Dripping Water Decorates the Cave
We have seen that seawater can create limestone, and that freshwater can later dissolve the limestone to create caves. Well here’s another twist to the story: some of the same water that dissolves limestone can then redeposit it as the mineral calcite. Calcite is a crystalline form of calcium carbonate, which is mostly what limestone rock is made of. Okay, so what’s the difference between a rock and a mineral? A rock has different materials in it, sort of like a pizza that has cheese, mushrooms and pepperoni. A mineral has a definite crystal structure and so is made of only one material, somewhat like the pepperoni on the pizza.

Getting back to Diamond Caverns, we know that rainwater gets carbon dioxide from that atmosphere and also from the soil as it soaks into the ground. And we know that the carbonic acid made by combining carbon dioxide and water can dissolve limestone. Now here’s the fun part: if the water stays in contact with the limestone long enough, then it will reach a point where it can dissolve no more limestone. When this water seeps into the cave, it can lose some of its carbon dioxide to the air in Diamond Caverns. This is similar to what happens when your soft drink goes flat after losing its fizz. Less carbon dioxide in the water means less carbonic acid, and so the water cannot keep as much dissolved limestone as before. So, some of the dissolved limestone crystallizes out as calcite on the ceiling, floor, and walls of Diamond Caverns. This can happen as a purely chemical reaction, but microbiologists who study caves have shown that bacteria are present on cave formations and may be involved in helping the calcite to crystallize. If the calcite deposits on the ceiling at a spot with an active drip, then a stalactite may form. Water dripping from the ceiling may form a stalagmite on the floor, and if water runs down an overhanging wall, then if may form a thin ribbon called a “drapery”. If the drapery is thin enough for light to shine through, and it is banded from changes in water chemistry as it grew, then it is called a “bacon formation” because that’s what it looks like. Diamond Caverns has excellent examples of bacon formations. If water seeps into the cave and evaporates at about the same rate as it seeps in, then “popcorn” can form. Of course it is made of calcite instead of corn.

How long does it take a stalactite or stalagmite to form? Until recently we thought that it takes a century for a cubic inch to form. In a few cases this might accidentally be right, but usually it is wrong. Fact is the growth rate varies a lot, but in human terms it is slow. One stalagmite from Diamond Caverns has been studied by scientists who can actually date the calcite layers. This stalagmite is only about 9 inches tall, and yet the youngest part of it at the top is approximately 170,000 years old, and the bottom is roughly 306,000 years old. Based upon these dates, this stalagmite took 136,000 years to grow, and then it sat in the dark for 170,000 years until cave explorers first saw it!

So, how long did it take for a cubic inch of this stalagmite to form? We know that it is 9 inches high, and let’s assume it has an average diameter of 3 inches. Recalling from geometry class some years ago, we know that the volume of a cylinder is Pi times the radius squared times the height of the cylinder right? Ok, I cheated and looked it up on the internet! If we square the radius (1.5 inches), we get 2.25. Then we multiply by Pi (3.14) and then the height of the stalagmite (9 inches) to arrive at a volume of 95 cubic inches. Because it took 136,000 years to form, we divide this by our volume of 95 cubic inches, and this tells us that a cubic inch of the stalagmite took about 1430 years or 14 centuries to form. As you can see, this is about 14 times slower than what is commonly assumed.

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Historic Diamond Caverns is geographically in the center of the Mammoth Cave Area International Biosphere Reserve.