If you've ever been inside an underground cave, you've probably seen many different rock formations. Did you wonder what they were called or how they were formed? The general name for cave features is speleothems. Cave features are usually formed by slow-moving water that has a high calcium carbonate content. Chemical changes inside the cave make the minerals harden and form deposits, such as icicle-like stalactites (which hang from the ceiling) and stalagmites (which rise up from the ground). Columns are created when a stalactite and stalagmite join together. Cave pearls are smooth, rounded speleothems that form in shallow hollows where water drips. Curtains are folded sheets of hardened mineral from the ceiling or wall of a cave, often so thin that they are translucent. Some of them look like large stone strips of bacon, while others look like sheets of long icicles.
Dripstone is the term for calcium carbonate deposits such as stalagmites, which were formed when water dripped through a point of aeration. Flowstone is the term for mineral deposits that were formed by water flowing along the floor and sides of caves. Other minerals, such as gypsum, also form deposits. When calcium carbonate crystallizes, it forms the minerals calcite and aragonite (also vaterite, which is less common). Most caves are formed from limestone, a sedimentary rock made primarily of calcite.
For a great hands-on project related to caves, please see our Making Stalactites and Stalagmites Science Project.