Stalagmites and Stalactites – How They Form and More

Stalagmites and Stalactites

Stalagmites and stalactites are fascinating natural formations in caves. These formations, along with others, are collectively called speleothems. Understanding their formation and characteristics provides insight into geological processes and the history of the Earth’s environment.

What Are Stalagmites and Stalactites?

Speleothems are mineral deposits that form in caves due to the deposition of minerals from water. The term “speleothem” comes from the Greek words “spelaion” (cave) and “thema” (deposit). These formations include stalagmites, stalactites, and other cave decorations.

Stalagmites are upward-growing formations found on cave floors. The term “stalagmite” derives from the Greek word “stalagma,” meaning “drop” or “drip,” reflecting their formation process from dripping water.

Stalactites are downward-growing formations hanging from cave ceilings. The word “stalactite” also originates from the Greek word “stalaktos,” meaning “that which drips.”

Differences Between Stalagmites and Stalactites

The primary difference between stalagmites and stalactites is their orientation:

  • Stalagmites grow upward from the cave floor.
  • Stalactites hang downward from the cave ceiling.

How to Remember Which Is Which

A simple way to remember the difference is:

  • Stalactites hold “tight” to the ceiling. Also, the word “stalactite” contains the letter “c” as in “ceiling”.
  • Stalagmites might reach the ceiling if they grow tall enough. The word “stalagmite” contains the letter “g” as in “ground”.

How Stalagmites and Stalactites Form

Speleothems form through the deposition of minerals from water. It all starts with water making its way through rocks in the ground. Trickling water picks up carbon dioxide from air, forming carbonic acid and making the water acidic. The acidic water dissolves minerals in rocks. This mineralizes the water and also slowly forms new cracks. Acidic water dissolves calcium carbonate in limestone, forming calcium bicarbonate [Ca(HCO3)2[. When water containing dissolved calcium carbonate (CaCO₃) drips into a cave, it loses carbon dioxide (CO₂) as it enters the cave’s lower pressure environment. This loss of CO₂ causes the calcium carbonate to precipitate, forming various speleothems.

The chemical reaction between water and limestone is:

CaCO3(s) + H2O(l) + CO2(aq) → Ca(HCO3)2(aq)

Then, once the mineralized water hits air, the reaction reverses and deposits calcium carbonate on the cave surface:

Ca(HCO3)2(aq) → CaCO3(s) + H2O(l) + CO2(aq)

  • Stalactites form as mineral-rich water drips from the ceiling, leaving behind a small ring of calcite. Over time, these rings build up and form a downward-pointing cone.
  • Stalagmites form where the water droplets hit the cave floor, depositing calcite and growing upward.

Other speleothem formations include:

  • Columns: When stalactites and stalagmites meet and join.
  • Flowstones: Sheets of minerals deposited by flowing water over cave walls or floors.
  • Helictites: Twisted, spiraling formations that grow in various directions due to capillary forces and air currents.
  • Soda Straws: Thin, hollow tubes that can eventually develop into stalactites.

Correspondence Between Stalactites and Stalagmites

Not all stalactites have corresponding stalagmites. A stalagmite only forms beneath a stalactite if the water dripping from the stalactite consistently lands on the same spot on the cave floor. Environmental conditions such as air currents and floor slope affect this process. If the water drips from the stalactite into a pool of water, a stalagmite does not form.

How Quickly Do Stalagmites and Stalactites Grow?

Speleothems grow very slowly, typically at rates of 0.1 to 3 millimeters per year. Factors influencing growth rates include the concentration of minerals in the water, temperature, humidity, and CO2 levels. Under ideal conditions, where the drip rate is slow and the water is rich in both dissolved calcium carbonate and carbon dioxide gas, stalactites and stalagmites grow up to 3 millimeters per year.

Chemical Composition and Colors of Speleothems

Most stalagmites and stalactites consist of calcium carbonate (CaCO3), which is white or translucent when it is pure calcite. Impurities produce colors. For example, iron oxide produces red or orange hues, manganese adds black or gray tones, and copper adds blue or green color.

The water that drips through the rocks contains several ions, mainly Ca2+, Mg2+, Na+, HCO, SO2-4, and Cl. So, other minerals form besides calcite, including other carbonates, limonite, opal, and sulfides.

Non-Speleothem Stalactites and Stalagmites

Stalactites and stalagmites also form outside of natural caves:

  • Concrete Stalactites and Stalagmites: Calcium hydroxide that leaches from concrete reacts with CO₂ and forms calcium carbonate.
  • Ice Stalactites and Stalagmites: Dripping water freezes into icicles (stalactites) and ice mounds (stalagmites).
  • Lava Stalactites and Stalagmites: Lava stalactites form when molten lava drips from the roof of a lava tube, solidifying as it cools. Lava stalagmites form where these drips hit the floor and build up. These stalactites and stalagmites are typically silicates rather than carbonates.

Additionally, crystallized urine, pitch, mud, and other melts, suspensions, or colloids form stalactites and stalagmites.