Cave formations

Caves are full of mineral deposits that form unique shapes, such as the icicle-like stalactites (which hang from the ceiling) and stalagmites (which rise up from the ground). Have you every wondered how they are formed? Learn about cave formations and then make your own!

Cave Features Science Lesson

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.

Cave Features Science Project

Making Stalactites and Stalagmites

(Adult supervision recommended.)

What You Need:

What You Do:

  1. Cut two 24′ pieces of cotton string or wool yarn, and tie one metal washer to each end to act as a weight.
  2. Fill four small glass jars or beakers 2/3 full with very warm (120-140 °F) water. Stir in as much baking soda into two of the jars as will dissolve; do the same with Epsom salt in the other two jars. Put the jars in a warm place where they can sit undisturbed for about a week.
  3. Next, place a small plate between each set of jars and put the ends of one string in the baking soda jars with the middle of the string draping in a ‘u’ shape just above the plate. Do the same with the Epsom salt.

After a few days, check to see if any material has built up on the string. An icicle-like formation should form downward on the string and upward on the plate. Real stalagmites and stalactites are formed in almost the same way, by mineral-containing waters dripping in caves until a deposit of calcium carbonate builds up…or down, as the case may be.