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Activity 11: Water Erosion

WATER, MOTHER NATURE'S CHISEL

Summary:

Many landforms on Earth are the result of erosion and weathering. This activity consists of a series of smaller activities used to better illustrate the physical and chemical forces that create landforms.

Instructional Method:

Experiment

Goal:

To teach students about different types of water erosion.

Objectives:

Students will be able to:

  • Explain the difference between weathering and erosion
  • Give two examples of water erosion
  • Develop strategies to slow down erosion where man is involved

Time:

Set up: 45 min.
Activity: 1 hr.
Discussion: 20 min.

Materials Needed:

  • Styrofoam peanuts
  • Two paper bags
  • Small bucket of gravel
  • Sand
  • Water
  • Grass clippings or shredded paper
  • Toothpicks
  • Water sprinkler
  • Jar
  • Sugar cubes

Vocabulary:

chemical change
dissolve
erosion
physical change
transport
weathering

Background:

Rocks break down from large pieces to smaller pieces by physical and chemical processes. When these processes change the size of rock pieces from large to small in place it is called weathering. When rock pieces are broken down and transported, it is called erosion.

Physical weathering and erosion occurs when a large piece of rock is broken into smaller pieces without changing the rock composition. An example of physical change is when you hit a rock with a hammer and it breaks into pieces. The pieces are still the same rock type, but the size has changed.

Chemical weathering and erosion occurs when rock is changed into smaller pieces by chemical reactions. Resulting rock is not the same as the original. For example, if acid is poured on chalk (CaCO3) it will fizz. Gases (CO2) are given off and the remaining rock piece is calcium (CaO). The chalk composition changes due to the chemical reaction. The rock had changed to a different chemical composition. The chalk has changed size and composition.

Processes that affect landforms are gravity, glaciers, wind, and water. This activity will discuss water processes only. Wind and ice will be discussed in a separate activity.

As water seeps into cracks, it enlarges them by washing pieces away and/or chemically dissolving the rock. A determining factor on how quickly rock will erode by water is the rock type. Harder rock, such as quartzite, takes longer to dissolve than softer rock, like mudstone. Quartz pieces are more likely to be transported than dissolved. Mudstone pieces can be transported but usually dissolve much faster during transportation. Water erodes rock easily when impacting the carried rock with stream bed rock. The broken rock pieces continue to flow and impact down stream. This is why river rock is rounded and smooth.

Rain splash is another form of erosion that results from water. When rain hits the ground, it acts like tiny bombs dropped from space. Each impact pushes small sand sized or dust sized particles down slope. When it rains, this happens billions of times. Even though the particles move millimeters an impact, over one rainstorm a single particle may have moved a few centimeters.

Instructional Procedures:

Stream action erosion

  1. Divide the Styrofoam cubes evenly. Have the students examine the Styrofoam cubes. Draw what they look like and describe them on a piece of paper.
  2. Place half the cubes in a paper bag. Place the other half in another other bag, and add the gravel sized rocks into the second bag. Ask the students to predict what will happen if we shake the bags; what will the foam pieces look like?
  3. Shake both bags for a few minutes. Pour out and place them on the table for inspection. Have the students examine the difference between the two bags. Notice the crushed or more rounded cubes in the bag with the rocks.

Rain splash and water erosion

  1. Make three small hills of sand, one on each tray or on the sidewalk. Compact it and place toothpicks in the hillside. Note the placement of the toothpicks.
  2. Pour grass clippings on one hill making sure the toothpicks are seen. Leave the other two hills without grass. Ask the students to look at the first two piles and predict what would happen if it rained on each of the piles right now. Have them write down their predictions.
  3. Using a watering can, sprinkle water on the first hill with the clippings. Have the students observe how much sand is washed away and if the placement of the toothpicks has changed. Have the students draw what they see with a short explanation of their observations. Next, sprinkle lightly one of the barren hills. Note the spots where the water has impacted. Did this hill erode slower or more quickly than the one that was protected by the grass? Again, have the students draw and describe their observations. This was a small rain storm, what if there was a huge rain storm that came through and dropped a lot of water in a short time. What would happen to the last hill of sand? On the last barren hill rapidly sprinkle water on it. Watch how much erosion occurs and if the toothpicks move. Have the students observe and draw or describe what they observed.

Dissolving of solids

  1. Line the bottom of the jar with sugar cubes.
  2. Slowly drop water on the cubes until a hole forms in the cubes. Let the jar sit until the water evaporates. (If the cubes are two layers thick you can see how caves form).
  3. Explain to students that this is what happens to rocks over long periods of time. Repeat this experiment with rock salt.
  4. Explain to students that salt is actually a type of rock. When water reacts with it, it will dissolve.
  5. Repeat again with pieces of chalk that are made of calcium carbonate (limestone) and drip water and then vinegar or lemon juice on them

Discussion:

Stream action
What would happen if the bag without the rocks were continually shaken for an hour. Would the cubes be more rounded or would nothing change them? Was this weathering or was this erosion? What is the difference? Did we move the pieces? (No, so technically this is weathering.) This experiment was supposed to represent what happens in a river. If we were in a real river would this be weathering or erosion? Why? (Erosion, because the current would have moved the pieces.) Why are river rocks so round and smooth? On a piece of paper, explain how stream action erosion occurs, including how rocks are broken down and rounded, and to where they are transported.

Rain splash
Why did the last hill erode the fastest? Why did the first hill erode the slowest? Where do you see this same process happening in nature? How are people involved in this kind of erosion? (Clearing land, or having property below and area that has been cleared.) Why could this be a problem? What can we do to make our environment respond more like the first hill than the last one? (Reseed quickly after clearing land, or keep a cover crop of residue on fields).

Dissolving of solids
Why did the water create a hole in the sugar? Why did it create a hole in the salt? Is this an example of physical or chemical weathering (chemical)? Did the water create a hole in the chalk too (over time it will)? What happened when we dripped the vinegar on the chalk (it dissolved faster)? Explain that many different types of rock are made of the same stuff as chalk, and water can dissolve chalk over time. But lately, especially around urban areas, water has been becoming more acidic and we call it acid rain. Would acid rain weather the chalk faster or slower than normal rain? Why could this be a problem? (Buildings and statues from ancient times are being damaged by the rain at a much faster rate than before. Landforms can also be effected.) This question may be a good research question for older students: investigate if acid rain is a problem in your town. If it is, find out why it is a problem and where the pollution is coming from, as well as what is happening in the community regarding this issue.

Extension:

Have students research National Park Service Sites to find examples of each of these processes. Have them present their findings to the class and defend their position as to why each NPS site presented is a good example.

Included National Parks and other sites:

Bryce Canyon National Park
Canyonlands National Park
Capitol Reef National Park
Grand Canyon National Park
Zion National Park

Photos:

Bryce Amphitheater
Rock slide at Bryce Canyon

Utah Science Core:

2nd Grade Standard 6 Objective 1, 2, 3
5th Grade Standard 1 Objective 1, 2, 3
5th Grade Standard 2 Objective 1, 2, 3

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Did You Know?

Visibility from Yovimpa Point, looking south towards Arizona

On a clear day, the visibility at Bryce Canyon National Park often exceeds 100 miles! This is due to our exceptional air quality, low humidity and high elevation. More...