Study guide for Exam 2, GS 100
Chapter 5-9 Lutgens and Tarbuck
Answers to the Review Questions
1. Weathering, mass wasting, and erosion are all integral processes involved in the rock cycle because of
their relationship to the formation of sedimentary rocks. In the rock cycle, the first step in the formation of
sedimentary rocks involves the derivation of sediment from pre-existing igneous, metamorphic, or
sedimentary rocks. Solid rock is transformed into sediment by the collective processes of weathering, mass
wasting, and erosion. Therefore, the external processes provide the important first step towards the
formation of sedimentary rocks.
2. A rock exposed to mechanical weathering would be broken into smaller and smaller pieces. Therefore, the
same rock following mechanical weathering would look identical to the original rock, only in smaller
fragments or pieces. Chemical weathering involves various processes that convert the minerals in a rock
into new minerals or release them to the surrounding environment. Because of these chemical changes, the
same identical rock above, having been chemically weathered, will most likely look considerably different
than the mechanically weathered rock.
3. Reduction in particle size produces an enormous increase in surface area of the material being weathered.
For very small particles, water, oxygen, and other important chemical-weathering agents have direct access
to nearly the entire grain, while uncracked interiors of coarser particles are protected, at least temporarily,
from contact with the main chemical-weathering agents.
4. Exfoliation domes form when the reduction in pressure that accompanies unloading leads to sheeting.
Fractures typically develop parallel to the surface and give these exhumed granite masses a domed shape.
Continued weathering causes the slabs produced by sheeting to separate and spall off. Examples include
Stone Mountain, Georgia, and Half Dome and Liberty Cap in Yosemite National Park.
5. (a) Moisture and warm temperatures accelerate rates of chemical reactions, thus chemical weathering will
predominate. Also, mechanical weathering processes dependent on freezing and thawing will not be
(b) Basalt would probably weather more rapidly. Ferromagnesian minerals would be rapidly oxidized and
decomposed under these conditions, and basalts have much larger percentages of these minerals than
6. In general, high temperatures do raise chemical weathering reaction rates, but most of these reactions take
place in an aqueous (watery) media or on the moist surfaces of rock and soil particles. Reaction rates
decrease drastically under very dry conditions.
7. Carbonic acid is a very weak acid formed by the solution of carbon dioxide (C0
) in water. Carbon dioxide
is a minor component of the atmosphere but is often enriched in soil gases by the oxidation of organic
matter. The common cations of feldspars (K = +1, Na = +1, and Ca = +2) are fairly soluble in acidic
solutions, so reaction of carbonic acid with potassium feldspar causes the feldspar to chemically