Stalactite: Definition & Formation

Instructor: Danielle Reid

Danielle has taught middle school science and has a doctorate degree in Environmental Health

If you walk into a cave there is a great chance you will see a stalactite hanging from the ceiling. Discover what a stalactite is and how nature uses rainwater to form these structures.

What Is a Stalactite?

Imagine you enter a cavern and look up to see a beautiful collection of minerals hanging like frozen teardrops from the ceiling. The beauty of nature amazes you as you stand in awe looking at all of the gorgeous stalactites. Time out, wait just one minute, a stal-act-what? Oh yes, you read that word correctly. Stalactites are structures formed from mineral deposits that hang from caves. A great way to remember the term stalactite is to think of the phrase, 'hang tite'--afterall, these formations hold tightly to the ceiling and hang down.

Stalactites inside a Bermuda cave

There is a very important point regarding stalactites: Be careful not to confuse them with their sisters, stalagmites. As opposed to hanging from the ceiling, stalagmite structures are formed from the cave floor that point upward. A great memory tool to ensure you never confuse the terms is to remember the 'c' in stalactite stands for ceiling, while the 'g' in stalagmite stands for ground. In the image shown below, test your ability to identify a stalactite from a stalagmite.

A cave containing both stalactites and stalagmites

Going back to stalactites, if you happen to travel to different caves, you may encounter different types of stalactites. Stalactites come in a wide variety of types, such as deflected stalactites (that form in a curve) and elephant's foot stalactites (that takes on a flatter form).

In All Its Glory: Formation of Stalactites

Nature's way of forming stalactites is very fascinating. The shape of stalactites can be thought of as an icicle-shape structure, as shown in the figure below. They tend to have pointed tips like the end of a pen or sharpened pencil.

A single stalactite

As we saw in the definition of stalactites, these structures are composed of minerals. Examples of minerals found in the stone of caves containing stalactites range from opal and limonite to carbonate and chalcedony. The most common stone found in caves with stalactite is limestone, which contains the mineral calcite. In fact, this mineral is found not only in limestone, but also in most sedimentary rocks.

Examples of the mineral calcite in the form of a crystal

Can you guess what calcite is made of? Right on! Calcite is made of calcium and carbonate ions. A fancier, chemically correct way of naming calcite is 'calcium carbonate' The molecular formula for calcium carbonate is CaCO3. Let's say you enter a cave to escape a rain cloud. Inside the cave, you hear and see rainwater flowing through the cave. This rainwater plays a very important role in forming stalactite. When the water trickles through the rocks, inside the cave, minerals from the limestone as well as carbon dioxide in the air is picked up by the water. As shown in the equation below, when carbon dioxide mixes with rainwater, carbonic acid is produced. Interaction between calcite and carbonic acid results in the formation of calcium bicarbonate.

Chemical equation used to produce calcium bicarbonate from water, air and limestone

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