Written by Jenny
Cave of the Mounds has many colorful speleothems. Massive stalagmites, colorful stalactites, hidden helictites, and glossy flowstone astonish visitors on a daily basis. Guests wonder how the cave formed, how the formations develop, and what creates the different shapes.
During a tour, guest comments often shift from generalized to specific questions. At the start of the tour questions revolve around the geology of caves and how they form. But as we move through the passageways and begin to see more and more formations the questions shift to wondering about how or why different shapes occur. What determines if the minerals form a stalactite, a stalagmite, or flowstone? Why do some areas of the cave have more of one kind of formation than another?
At Cave of the Mounds all the formations are made of calcite, which consists mostly of calcium carbonate (CaCO3). They take shape when dissolved minerals are deposited within the cave. First, rainwater and melting snow combine with carbon dioxide in the air and the carbon within the soil to form carbonic acid (H2CO3). This acid dissolves the minerals in the limestone above the cave. As the mineral rich water enters the cave, the change in pressure releases the carbon dioxide much like opening a can of soda causes pressure changes within the can. Once the carbon dioxide is released, calcite is deposited.
Pure calcite is white or colorless, but impurities in the calcite result in a wide variety of colors. At Cave of the Mounds, we see rich browns and oranges from the iron oxide present, and deep grays and black from manganese oxide. These additional compounds influence the colors of the formations, but the shapes depend on different factors. The amount of water, drip rate, and how the water moves determine the shapes that form.
Dripstone forms where water drips from the ceiling. Steadily dripping water brings mineral rich droplets to the same location over and over. With each drop a tiny amount of calcium carbonate precipitates out of the solution. This precipitate, or solid matter, creates a buildup of crystals. Over time these tiny crystal deposits grow into pointed stalactites that resemble icicles, and tall stalagmites that reach up from the ground.
A stalactite is likely to form when the water enters the cave at a slower rate. A slow drip rate allows the calcite to crystallize before the water drops to the ground. This happens because the carbon dioxide (CO2) has time to release from the carbonic acid. Then the calcium carbonate crystals are deposited before the water drops. Over hundreds of years, the process repeats again and again until a thin, hollow soda straw stalactite starts to appear.
Stalagmites form when water is moving more quickly and steadily. Fast, steady drops do not have sufficient time for the chemical reactions to occur before falling from the ceiling. Therefore, the minerals precipitate out after falling to the ground. Over time the solid calcite crystals build up on the same location, giving rise to tall pillars or posts.
Flowstone formations are created when water continuously flows over the same location. Instead of crystals forming on a focused point, the minerals precipitate out over a larger surface. These formations have more variance in shape. Ribbon stalactites, also known as cave bacon, are formed by this process. Water enters the cave and runs down the sloping ceiling instead of falling immediately. Over time calcite builds up on the path where the water flows. This forms a ribbon shape, rather than a pointed shape.
The flowstone in the North Cave and the Painted Waterfall form when water enters the cave and flows over the surface of the floor below. Without a focal point to build on, the calcite crystals cover the underlying limestone rubble continuously. It looks like a smooth, slippery, soft layer but flowstone is gritty like fine sandpaper and as hard as solid rock. The shape of flowstone is as varied as the surfaces it covers.
Dripstone and flowstone can form in the same space. The Beauty Rooms are the perfect location to see multiple examples of each. Rimstone dam flowstone on the cave floor, ribbon stalactites so thick they hang like drapery, and tiny soda straws forming delicate colonies in the recessed walls create a masterpiece of natural beauty. There are also thick stalactites and stalagmites that have grown together to create solid columns.
Cave of the Mounds has earned the nickname of the Jewel Box of America’s Show Caves for the variety and delicacy of formations. Observant guests notice the similarities and differences between areas of the cave. They spot the many shapes and colors throughout. What will you notice on your trip underground?