Lab: Exploring Mechanical Weathering
Background Information
The Earth’s surface is constantly undergoing a process of change. Rock is broken down and
disintegrated, moved to lower elevations by gravity, and transported by water, wind, or ice. These are the
processes of weathering and erosion. Weathering is a slow, continuous process that breaks down rocks at
or near the Earth’s surface in response to air, water, living matter, and the force of gravity. Erosion is the
process by which weathered rock and soil particles are transported from one place to another by water,
wind, or ice.
There are two general types of weathering: mechanical and chemical. Mechanical, or physical
weathering, is the disintegration process by which rock is progressively broken down into smaller and
smaller fragments as the result of physical forces. In chemical weathering, the chemical makeup of the
original Earth materials is actually changed by the process. In this activity, we will concentrate on the
process of mechanical weathering.
During mechanical weathering, physical forces acting upon surface or near surface rocks
eventually break these materials down into smaller and smaller pieces. In nature, several key physical
processes lead to the disintegration of Earth’s crustal rocks. These are ice wedging, abrasion, exfoliation or
sheeting, thermal expansion and contraction, and organic activity. (by living things)
In ice wedging, water which has worked its way into cracks and voids in rock, expands and
wedges the rock apart upon freezing. Water has the unique property of expanding about 9% as it freezes.
This expansion in volume exerts a tremendous outward force. A daily freeze-thaw cycle, particularly
common to many mountainous regions in the middle latitudes, works constantly to break apart surface and
near surface rocks. This is the process behind the many cracks and potholes seen in roadways and
driveways during the winter months in colder regions. Surface rocks are also broken down by the process
of abrasion. The agents of abrasion include gravity, running water, wind, and ice. Gravity causes loose
soil and rock to move downslope from higher elevations. As these materials fall, they collide with each
other and with other rocks in their paths, and break into smaller pieces. Running water carries loose rocks
and other particles along its course. These materials collide, scrape against one another, abrade surface
rocks they come in contact with, and eventually break. In similar fashion, the wind easily picks up and
carries sand particles which impact and abrade exposed surface rocks. Particles of loose rock and soil may
also be transported by moving glaciers, effectively scouring and breaking off small pieces of surface rocks
in the process.
Physical changes within the rocks themselves may also aid mechanical weathering. For example,
when large igneous bodies (most notably granites) are exposed at the Earth’s surface by erosion and/or
uplift, the pressure on the once confined rocks decreases. As a result, researchers believe the exposed rocks
may actually expand slightly, causing long curved cracks to develop parallel to the rock’s surface. These
surface cracks, called joints, cause layers or sheets to break loose over time and peel away from the parent
rock. This type of breaking off of rock is called exfoliation or sheeting.
Rapid and significant changes in temperature are also believed to play a part in the mechanical
weathering of rock. Rapid heating and cooling of surface rocks from day to night or from summer to
winter cause thermal expansion and contraction of the rocks’ outer surface. This may weaken the bonds
between individual mineral grains or actually cause particles on the rock’s surface to crack or flake off.
While laboratory evidence to support this process has been inconclusive, it is believed that heating and
cooling can add significantly to the rate and effects of the weathering process.
The activities of plants and animals are also responsible for the mechanical weathering of rocks.
Plant roots in search of minerals and nutrients can work their way into cracks and crevices in surface rocks.
As the roots grow and expand, they create pressure that wedges the rock apart. Burrowing animals also
contribute to mechanical weathering by removing surface cover and exposing new rock surfaces to the
forces of weathering. Human activities in mining, road and building construction, and even rock-climbing,
all have a physical impact on rocks at or near the Earth’s surface.
Rates of Weathering
While the processes of weathering generally work very slowly, the rate at which rock weathers
depends upon many factors. Mineral composition, particle size, and the strength of bonding between
grains, all influence the rate of rock weathering. Rocks composed of some harder minerals, like quartz, are
generally more resistant to both mechanical and chemical weathering. In some cases, however, the bonds
which hold the mineral grains together may be weakly cemented. Many sandstones, siltstones, and shales
fall into this category. While containing some hard quartz grains, the cementing agent between the grains
in these rocks may be weak, allowing them to break up more easily into clay and sand particles under
mechanical weathering. In contrast, some conglomerates and sandstones are more strongly cemented by
silicate minerals, and will resist weathering much longer.
The amount of surface area and the length of time a rock has been exposed at the Earth’s surface
will also influence the rate of weathering. The greater the exposure, the faster the material will weather.
Further, as rocks are broken down into smaller and smaller pieces, more rock surfaces are progressively
exposed to the weathering process.
General climatic factors, most notably temperature and moisture, will also significantly affect the
rate of rock weathering. In hot, dry climates, the rate of weathering generally takes place more slowly, as
lack of water limits the rate of many chemical and mechanical weathering processes. Climates in which
variable weather conditions persist, especially freeze-thaw cycles, will advance the rate of weathering
through greater ice wedging and thermal expansion and contraction.
The ongoing interaction of these and other physical and environmental factors determines the type
and rate of rock weathering in a given area.
Overview:
In this activity, you will conduct an investigation of rock abrasion and the rate of mechanical weathering as
influenced by mineral composition, amount of surface exposure, particle size, and grain integrity. Using
several rock chips from three common rock types, you will compare the effects of physical abrasion on
each sample by shaking the chips in a sealed container that has been partially filled with water. The rock
samples include a weakly cemented sandstone (#1), a quartz pebble conglomerate (#2), and a coarsegrained granite (#3).
With progressive shaking and abrasion, you should see the progressive disintegration of the rock samples
as evidenced by the gritty residue created in each jar and the rounding of the edges on the sample chips.
You should also observe some variation in the rate of weathering between the samples, due to differences
in mineral composition and bonding strength between grains.
Materials Required:
3 groups of rock sample chips
3 labeled plastic jars and lids
1 unlabeled plastic jar
1 glass, 16 oz.
1 strainer
1 magnifier, 3x
Water
Procedure: (Please read through all procedures BEFORE you construct a data table. It will help you to
determine how many data tables, and how to construct them!)
After you’ve completed the reading, and designing your data table…generate a hypothesis. The
hypothesis should be well constructed. Make it an “If…then…” statement. The hypothesis should be
about what you think will happen to the three samples you’ll be shaking. Which will hold up best?
Which will hold up worst? Etc…
In this investigation, you will explore several key aspects of mechanical weathering by duplicating physical
processes which influence the break down of rocks at or near the Earth’s surface.
1. Create a data table that will hold qualitative and quantitative data for your three samples,
for four trials each. Examine the 3 bags of rock sample chips labeled sample 1, 2, and 3.
Observe and note the typical surface shape, texture, and grain size of the average chips in
each sample. Use your hand lens. Record this data in your data table. Determine the
combined mass of rock sample chips in each container. Record this data in your table as
well.
2. Empty approximately one half of the contents of each sample bag into its matching
plastic jar. Count the number of rock chips you have, and record this information.
3. Starting with sample 1, add sufficient water to the jar to just cover the rock chips. Screw
the lid on TIGHTLY, and shake the jar vigorously 100 times.
4. Pour the water and the rock chips slowly from the sample jar into the strainer. Position
the empty unlabeled plastic jar from your kit under the strainer to catch the wastewater as
it passes through.
5. Now run your finger around the inside of the empty sample jar, and record what you feel.
Look closely at the residue in the jar and write down your observations.
6. Use the hand lens to examine the rock chips in the strainer and record any observed
changes in physical appearance in your data table.
7. Now place the rock chips from sample 1 and the wastewater from the plastic sample jar
back into sample jar 1. Retighten the lid and shake the container again 100 more times.
Repeat steps 4-7, continuing this process three more times. Make sure you make
notations of ALL qualitative data in your data table.
8. After all four trials, pour the waste water into the clear glass jar and examine it with your
hand lens. Note its appearance descriptively and record in your data table. Photographic
evidence is an excellent form of qualitative data. If you have a cell phone and can
take and print pictures, it is good for extra credit.
9. Repeat steps 3-8 for samples 2 and 3.
10. Set the rocks out to dry until tomorrow, and re-mass them then. Record in data table. Recount the number of rock chips you have, and record this number
Part B: Inquiry Investigation
Question: How acid/basic rain affect the rate of weathering?
In your lab groups, plan an investigation to determine the answer to that question. You will have the same
materials you had for the above lab, and anything else you may need…just ask. Things to remember:
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Write the question being investigated in part B (which is written above) You choose acid, or base
Write a hypothesis as to what you think will happen (please note, your hypothesis does NOT have
to be in agreement with your other lab partners)
Write the first 3-4 steps in your procedure.
Design any data tables you may need to collect data during your investigation.
Analysis Questions:
1. Did the appearance of the rock chips in samples 1-3 change after repeated shakings? Explain.
2. Which of the three rocks samples appears to have changed the most? Which sample has changed
the least? What might explain this difference?
3. Describe how the appearance of the wastewater changed with repeated shakings.
4. What primary agent of mechanical weathering was at work in this exercise?
5. What situation in nature do you think this exercise most closely simulates?
6. How did your group choose to investigate acid/base-chemical weathering?
7. What conclusions were you able to make about how acids/bases affect the rate of weathering?
8. List and describe 2 other examples of mechanical weathering you have observed around the school
grounds, or in your neighborhood. Describe which agents may have been responsible for each of
your two examples.