken far deeper than the same structure would need in a temperate climate.

Answer: D

Correct answer: formed in place from the rock beneath it

A residual soil is the weathered product of the rock directly beneath it, so its character can be anticipated from that rock and it grades downward into it through the weathering profile. A transported soil has been carried from elsewhere by water, wind or ice and bears no relation to what lies below. The distinction matters in site investigation, since a residual profile can be predicted and a transported one cannot.

Answer: A

Correct answer: thoroughly decomposed rock retaining its original fabric

Saprolite is rock that has been chemically decomposed in place to the strength of a soil while keeping the texture, the relict joints and often the appearance of the parent rock. It can be cut with a spade yet still shows the original mineral outlines, and its relict discontinuities are planes of weakness that behave as joints do - which is what makes it treacherous in slopes and excavations.

Answer: B

Correct answer: soil at the top through weathered rock to fresh rock

The profile is a gradation, since weathering works downward from the surface: residual soil at the top where decomposition is complete, then progressively less altered rock, then fresh rock at depth. The intermediate zone commonly contains core stones of fresh rock within decomposed material. The boundaries are gradual, which is precisely why defining rock head for a foundation is so difficult.

Answer: C

Correct answer: fresh rock remnants surrounded by decomposed material

Core stones are what is left of the original blocks after spheroidal weathering has attacked them from every joint face - rounded masses of fresh rock isolated within saprolite. They are the reason a borehole in a weathered profile can strike hard rock at a shallow depth and still not have reached rock head, since the drill may have entered a boulder with soft material beneath it.

Answer: D

Correct answer: running water

Rivers and the runoff feeding them move more material from the land surface than all other agents combined, because water falls everywhere, flows everywhere and never stops. Wind dominates locally in deserts, ice in glaciated regions and waves at the coast, but each is confined to its own environment while running water operates over almost the whole land surface of the earth.