How do stalactites and stalagmites form in caves? — GS, Conroe, TX
They form when various minerals come out of solution in water and crystallize on the surfaces of a cave. To understand how this process occurs, we must look at the interface between the water and the cave surface. Whenever water is in contact with a mineral surface, there is a chance that an atom of the surface will suddenly leave the surface and dissolve in the water. If there are atoms already dissolved in the water, there is also a chance that one of them will suddenly come out of solution in the water and attach to the surface. Atoms leave and return to cave surfaces all the time as water drips from the ceiling of a cave to its floor.
What is important for the growth of stalactites and stalagmites is that more atoms stick to the cave surfaces than leave those surfaces. That is exactly what happens and it does so because the water has already picked up more than enough dissolved atoms before it reaches the stalactite. Either because of temperature changes or because of evaporation, the water that runs across the cave roof and down the sides of a stalactite deposits more atoms on the stalactite’s surface than it removes. The same goes for the stalagmite after the water drips down to the cave floor. As the atoms build up on the cave surfaces, the stalactites grow down and the stalagmites grow up.