Chemical and Mechanical Weathering of Rock
Perform a series of hands-on experiments to understand the chemical and mechanical processes involved in rock formation and decomposition.
- Chemical weathering
- Mechanical weathering
Three general processes contribute to the breakdown of rock—physical or mechanical processes, chemical processes and organic processes. Examples of physical or mechanical processes include glacial activity, water erosion, wind erosion and ice expansion.
Thousands of years ago, glaciers covered much of the Earth. Glaciers are thick ice masses that originate on land from the accumulation and compaction of snow. Most of the soil present in North America today was formed and deposited by glacier movements. As these large glaciers moved (or flowed), they exerted a great amount of force on the rocks and other surfaces below them. Glaciers erode land in two ways—plucking and abrasion. Plucking occurs when a glacier moves over a fractured rock surface and loosens and picks up large pieces of rock. Abrasion occurs when a glacier and its load of rock pieces move along and grinds away at the surfaces of the Earth. Glaciers are responsible for geological formations such as the Alps and Yosemite Valley.
As each drop of rainwater hits the surface of the Earth, small amounts of soil and rock particles are moved. As the rainwater moves over the surface of the Earth, it carries small bits and pieces of soil and rock fragments that eventually wear away or erode the Earth’s surface. In early stages, the water flowing across the Earth’s surface is originally in the form of thin sheets of water. This movement of thin sheets of water is known as sheet erosion. After flowing for relatively small distances sheet erosion generally develops into small channels known as rills. As the water moves through the rills, still larger depressions or cuts in the soil form and are called gullies. As water moves through gullies, a large amount of the dislodged rock and soil particles are deposited in a new location. The newly deposited rock and soil fragments are called sediment. The most well-known example of water erosion is the Grand Canyon.
Moving air, just like moving water, is capable of picking up particles and causing erosion. Wind erosion is most prevalent in arid regions where particles are not likely to bind to surrounding vegetation. A great example of wind erosion occurred during the 1930s in many parts of the Great Plains states. Extensive grazing and plowing over of vegetation followed by severe drought and high winds led to the right conditions for a major dust storm called the Dust Bowl. During the Dust Bowl the high winds and flying particles were so overwhelming that at times, no sunlight struck the surface of the Earth.
Ice expansion is another form of a physical change. Ice is less dense and takes up more room than the same amount of water. This explains why ice cubes float in a glass of water. When water freezes it expands. If water seeps down into small crevices or cracks of rock and then freezes, a great amount of pressure will be exerted on the rock. An example of this pressure is seen in the formation of potholes in roads. Rainwater and snow seep into small cracks in the road. As the water freezes pressure is exerted on the road and potholes and cracks will form.
Chemical processes break down soil and rock as well. Water plays the most important role in this breakdown. Although water itself is not generally chemically reactive to rock, a small amount of dissolved material can cause a major impact. For example, oxygen, O2, dissolved in water will oxidize certain materials. Rocks containing iron-rich materials will actually develop a rust layer due to the dissolved oxygen. Carbon dioxide, CO2, dissolved in water, H2O, forms carbonic acid, H2CO3. Carbonic acid ionizes to form very reactive H+ ions and the bicarbonate ion, HCO3–. Hydrogen ions attack and disrupt the chemical makeup of various types of rock.
Weathering also occurs due to organic processes. Activities of organisms, such as plants, animals, and humans, can cause stress on the Earth’s surface. Rapidly growing plant roots looking for minerals and water can break apart rocks. Animals that dig tunnels can disrupt soil and fragment rocks as they burrow. Decaying organisms can also be acidic and can further lead to the chemical decomposition of rock. Humans also destroy soil and rock formations in the search for minerals or during road construction.
Calcium sulfate, CaSO4, 2 spoonfuls
Hydrochloric acid solution, HCl, 1 M, 15 mL
Balance, 0.1-g precision
Bean seeds, 10
Cups, plastic, small, 3
Forceps or tweezers
Granite chips, 20 g
Halite chips, 15 g
Local rock samples, 2
Marble chips, 25 g
Paper, white, 8½" x 11", unlined, 1 sheet
Pipets, Beral-type, graduated
Pyrite pieces, 2
Sand, 6 spoonfuls
Stopwatch, or watch with secondhand
The hydrochloric acid solution is corrosive to skin and eyes. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the classroom. Please follow all laboratory safety guidelines.
Part 1. Mechanical Weathering
Part 2. Geological Changes
- Obtain approximately 15 g of marble chips. Record the mass of the marble chips in the Mechanical Weathering Data Table.
- Place the marble chips into a sample container.
- Add water to the sample container until the marble chips are completely covered with water. Record all observations in the Mechanical Weathering Data Table.
- Screw on the lid and shake the sample container for approximately three minutes.
- After the three minutes have elapsed, record all observations in the Mechanical Weathering Data Table.
- Unscrew the lid and carefully pour off the water so that none of the marble chips are poured out of the container. Use paper towel to dry the chips.
- Using a balance and a weighing dish, mass the marble chips left in the sample container. Record the mass of the marble chips in the Mechanical Weathering Data Table.
- Using a magnifying glass, observe the condition of the marble chips. Look closely at the edges and the surface of the marble ships.
- Place the marble chips back into the sample container and repeat steps 3–8 three more times for a total of 12 minutes of shaking. Record all observations in the Mechanical Weathering Data Table.
- Repeat steps 1–9 for the halite chips and the granite chips.
- Clean the sample container with water after testing has been completed.
- Answer the questions for Part 1—Mechanical Weathering.
Part 3. Glacial Changes
- Obtain two local rock samples and a white sheet of paper.
- Simulate geologic changes on a miniature scale by rubbing the two rocks together over the white sheet of paper. Record all observations in the Geological Changes Data Table.
- Answer the questions for Part 2—Geological Changes.
Part 4. Ice Expansion—(Teacher Demonstration)
- Obtain an ice cube, sand, a paper towel and a polystyrene tray.
- Sprinkle a small amount of sand onto the tray.
- Using a paper towel, hold onto the ice cube.
- Slowly move the ice cube over the sand while pushing the ice cube down on the tray. The weight being placed on the ice cube simulates the many feet of ice and snow in a glacier.
- After the ice cube has been moved over the surface of the tray, lift it up and record all observations in the Geological Changes Data Table.
- Gently wipe the sand off of the tray. Describe the surface of the polystyrene tray in the Glacial Changes Data Table.
- Answer the questions for Part 3—Glacial Changes.
Part 5. Expansion and Contraction Effects—(Teacher Demonstration) Note:
- Fill one vial to the top with tap water. Let this vial sit for a minute to allow the gas bubbles to escape. Leave the other vial empty.
- Seal the water-filled vial tightly to be sure there is no air left in the container.
- Wrap the water-filled vial with a paper towel.
- Place both the water-filled vial and the empty vial in a freezer.
- The following class period, observe what happened to each of the vials.
- Have students record all observations in the Ice Expansion Data Table.
- Answer the questions for Part 4—Ice Expansion.
Demonstrator should wear safety glasses.
Part 6. Chemical Weathering
- Use the empty vial from Part 4, a heat source, 500-mL beaker filled with cold water and tongs.
- Remove the top from the empty vial.
- Grasp the vial with tongs and heat for one minute.
- After one minute of heating, drop the vial into a beaker of cold water.
- Have students record observations in the Expansion and Contraction Effects Data Table.
- Have students answer questions for Part 5—Expansion and Contraction Effects.
Part 7. Oxidation
- Place three pieces of marble into a small plastic cup.
- Place three pieces of granite into another small plastic cup.
- Using a graduated pipet, add enough 1 M hydrochloric acid solution to cover the samples in both small plastic cups.
- Observe and record the effects of the 1 M hydrochloric acid solution after 20 minutes has elapsed in the Chemical Weathering Data Table.
- Allow the rock samples to sit overnight and record all observations in the Chemical Weathering Data Table.
- Answer the questions for Part 6—Chemical Weathering.
Part 8. Organic Processes
- Place two pieces of pyrite into a small plastic cup. Record all observations in the Oxidation Data Table.
- Using a graduated pipet, add enough 1 M hydrochloric acid solution to cover the pyrite pieces in the cup.
- Allow the cup to sit for 2 to 3 days. After 2 to 3 days, use forceps or tweezers to remove the pyrite pieces from the cup and place them on a piece of paper towel.
- Use a magnifying glass and record all observations in the Oxidation Data Table. Answer the questions for Part 7—Oxidation.
- Obtain an aluminum dish.
- Using a marker, label the bottom of the dish with your group’s initials.
- Place two spoonfuls of calcium sulfate in the dish.
- Add water until the calcium sulfate forms a thin paste.
- Add and submerge 10 bean seeds to the calcium sulfate paste in the dish. This mixture represents a simulated rock.
- Observe the simulated rock for the next two weeks and record all observations in the Data Table.
- After two weeks have elapsed, answer the questions for Part 8—Organic Processes.