Weathering

Summary

Key Concepts

Chemical weathering, physical weathering, chemical reaction

Credits

Sabine De Brabandere, PhD, Science Buddies

Introduction

Weathering, or the wearing away of rock by exposure to the elements, not only creates perfectly smooth boulders, sinkholes to swim in, and caves to explore; it also eats away statues and buildings. Try it out on a sugar cube and feel how destructive weathering can be.

Background

Rock is always being formed and broken down. We usually do not notice it because the process happens slowly over millions of years.

Rock forms by cooling magma, the hot fluid that flows out of volcanos. Obsidian and granite are examples of rock formed by cooling magma. Layers of debris compressing together is another way rock forms. Limestone and chalk are formed this way. They are made from compacted layers of shells and marine animal skeletons.  Rock can also transform. This mainly happens when the rock is buried and thus exposed to extreme temperatures and pressures.

Rock is also constantly broken down in nature. This is called weathering. Scientists categorize the processes that break down rock in two groups: physical and chemical weathering. Physical (also called mechanical) weathering happens when forces (pushes and pulls) repeatedly act on the rock. An example is wind repetitively gliding against a rock taking rock particles with it. Over many years this makes the rock look and feel smooth. Moving water can have the same effect. Waves that bang against a rock or smash rocks together are other examples of physical weathering, so is freezing water that pushes a crack open as it expands.

In chemical weathering, the rock disintegrates or even dissolves because a chemical reaction changed the composition of the rock. Rusting is an example. If iron in rock comes in contact with oxygen in the atmosphere, a chemical reaction called rusting occurs. The result is rusted iron, a substance that has a weaker structure and thus breaks down more easily.  You can recognize it by its typical orange-red-brown color.

Some rock even dissolves as the result of chemical weathering. When certain types of rock like chalk or limestone come in contact with rainwater – which is usually slightly acidic – a chemical reaction occurs, slowly transforming the rock into substances that dissolve in water. As the products dissolve, they are washed away with the groundwater. It is like the rock vanished! This process even happens underground where it creates sinkholes (also called cenotes) or caves.

The reason rain and stormwater is little acidic is because it collects impurities like carbon dioxide(CO₂) from the atmosphere as it falls.  Pollution increases the amount of CO₂ and other impurities in the atmosphere also increase, making rainwater more acidic than it naturally would be.  The more acidic the water is, the faster these types of rock dissolve. It isn’t just rock out in nature that are impacted though, statues and buildings made with rock that react with acidic water suffer too.

In this activity, you will model physical and chemical weathering on a sugar cube.  Will your sugar sculpture survive rain?

Materials

• A few sugar cubes
• Water
• Dark colored paper or countertop
• Glass
• Dropper
• Work area that can get wet
• Clay (optional)
• Spray bottle (optional)
• Nail file (optional)
• Baking plate (optional)

Observations and Results

Was breaking a sugar cube by smashing, crushing or grinding it easy?  Rock breaks down in a similar way but a lot slower.  Forces in nature like wind, flowing water, expansion and contraction due to temperature changes, or the push of plant roots making their way in cracks push and pull on rocks. These forces wear the rock down. Eventually, it breaks or pulverizes. The result is smaller pieces of rock, just like you were left with smaller pieces of sugar. Scientists call this type of weathering physical weathering.

Did the sugar cube become weak and eventually dissolve in the drops of water? That happens to some types of rock too. Some minerals in rock react with rain or the gasses in the atmosphere, changing the nature of the rock. The new substance is often weaker, and sometimes even dissolves in water. Scientists call this type of weathering chemical weathering.

Your sugar cube dissolved in water too, and, after you applied enough water, you probably did not have any sugar cube left, it was all carried away with the water. In a similar way, rocks can dissolve in rain or stormwater and be carried away with the groundwater. Caves and sinkholes form this way.

If you tried to build a sugar statue and exposed it to water, you probably saw it slowly disappear. Some statues and buildings undergo the same fate.

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Additional Resources

Holey porous Rick Science, from Science Buddies

How Dirt Cleans Water, From Scientific American

Acid Rain, from Kids Ecology Corps

Science Activities for All Ages!, from Science Buddies