Rates of Weathering
By Trista L. Pollard
As dramatic as the process of weathering sounds, it
does not happen overnight. In fact, some instances of
mechanical and chemical weathering may take hundreds
of years. An example would be the dissolving of limestone
through carbonation. Limestone dissolves at an average
rate of about one-twentieth of a centimeter every 100
years. If you want to see a layer of limestone (about 150 meters thick) dissolve,
plan on watching that layer for about 30 million years.
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Where we see the effects of weathering often is on our stone monuments and
buildings and large rock structures. However, before you can analyze the rate at
which these structures are weathering, you need to understand the factors that
affect weathering rates. The weathering rate for rocks depends on the composition
of the rock; the climate of the area; the topography of the land; and the activities of
humans, animals, and plants.
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A rock's composition has a huge effect on its weathering rate. Rock that is
softer and less weather-resistant tends to wear away quickly. What is left behind is
harder, more weather-resistant rock. This process is called differential
weathering. Quartz is one type of rock whose composition, especially its
crystalline structure, makes it resistant to mechanical and chemical weathering.
This is why quartz remains unchanged on the Earth's surface after surrounding
sedimentary rock has been eroded. There are some rocks, like limestone, that
weather more rapidly. Limestone has the compound calcite. It is the carbonization
of calcite that causes the increased rate of weathering of limestone. The material
found in sediment grains also affects the rate of weathering. The mechanical
weathering of rocks like shale and sandstone causes their grains to break up over
time and become sand and clay particles. Why? Well, the grains in these two types
of rocks are not cemented together firmly. Rocks like conglomerates and
sandstones have grains that are cemented strongly with silicates. These rocks and
other similar types tend to resist weathering. Geologists have also found that they
may resist weathering longer than some types of igneous rocks.
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A rock's exposure to the weathering elements and its surface area can affect its
rate of weathering. Rocks that are constantly bombarded by running water, wind,
and other erosion agents, will weather more quickly. Rocks that have a large
surface area exposed to these agents will also weather more quickly. As a rock
goes through chemical and mechanical weathering, it is broken into smaller rocks.
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As you can imagine, every time the rock breaks into smaller pieces its surface area
or part exposed to weathering is increased. Think about a cube, which has both
volume and surface area. To find the surface area of a cube, you need to calculate
the sum of the areas for all six sides. Let this cube represent our rock that is
exposed to weathering. Already our cube has six sides that are exposed to the
elements. If we split our cube into eight smaller cubes, then the total surface area
would be doubled. Although the surface area increases, the volume remains
constant. Splitting the eight smaller cubes in the same way would have the same
effect; the surface area would again be doubled. Increased surface area causes
rocks to weather more rapidly.
There are very few smooth rocks on our planet. So it should not surprise you
that the majority of our rocks have fractures and joints. Both increase the surface
area of rocks because they are natural zones of weakness. Fractures and joints
provide new natural pathways for running water. Once inside, this water penetrates
rocks further causing the rocks to break apart. Water is the number one ingredient
in ice wedging; fractures and joints speed up that process. Chemical weathering
also occurs more rapidly. As the water and other compounds enter the rock, more
material is removed from the fractures and joints. This causes the structure to
become weaker, which increases its rate of weathering.
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Climate and topography are other influences on weathering. In areas where the
climate changes from hot weather to cold weather frequently, rocks tend to weather
more quickly. A major component of ice wedging is the freezing and thawing.
Once these rocks are broken down, then chemical weathering begins its job. High
temperatures also increase the rate that chemical reactions occur. Rocks found in
warm, humid climates tend to go through chemical weathering rapidly. However,
not all hot climates cause an increase in the weathering rate. Geologists have found
that weathering rates are slower in hot, dry climates.
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Quick Drop: Why do you think the rates of weathering are slow in hot dry
climates?
Weathering rates also occur more slowly in very cold climates. The impact of
climate can be seen on Cleopatra's Needle in New York City. This structure or
obelisk is made of granite. Prior to its journey to America, it was located for 3,000
years in Egypt. Pictures of the structure before 1880 showed ornate markings and
drawings on the obelisk's surface. Once it was brought to New York City in 1880,
it became exposed to the city's climate, pollution, ice wedging, and acid
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precipitation. All it took was 100 years for the rapid weathering to wear away this
ornate surface.
A land's elevation and slope of its surface or topography affect the weathering
rates of rocks. Rocks found at higher elevations will weather most often from ice
wedging than rocks at lower elevations. The rate of weathering on mountainsides
and steep slopes occurs more rapidly than rocks located on flatter land surfaces.
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Quick Drop: How does the slope of the land surface cause the rocks to weather
more rapidly?
The last two factors that influence weathering rates are human activities and
plant and animal activities. We cause chemical and mechanical weathering of
rocks through mining and construction practices. Both expose new rock surfaces to
weathering agents. Mining companies use acids and chemical compounds when
extracting ores. These acids and compounds, when combined with water and other
solvents, cause chemical weathering to occur in the area. Soil is removed on
construction sites to make way for new buildings and structures. Once the soil is
removed, rock surfaces that weren't exposed become exposed to the elements. The
use of recreational vehicles and other activities also speed up the weathering
process. When we ride all-terrain vehicles and hike on trails, we expose new rock
surfaces to weathering. Human activity weathers rocks more rapidly than rocks in
undisturbed areas. The same can be said for rocks disturbed by plants and animals.
As plants and trees grow, their roots break apart neighboring rock. Animals that
make their homes by burrowing into rocks and soil disturb rock and expose new
surfaces. These exposed surfaces weather more rapidly. Animal wastes also
accelerate the rate of chemical weathering. Some forms of animal wastes, like bat
guano, attract millipedes and other insects. These creatures speed up the process of
mechanical weathering on the surfaces below the guano. The compounds in the
waste also speed up the chemical weathering process. All of these factors influence
weathering processes that usually occur at a very slow pace.
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Copyright © 2013 edHelper
Name _____________________________
Date ___________________
Rates of Weathering
1. True or False: Sediment grains that
are held together strongly tend to
resist weathering better than
sediment grains that are held together
loosely.
False
True
3. ______ increase the surface area of
2. Describe the effect climate change
had on Cleopatra's Needle in New
York City.
4. What is differential weathering?
rocks and, as a result, the rocks'
weathering rate.
Mineral composition and
structure
Joints and nodules
Fractures and joints
Fractures and nodules
5. Describe how rock composition
affects the rate of weathering.
6. Which do you expect to weather at a
faster rate- large boulders or smaller
stones? Why?
7. Rising temperatures cause the rate of 8. True or False: Ice wedging
chemical weathering to ______.
Accelerate
Decelerate
Stay constant
None of the above
commonly occurs at lower
elevations.