"How Caves Are Formed" Lessons

A. Acidic water make caves: Discuss with the class the fact that the types of caves you're studying are formed when acidic water slowly dissolves the rock around small cracks away, until the once-tiny crack is big enough to crawl through and be called a cave. Point out that some areas of the world have no caves, while in other places it's hard to walk without falling down a hole. Ask why that might be. When you arrive at the possibility that the different types of rocks that make up our earth might make a difference, do the Acid Rock Lab. Sneak up on them with it . . . say that you just happen to have some rock samples from your travels, and that you want to test them with a weak acid (vinegar) to see if they dissolve. Mention that water in the ground can be acidic, which is why you picked vinegar, but don't go into it yet. If you haven't studied acids yet, ask what they know about acids, do they dissolve everything, etc. Then do the Acid Rock Lab.

B. Acid Rock Lab: this activity beautifully demonstrates why some areas of the world have more caves than do others, and how weak acids can dissolve away enough rock to form a cave. It's easy and fun, and is crucial to most of the rest of the unit. All you need is vinegar and small samples of rocks. One of those samples needs to be limestone or marble (if you don't have these locally, call up your nearest monument company (you know, headstones) from the yellow pages and ask if they have any marble chips or chunks you can have. Or use chalk, which is also made of calcite.

C. Where does the acid come from? Ah. Now they know that some rocks dissolve in weak acids, while other rocks don't. And they want to know where the acidic water comes from. Many kids will think its acid rain, but remind them that caves can be thousands of years old, and that we only recently caused acid rain with our air pollutants. Start by asking what gas we exhale (carbon dioxide, CO2), and then tell them that all animals, including the tiny ones in the ground, also exhale (or give off) carbon dioxide. Carbon dioxide, when it mixes with water, forms a weak acid called carbonic acid (H2CO3). So as rainwater sits around or percolates through the soil, it picks up CO2 and gets acidic. As that weak acid drains through the cracks in the rock, it dissolves it (the rock goes into solution) and carries it away. After enough time, the tiny cracks can grow big enough to crawl, walk, drive a bus, or fly a plane through. They can be really long. Mammoth Cave in Kentucky is over 400 miles long!