How Do Formations Grow?

There are some really amazing things that "grow" in caves. Remind the students of the water working its way down through cracks, picking up minerals as it goes. If the water happens to come out inside a cave, it can leave some of those minerals behind. Depending on their size and shape, we call them soda straws, curtains (or bacon), stalactites or stalagmites (one hangs "tite" to the ceiling, the other "mite" make it to the ceiling. Or, if you live in an area with chiggers, "when the mites go up, the tites come down"). There are many other types of formations (explore the Virtual Cave Tour), some of which are not depositional, but most of the common formations are formed in this manner.

Here's the tricky part, the "why minerals are left behind now when it was just dissolving some of the rock above it away" question. As the drop hangs from the crack in the ceiling, it can lose some of its CO2. This, of course, makes it less acidic, so it can't hold all the calcite it picked up. It then leaves a few calcite cyrstals behind. If enough drops fall from that spot on the ceiling, a stalactite forms. If it runs down the wall, it leaves a ridge of tiny crystals behind. If enough drops run down that ridge, leaving crystals along the way, a curtain can form. If it runs all over down a wall or floor, the whole area can be covered in calcite and is called flowstone. And no, we don't know how all formations grow. Some are just plain weird. It would really help if you pass around some pictures of the formations, if you can't have students use the Virtual Cave Tour. (Library book? Booklet from some tour cave? Mammoth Cave's booklet "The Story Behind the Scenery" works very well for illustrating many aspects of caves, and, at $8.00, is cheaper than most books.) Now do the awesome Crystal Drip lab exercise. You just need epson salt (big cheap box from the pharmacy or grocery store), string, and cups.