Understanding Weathering and Erosion in the Environment

Last fall, our school’s parent organization raised funds to beautify the courtyard between the two main buildings on our school campus. In addition to new foliage, a natural stone path was created from large rocks leftover from a construction dig. While walking through the courtyard this spring, my students noticed that the rocks in the path appeared to be cracked in several places, and they became curious as to what might cause this to happen. Sensing a teachable moment, I explained the process of ice wedging (a type of mechanical weathering), which occurs when water seeps into cracks in the rock, freezes, and expands over a period of time. Eventually, the cracks widen and become very visible. The process can also be seen in sidewalks, especially in areas that experience extreme periods of freezing and thawing.

Walk outside at any time of the year, and you can find examples of weathering. It can be seen in pitted rocks shaped by blowing sand, smooth pebbles in a stream, and the wear and tear on marble buildings and monuments.  Mechanical weathering is the breakdown of rock into smaller pieces by physical means, which can include ice, wind, water, gravity, and even plants.

Weathering

A simple but effective way to teach the concept of mechanical weathering involves a hands on lab using the following materials:

• soft rocks (limestone, sandstone siltstone, etc .  . .)
• a plastic juice bottle with a lid
• an electronic balance (these can usually be borrowed from middle or high school science teachers if you don’t have one).

Once you have the materials, you can start the experiment.

• Give each team of three students a rock and ask them to find its mass. One student will be the timer, one the shaker, and one the recorder. 
• Fill the juice bottle with water about a quarter full and place the rock inside. 
• The shaker should shake the bottle vigorously while the timer times for one minute. 
• When the time is up, the recorder should remove the rock and again find its mass. 
• Repeat the process for twenty minutes and graph the results. 
• Students may be surprised if the mass of the rock actually increases during the first minute or two; this is a result of the rock soaking up some of the water inside the bottle. Eventually, small pieces of rock will begin to flake off due to “weathering,” and students will have created a model of what occurs in nature over longer periods of time.

Chemical Weathering

Chemical weathering is the chemical breakdown of rocks and minerals into new substances by water, acids, air, and soil. This can be readily seen in areas with lots of acid rain, as local buildings and monuments will likely show evidence of damage. Since natural rainwater is also slightly acidic, older gravestones made from limestone also display excellent examples of the damage that can be done by chemical weathering.

Students can easily create a model of chemical weathering by finding an old penny and rubbing it with ketchup or vinegar for several minutes. Both ketchup and vinegar are acidic, and they eat away the corrosion on the coin.  The penny should be relatively shiny again after rinsing!

For more weathering activities, try one of the following lesson plans.

Weathering and Erosion Lessons and Activities:

Weathering Rocks

In this lesson students discuss the kind of natural events that cause rocks to weather. They conduct an experiment using plaster of paris to analyze which rocks are hard, and which rocks break easily. They also observe a demostration of how erosion works on sand and water.

Weathering and Erosion

Students discuss weathering and erosion, and identify the similarities and differences between the two phenomena. They conduct an experiment to analyze what happens when different materials are explosed to weathering and erosion. They record their results.

You've Been Weathered!

This lesson requires students to identify the different types of weathering, chemical and mechanical. At the end of the lesson, students demonstrate an example of weathering for the class.