Cave Formations

When most people think of caves, they imagine rooms filled with beautiful stalactites and stalagmites. Those are some of the more common formations, but there are many other types. Some of them are very strangely shaped. For instance, that round formation in the photo is called a shield, and has curtains hanging from the bottom of it. You can also see some thinner curtains (or bacon) hanging to the right of the shield. Of course there are many miles of cave that have no formations at all, maybe just rocks and mud.

What causes these strange things to grow, and why in such odd shapes? They're not alive, of course, as they're just made of minerals, but many of them do get larger over time. Let's take a look at this process.

Remember that caves are formed by cracks being dissolved away by acidic water. Some caves then drain of that water, leaving air-filled passage. Most types of formations cannot really begin to grow until that time.

The cracks in the walls and ceiling of those passages may drip with water that began as rain on the surface above. Like the water that formed the cave itself, this dripping water is mildly acidic. (Do you remember why? CO2 + H2O ---> carbonic acid (H2CO3)!) As it moves through the cracks in the rock on the way to the cave, the acidic water is still dissolving rock (future cave!) and carrying it along with it. When this mineral-rich water emerges from the crack onto the cave ceiling, it may leave a deposit of minerals there. This is similar to the crusty deposits you've seen on older faucets. Over time, these cave deposits can build up to create formations as large as a big tree. (See photo at right.)

Now, exactly why these deposits are left behind is sort of tricky. Follow closely. As the drop of water hangs from the ceiling, it loses some of its carbon dioxide. This makes it less acidic. That means that it can't hold all of the dissolved minerals that it had picked up, on its way through the cracks above. The drop leaves a few crystals of the mineral calcite behind, and a stalactite begins to grow. Drop after drop, over hundreds and thousands of years, can end up leaving some big stuff! Our new cave is filling back in with rock from above, and tiny cracks up there are growing into more new cave!

Here's the chemical formula that represents that dissolved mineral (calcite) precipitating out of the water :

Ca(HCO3)2 -----> CaCO3 + H2O + CO2

calcium bicarbonate ----------> calcite + water + carbon dioxide

If that water runs down a wall instead of dropping straight off the ceiling, it can leave a whole row of those crystals behind. If each successive drop runs down the same spot, leaving more crystals, the ridge hangs down longer and longer and becomes a curtain (or bacon or drapery)(see bacon photo at left). Where the drop falls to the floor and leaves crystals behind, a pile of crystals can grow upwards to form a stalagmite (see photo above right). (Stalactites hang tight to the ceiling, while stalagmites might make it to the ceiling.) If they meet, then the formation is called a column (see photo below right).

Can cave passages completely fill in over time? You betcha'! Most cavers have been stopped from continuing down a passage by formation growth.

How fast do these formation thingies grow? In wet, active spots, some formations have been known to grow about 1 inch in 100 years. That's screaming fast, huh. And formations can grow during wet years, then stop during more dry years. And heck, they can grow for hundreds of years, and then have their water supply cut off for some reason, and sit, unchanged, for thousands of years. For those reasons,cavers are super careful to not break anything. Unfortunately, as the bumper sticker says, guano happens, and most of us have broken a thing or two. I know of 2 stalactites that I broke many years ago, and it still bothers me when I think of them :-(

Water can do some very amazing and spectacular things over time. Below are some names of other types of formations. Click on the ones on the left to see a photo. Click on Dave Bunnell's Virtual Cave Tour to see photos of the ones in the right list, along with many other beautifully photographed formations.

Showerhead . . . . . . . . . . . . Cave Coral

Flowstone . . . . . . . . . . . . . .Helectites

Soda Straws . . . . . . . . . . . . Cave Pearls

Shield . . . . . . . . . . . . . . . . . Rimstone