Our Weathering/Erosion Experiment

(From our mini-unit on weathering and erosion.)

We conducted this experiment in our backyard:

Michael mounded dirt in an area of the garden. He insisted on putting a chunk of dirt on the top of his "mountain". I felt it would fall off... hmmmm, another experiment!

Once the mountain was made, we now had to exercise patience to watch it erode. We also had to keep the neighbor kids off of it!

Each week we would go out and measure that mountain. It started as 42" wide, 19 1/2" tall, including the chunk. We had a chart we filled out. On this, we would log the date, height, width, observations, and make a little drawing of the mountain on our chart in a 1" x 2" square.

After just two weeks, the dirt mound had spread out more than 2 inches and had worn down about 3 inches.

After a few weeks (and several rainstorms), the mountain showed clear signs of weathering:

The chunk remained on top! The entire pile of dirt had weathered down. (The photo shows one of Michael's green beans, which he grew, protruding from the top. In his mind, it climbed the mountain and was victorious!)

We would have continued with the measurements for the twelve weeks the experiment suggested, but something or someone got into the dirt mound during the night and we could no longer take accurate measurements. But we did get to see the effects of the weather on that dirt mound!

By the way, Michael thought it was important to wear the same clothes for each photo. :-)

More on weathering and erosion. We conducted many of these experiments, too!

Weathering - When rocks are worn down by water, wind, or other means, that is weathering. Soil is made up of weathered rock particles, so without weathering, there would be no soil. Try the following activities to see how rocks can weather.

How Do Chemicals Weather Rocks?
Place an equal number of limestone or marble chips in each of two jars. Cover the stones in one jar with water and those in the other with vinegar; then screw lids on the jars. Allow the jars to stand overnight. You may notice some bubbles forming in the jar with the vinegar. Ask the students to speculate about what this might mean. The next day, pour out the liquid from each jar into separate bowls. Label the bowls "water" and "vinegar". Allow the water in the bowls to evaporate. Compare the amount of solid material remaining in the two bowls. The bowl that contained vinegar will have a larger amount of solid material. (Limestone, marble, and other carbonate rocks react with acid to form carbon dioxide and soluble salts. Rainwater is often acid and can erode limestone easily. Acid groundwater dissolves limestone underground, forming caves and sinkholes. Study or visit any caves or sinkholes in your area. Examine other types of rock to see if they react to vinegar in the same way as carbonate rocks.

How Do Streams Weather Rocks?
Place some small, freshly broken pieces of rock or brick in a large plastic jar. Softer rocks such as sandstone, shale, or limestone work best. Fill the jar about halfway with clear water. Put aside some other pieces of the broken rock. Close the lid of the jar and shake it 1,000 times. Remove the rocks and note any changes in their appearance compared with the rocks that were not shaken. Describe these changes. Filter the water through a piece of filter paper or funnel lined with paper towels. What do you see in the filter paper? You may want to experiment with different types of rock to see what types will change more. What might happen to rocks in a stream? (Rocks in streams are weathered by water and movement.)

How Does Ice Weather Rocks? (Teacher Demonstration)
Completely fill a glass jar with water and cap it tightly. Place the jar in a resealable plastic bag. Put the jar and the bag in a freezer overnight. When you remove the jar, it will be broken. Handle and dispose of the broken glass carefully. Ask students to predict what this demonstration might tell us about rocks and weather. (When water freezes, it expands. When it freezes in cracks in rocks, the expansion can cause the rock to break.)

Erosion - When particles weathered from rocks are carried away, it is called erosion. Erosion is responsible for wearing down exposed places and depositing sediment in level places.

What Happens To A Mountain?
Build a "mountain" of soil 50 cm high in an undisturbed location in your schoolyard. Observe the mountain once a week for three months. Have students measure its height and width and note any changes in its surface. After observing the mountain for an extended period of time, ask students to suggest what forces might have caused changes in your mountain. How long do they think the mountain would remain if you left it there? (Our experiment above.)

How Does Vegetation Affect Erosion?
Punch holes in the bottom of a plastic cup to make a sprinkler. Place three to five centimeters of soil in each of two planting flats. Sprinkle grass seed on the soil in one of them and cover lightly with soil. Water both flats every day with the sprinkler until the grass is five centimeters tall. Prop the ends of both flats up at a moderate angle with bricks or blocks. Sprinkle each flat with water equally until you observe soil erosion. In which tray did the soil erode more? What might be the effect of removing plants like trees from a steep mountainside? How could soil erosion be controlled in steep places? What other factors can students think of that would affect the amount of erosion? Have students identify any areas in your community that are steep. Take a field trip to look for signs of erosion.

Wave Action - How Beaches Are Formed
Slope a generous amount of sand against one end of a dishpan. Add water until the sand is about half covered. Use the side of a ruler to generate steady, even waves in the tray. Have students observe the action of the waves on the sand. They should see that the water removes sand from the upper part of the "beach" and deposits it below the waterline. This is how sandbars and barrier islands are formed. If you can make very small, rapid waves, you may see that the sand is redeposited on the beach.

How Does Water Carry Particles Of Rock And Soil?
Mix some gravel, sand, mud, silt, and clay in a large jar. Add water to the jar. Cover the jar with the lid; then shake the jar vigorously. Have students observe the jar over a period of time and note how long it takes for the different materials to settle on the bottom. Do they see different layers on the bottom? What is different about them? Have students think about different kinds of streams and rivers. Where would they expect to find a rocky or gravel bottom? Where would there be a muddy or silty bottom? Which type of material might be carried for the longest distance? (The smaller particles remain in suspension in the water for the longest time. They have a larger ratio of surface area to mass and therefore experience more resistance from the water as they sink.)

Model of the Grand Canyon
Cut out the middle section of one of the short sides of a cardboard box. (The lid of a paper box works well.) Cover the bottom of the box with a layer of mud or wet soil. Allow the mud to dry. Cover this layer with another layer of a different color. Repeat with many layers until the box is full, allowing each layer time to dry. Then tilt the box under an outdoor faucet with the open end downward, and run water in a steady stream onto the top end. Have students observe how a canyon is formed. Ask students to describe the effect the different layers have on the erosion of the canyon.

Book List:
Geography From A to Z: A Picture Glossary by Jack Knowlton
The Beginning Of The Earth by Franklyn M. Branley
The Magic School Bus Inside The Earth by Joanna Cole
Surtsey: The Newest Place On Earth by Kathryn Lasky
The Rock by Peter Parnall

(All information for this mini-unit came from a Mailbox Magazine - issue date is unknown.)

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