Geoscience Reference
In-Depth Information
VISUAL CONCEPT CHECK
14.1
This image is of a rock in the southwestern
United States. Given what you see, and the
location of the rock, which one of the following
weathering
processes
produced
the
rock
pattern visible here?
a)
Hydrolysis
b)
Carbonation
c)
Freeze-thaw activity
d)
Thermal expansion
e)
Oxidation
at least as far as the United States and Canada are concerned.
Before the first federal regulatory controls on air quality were
passed in 1970, the amount of pollutants emitted into the air
had risen steadily since the onset of the Industrial Revolution
in the 19th century. These emissions peaked at 28.8 million
tons in 1973 in the United States alone. Given growing con-
cerns about acid rain in the 1970s and 1980s, the Clean Air Act
was amended in 1990 to give industries more pollution control
options such as switching to low-sulfur coal or adding scrub-
bers to control acid emissions. In more extreme cases, plants
were ordered closed because they could not comply. The 1990
amendments clearly had a positive effect on acid emissions,
with a 32% decrease to about 18 million tons in 1995. By 2010
the amount had dropped to about 11 million tons. Although nitro-
gen dioxide emissions remain unchanged, we appear to have at
least turned the corner with respect to acid rain in North America.
Major problems remain in the former Soviet bloc of eastern
Europe, however, where virtually no environmental regulations
were in effect during the Communist era. As a result, the rela-
tively new democratic governments are struggling with how to
cope with the serious environmental degradation in these areas.
KEY CONCEPTS TO REMEMBER
ABOuT WEATHERING
1.
Two general kinds of weathering processes take place:
mechanical (physical) and chemical. Mechanical weath-
ering causes rock bodies to break into smaller rock
fragments, whereas chemical weathering causes the
rock to change chemical composition.
2.
Although mechanical weathering and chemical weath-
ering can occur anywhere, most mechanical weathering
occurs in cold environments, whereas chemical weath-
ering is more pervasive in warm/humid climates.
3.
The three major kinds of mechanical weathering are
frost wedging, salt-crystal growth, and exfoliation. Frost
wedging involves water that expands and contracts
upon freezing and thawing in joints and cracks in rock.
Salt-crystal growth occurs when water seeps out of rock
in arid environments. Exfoliation happens when rock ex-
pands because of unloading and temperature changes.
Weathering
Now is a good time to review the various weathering
processes discussed so far in this chapter. Given that they
involve water movement and the gradual wearing down of the
landscape, it is useful to observe them in an animated format.
Go to the
Geo Media Library
and select
Weathering
. This
animation shows, for example, how frost wedging gradually
causes rock to split apart and how the surface area of a rock
mass increases with progressive mechanical weathering.
The animation also reviews the various ways that rock
decomposition occurs as a result of chemical weathering
processes such as hydrolysis and carbonation. As you watch
this animation, think about how weathering contributes to the
wearing down of rock masses and the time involved for this
erosion to occur on a noticeable scale. After you complete
the animation, be sure to answer the questions at the end to
test your understanding of weathering.
