Age: grades 1-4
Subject: rack weathering
Skills: observation, classification, inference
Duration: 3-10 days, 30 minutes/day
Group size: 5-10
Setting: outdoors or indoors
Key vocabulary: abrasion, angular, rounded, sediment, weathering
Materials:
A common "soft" red brick
1 quart jar with a tight-fitting lid, either plastic or strong-walled glass (check with the school kitchen or a local cafe)
Masonry hammer
Towel
Paper bag (for use with glass jar)
I gallon glass jar
Objectives:
Students will be able to:
1. Describe the effects of abrasion (weathering) on the shape and size of brick Fragments.
2. Classify the brick fragments according to the amount of shaking.
3. Infer where this process happens to real rocks.
Method:
Students will observe the effect of repeated shaking on the shape and size of brick fragments in a jar partially filled with water.
Overview
The physical weathering of rocks by the action of moving water is an important process. It shapes the landscape, forming features as impressive as the Grand Canyon and, more importantly, helps form some of the richest soils available for agriculture. This physical weathering is accomplished by the abrasion of rock against rock and sand against rack, powered by the force of moving water.
Too often students read and talk about how water wears away rocks without having any experience with what really happens. This activity gives them a chance to actively manipulate and observe this process in the classroom.
Procedure
1. Wrap the brick in the towel and, using the masonry hammer, break into pieces about 1-1712 inches in diameter.
2. Put 6-10 angular fragments in the I quart jar and add enough water to fill halfway. Fasten the lid tightly.
3. Ask 10 students to give the jar 100 shakes each. If using a glass jar, place it in a paper bag in case it cracks.
4. After 1000 shakes, let the students examine the fragments and observe the color of the water. Pour the water into the gallon jar and let the sediment settle.
5. Using the same fragments, have the students repeat the process, giving the fragments 1000 shakes each day until they are well rounded (5-10 days.) Have them record the number of shakes, note the changes in the fragments, and save the sediment.
Extension
1. A record of the pebble-rounding may be made with an overhead projector. The angular fragments are heavily crayoned on one side, placed an the projector with this side down, and projected an paper taped to the wall. Their silhouettes are traced exactly and then cut out. A new set of cutouts is made after each 1000 shakes, always with the Fragments in the same position and the projector at the same distance From the paper. The cutouts are then mounted to show the change in the shape of the "pebbles".
2. Add some sand to the fragments and water to see if this speeds up the process. Use one jar with just Fragments and water as a control and to the second jar add a cup of coarse sand. Follow the same procedure and keep a record of the results.
3. If there is a river nearby, bring in examples of water-smoothed cobbles. Break some into pieces with the masonry hammer to help illustrate how they originally appeared, before changing due to the action of abrasion in moving water.
Evaluation
1. Have the students describe or draw the changes in the shape and size of the brick pebbles.
2. Ask the students where this same process occurs with real rocks.
Source
1. Rockcastle, N., 6 Schmidt, V. (1968) Teaching Science With Everyday Things. New York: McGraw-Hill Book Co.