Soda Straws


Soda straws represent the earliest growth phase of stalactites. They are hollow, elongate, generally translucent tubes of calcite equal in diameter to the water drops conducted along their length. Drops that hesitate at the growing tip of a soda straw before falling to the floor lose carbon dioxide to the cave atmosphere and so precipitate some of the calcium carbonate they carry in solution. This calcium carbonate is added to the thin blades of calcite that jut downward from the soda straw growing tip like pointed teeth.
Some soda straws, like those in the second photo, are oddly deflected. Perhaps growth of the straws is sometimes favored on the windward side of air currents driven by convection, a chimney effect, moving water, or barometric changes on the surface. Alternatively, evaporation may cause solution impurities to be incorporated in the calcite matrix, leading to wayward crystal growth.

Soda straws may occur in large numbers together, as in third photo. Occasionally they develop to extraordinary lengths. Specimens close to 30 feet long have been observed, as noted on the page on the legacy Virtual Cave site devoted to the world’s largest speleothems.

Soda straws are sometimes present on the tips of stalactites, such as the dripping straws in photo #4.

Few cave formations are as delicate as soda straws, so great care must be taken when moving around beneath ones that are low-hanging.

AUTHOR: Djuna Bewley