Stalactite Cross Section
December 04, 2012
Photographer: David Lynch; Dave's Web site
Summary Author: David Lynch
Stalactites are a form of dripstone that occurs when water percolates through limestone and drips from the ceiling of a cave. The water carries dissolved calcite (CaCO3 – limestone). Calcite is dissolved by carbonic acid in the water (HCO3) which happens naturally when atmospheric carbon dioxide is dissolved in water. Indeed, even rainwater is slightly acidic because of the CO2 it contains.
When the water reaches the air in the cave, CO2 evaporates from the water, lowering its acidity. This forces the CaCO3 to come out of solution and deposit as calcite stalactites. At first, calcite forms a thin ring around the periphery of the drop. Over time it grows into a slender, hollow tube that grows lengthwise. These are called “soda straws,” one of many speleothems, or geologic cave formations. Water flowing down the outside of the soda straw also deposits calcite, and the formation grows outward layer-by-layer, somewhat analogous to tree rings. The graceful outward beauty of stalactites gives no hint as to what's found inside them.
I discovered this stalactite already broken, lying on the floor of a cave in southern Indiana in 1969. Cutting and polishing it revealed the delicate growth rings. Each layer is a slightly different color as the mineral content of the water changed over time. This stalactite started out as two soda straws, and then merged into one stalactite. Photo taken in Topanga, California on November 22, 2012.