It’s hard to believe, but the tiny grains of sand you see at the beach were once part of giant boulders. Over many years, these huge rocks were broken down into smaller pieces in a process called weathering. There are two main types of weathering: chemical weathering and mechanical weathering. In chemical weathering, the rock reacts with substances in the environment like oxygen, carbon dioxide, and water to produce new substances. For example, iron in rock can react with oxygen and water to form rust, making the rock reddish and crumbly.
During mechanical weathering, no new substances are produced. The rock gets smaller, but it stays the same kind of rock. For example, water sometimes gets into the tiny cracks in boulders. If that water freezes, it expands, opening the crack even more and eventually splitting the rock into pieces. In a stream, the moving water often knocks rocks against other rocks and sand. Over time, the rocks become smoother and smaller. This process is called abrasion. If you’ve ever rubbed a piece of wood with sand paper, you’ve seen abrasion in action.
Examples of mechanical weathering are easy to find in your own neighborhood. Look for cracks in the sidewalk caused by heating and cooling. Underlying tree roots can make whole slabs of sidewalk buckle. Old gravestones are often hard to read because weathering has worn away the letters.
Another concept related to weathering is erosion. Erosion is the movement of weathered materials to new places. Water, wind, gravity, and glaciers (rivers of moving ice) all cause erosion. Have you visited or seen picture of the Grand Canyon is Arizona? Over millions of years, water, wind, heat and cold weathered the rock, which was ultimately carried away by the Colorado River.
Finding evidence of weathering is fun, but it’s even more fun to cause weathering yourself!
How can we model mechanical weathering?
- Disposable plastic cup
- Disposable plastic spoon
- 6 tablespoons of plaster of Paris (available at hardware stores)
- Goggles or other protective eye ware
- 3 tablespoons cool water
- Four bean seeds
- Paper towel
- Spray bottle
- Presoak the bean seeds in water overnight.
- Cover your work area with newspaper, or work outside.
- Use the tablespoon to measure 6 tablespoons of plaster of Paris into the plastic cup.
- Using the disposable spoon to stir and your measuring spoon to measure, add one tablespoon of water at a time to the plaster of Paris, stirring after each addition, until you have added 3 tablespoons all together. Why don’t you add the water all at once?
- If your mixture still seems dry after you added all the water, add a teaspoon of water at a time until the mixture is a thick, smooth liquid.
- Tap the cup on the table to settle the contents, and tidy the sides.
- Carefully watch the plaster until it begins to solidify.
- Once the plaster of Paris begins to solidify, insert your four beans so that 2/3 of each bean is embedded in the plaster.
- Let the plaster solidify completely.
- Fold a paper towel in half twice.
- Moisten the paper towel until it is damp, but not dripping wet.
- Place the paper towel on top of the bean seeds to water them.
- Place the cup in a sunny window.
- Make sure to keep the paper towel moist.
- Observe both the growth of the bean plants and plaster.
As the beans seeds sprout, they crack the plaster, just as tree roots can eventually crack a sidewalk.
When making the plaster, you did not add all the water at once because you can make a more uniform mixture if you add it slowly. If you added the beans to plaster when it was still soupy, they may have gotten pushed or sunk to the bottom.
In your investigation, you got to see mechanical weathering in action. The growing plant root and stem pushed open bigger and bigger cracks. Water was able to move in, enlarging the cracks even further. The warming and cooling of the plaster caused it to expand and contract, further weakening the overall plaster “rock”. There was likely some chemical weathering in action, too. The growing bean seeds released substances that chemically broke the plaster down.
Once you are done with your experiment, you might move your sprouting bean plants to some soil. To do this, just crack the plaster a bit more. As you dig a hole to plant your beans in the soft earth, thank the process of weathering for having started the process of turning giant boulders in fertile soil, millions of years ago!