Download Igneous rocks outline – Day 1

Weathering & Erosion optional questions KEY
1) In the most general terms, why are some minerals very susceptible to chemical weathering? Consider how where they
formed is different from the weathering environment.
Minerals that formed under conditions different from Earth surface conditions are susceptible to weathering
because these minerals are exposed to different T, P, and to water and our atmosphere.
2) If all other factors are equal, how rapidly would a massive body of rock weather compared to the same amount of severely
broken up rock? Why?
A massive rock (one that is not broken up) has much lower surface area than a severely broken up rock. The
greater surface area of the broken rock allows chemical reactions to occur faster, and these reactions chemically
weather the broken rock faster.
3) Table16.2 (Stability of min’ls) gives relative rates of dissolution of several minerals. What does this table suggest about
the likelihood of finding A,B, and C, and why?
A. Olivine existing as beach sand? Where might expect to find olivine composing a significant portion of sand grains
on a beach (This phenomenon does occur, by the way)?
Olivine crystallizes from silicate melts at hi T & P, and is not very stable at the Earth’s surface, therefore it would be
unlikely to find an olivine beach unless there was a source of fresh rock containing olivine very nearby. Some beaches
in Hawaii are dark, containing lots of olivine because frequent new lava flows provide fresh basalt as a source for
olivine very near the beach.
B. A sedimentary rock composed of, among other minerals, a significant amount of amphibole grains?
Not likely. Amphiboles are mafic minerals which form at hi T & P, and they are not very stable at the Earth’s
surface. Therefore, because sediments from weathered rock are unlikely to contain amphiboles, amphiboles are
unlikely to be transported and deposited to be later buried and lithified, so almost no sedimentary rocks contain
amphiboles.
C. A beach composed of primarily grains of fine quartz sand?
Very likely. Quartz forms in igneous melts at relatively low T & P, and it also can precipitate from solutions very near
the surface at near Earth surface conditions. Therefore, quartz is very stable under surface conditions. It can be
broken physically by transport, but it very often “survives” to make it to deposited at the ocean by rivers. Most other
minerals are less stable, so lots of beaches are primarily quartz.
4) Granites in Georgia are fairly typical mineralogy: mostly quartz, K-spar, plagioclase feldspar, and minor biotite and
hornblende (an amphibole). Georgia is also known for the production of Georgia kaolin (kaolinite), which is used in making
paper (gives paper a glossy luster) and pottery. The kaolinite is mined from quite pure deposits that can also have some
quartz present, but not much else.
A. How do these deposits form from a granite? What happened to the other minerals?
Granites chemically weather, losing the less stable mafic minerals. The abundant feldspars weather to the clay
mineral kaolinite which stays in the soil. K+ or Na+ from the feldspars are released to solution, which is carried to
the sea.
B. How is this process analogous to making coffee?
Unweathered granite = fresh grounds
kaolinite ± quartz = grounds after coffee is made
rainwater = water for making coffee
coffee = soil water which makes it to the sea (‘though we’ve not discussed water movement yet)
C. There are granites of roughly equal composition in Georgia and New Hampshire (for example). Why does this process
occur in Georgia and not in New Hampshire?
Georgia is warm and humid, both of which speed up chemical weathering. New Hampshire is colder, which slows
down chemical weathering.
5) Laterite soils (composed of aluminum and iron oxides and little else) began as mafic igneous silicate rocks. They are most
often found near the equator where rainfall is very high? Why? Can you think of one or two reasons why there is little quartz
in such soils?
High temperatures and much rainfall cause rapid chemical weathering. The mafic minerals are very unstable at the
Earth’s surface, so they weather away quickly, leaving behind aluminum (from feldpars, CaAl2Si2O8, or micas) and
iron (from olivines, pyroxenes, biotite, amphiboles) oxides in the soil. The iron and aluminum oxides formed in the
weathering environment, so they are very stable.
There is relatively little quartz in most mafic rocks. What little quartz there is can also be weathered away.
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6) CO2 and weathering
A. Where does CO2 in rainwater come from? From atmospheric CO2 dissolving in rainwater.
B. Does CO2 dissolved increase or decrease rainwater acidity? It forms carbonic acid by the reaction CO2 (gas) +
H2O (liq) = H2CO3 (carbonic acid), so it makes rainwater more acidic. The pH of rainwater is lowered by the
reaction H2CO3 = H+ + HCO3-.
C. Why is this effect on acidity important for chemical weathering?
Most rocks are somewhat basic (i.e., they are alkaline - can react to neutralize ). The carbonic acid
chemically attacks rocks, weathering them.
D. What human-caused changes in the atmosphere, related to electric power generation, can alter rainwater pH? What
effect does this phenomenon have on weathering of rocks and building stone?
Acid precipitation is caused by the burning of high-sulfur coal. This causes rainwater to be acidic (have low pH). The
acid rainwater chemically attacks rocks and building stones.
7) Explain Fig. 16.1. The figure shows a slate (metamorphic rock with muscovite, biotite, quartz, and feldspars) gravestone
that is easily readable. It also shows a marble (metamorphic rock composed mostly of recrystallized calcite) gravestone of the
same age that is barely readable.
The slate contains minerals that are more resistant to weathering than the calcite that makes up the limestone, so the
slate gravestone has weathered more slowly.
8) Figure 16.3 is a cartoon illustrating the effect of breaking a rock up into smaller pieces on the total surface area of the rock.
A. As a rock is broken, what happens to its mass, as long as no rock is lost? Nothing; the mass doesn’t change.
B. How does breaking up a rock change its susceptibility to chemical weathering? Why? give an example from the
kitchen to illustrate this effect.
Breaking up a rock without mass loss increases the surface area, causing much greater susceptibility to chemical
weathering (faster rates of dissolution and weathering). Table salt or sugar dissolve more quickly quickly if the grains
are small.
9) The Great Pyramids are made of limestone. If they had been built in Mississippi instead, they would be in much worse
shape. Why?
Calcite weathers by dissolution, but if there’s little rain (as in the deserts of Egypt), the weathering is slow.
10) Explain Fig. 16.4 and the relationships between CO2, T, long-term climate changes, and weathering.
As CO2 in the atmosphere increases, temperature (T) increases because the “extra” CO2 acts to insulate the earth
from heat loss. As T increases, then chemical weathering increases (most chemical reactions speed up with increasing
T). As chemical weathering increases, more CO2 is consumed by the weathering of rocks, so CO2 in the atmosphere
decreases, causing T to decrease, which in turn causes weathering to decrease. These processes have occurred in many
cycles in the geologic past.
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