Stalagmite

A stalagmite is a specific type of rock formation that rises upward from a cave floor.

These formations grow because of the accumulation of material deposited by dripping liquid. Most stalagmites are classified as speleothems, which is a term for mineral deposits formed in caves. While they are usually made of calcium carbonate, they can also consist of other materials. These include lava, mud, peat, pitch, sand, sinter, and even amberat. Amberat is a specific material made from the crystallized urine of pack rats. When a stalagmite grows long enough to connect with a stalactite hanging from the ceiling, the two joined structures form a column.

Most stalagmites form in limestone caves through a complex chemical process. This begins when water containing carbon dioxide dissolves limestone. This reaction creates a calcium bicarbonate solution. For conventional growth to occur, the partial pressure of carbon dioxide in the water must be greater than the partial pressure of carbon dioxide in the cave chamber. As this mineralized water solution drips from the ceiling, it undergoes deposition. The minerals precipitate, or settle out, from the liquid onto the cave floor. This process builds the stalagmite layer by layer. Because this relies on slow mineral precipitation, it can take thousands or even hundreds of thousands of years to form.

Not all stalagmites are made of limestone or formed by water solutions. Lava stalagmites are a distinct type that forms inside lava tubes. These occur while molten and fluid lava is still active inside the tube. As the main lava flow passes and the temperature decreases, drops of molten lava solidify on the floor. These are composed of siliceous minerals, which are similar to obsidian, the main part of volcanic glass. Unlike limestone stalagmites, the material is transported in a molten state rather than a dissolved aqueous solution. This process is driven by gravity but does not rely on degassing. Because they depend on active lava, these formations grow very quickly, often in only hours, days, or weeks.

There are also seasonal or year-round ice stalagmites. These occur when water seeps from the surface into a cave where temperatures are below freezing. The water collects on the floor and freezes into a stalagmite shape. Deposition can also happen directly from the freezing of water vapor. Ice stalagmites are actually more common than ice stalactites. This is because warmer air rises to cave ceilings, which can keep those areas above freezing. Like lava stalagmites, ice formations can grow very fast, within hours or days. However, ice stalagmites can grow back if the temperature and water supply remain suitable.

Humans can also encounter stalagmite-like structures in man-made environments. These are called calthemites, which distinguishes them from natural speleothems. Calthemites form on concrete ceilings and floors due to concrete degradation. During this process, calcium ions are leached out of the concrete into a solution. This leachate solution carries the calcium to the ground under the structure. When carbon dioxide is absorbed into the alkaline leachate, chemical reactions cause calcium carbonate to deposit. These formations grow much more rapidly than natural cave formations. However, they rarely grow taller than a few centimeters.

Scientists and explorers have found incredible examples of these formations globally. The largest known stalagmite in the world is located in Sơn Đoòng Cave in Vietnam. In the Shapur cave in Iran, a massive stalagmite was used for art. A colossal statue of Shapur I, the second ruler of the Sassanid Empire, was carved from a single stalagmite. This 3rd-century statue stands on the fourth of five terraces in the Zagros Mountains. These examples show how stalagmites can be both natural wonders and subjects for human creativity.

It is very important to protect these formations when visiting caves. People should not touch stalagmites because skin oils can interfere with their growth. These oils alter the surface tension where the mineral water clings or flows. This change in tension affects how the minerals precipitate onto the surface. Additionally, oils and dirt like mud or clay can permanently stain the rock. This contact can change the color of the formation forever. Protecting these structures ensures that the slow process of mineral deposition continues undisturbed.