Stalactites have geometry as destiny

Scientists at the University of Arizona brought together ideas and techniques from physics, chemistry, and geophysics to derive a mathematical theory that explains the morphology of cave stalactites (the carrot-like formations hanging down from cave roofs). With an understanding of the growth of speleothems—the collective name for limestone cave formations—weather features from thousands of years ago could be deciphered from the layering in these underground repositories, much as tree rings or ice core samples contain clues to past climate. Stalactites are composed of calcium carbonate precipitated from water entering the cave after percolating through carbon dioxide–rich soil and rock. Treating stalactite growth as a free-boundary problem (meaning that no a priori assumptions about shape were made), the researchers linked the fluid dynamics and chemistry to determine a precipitative growth rate, which in turn led to the discovery of a universal geometrical shape. A quantitative comparison with real stalactites (see photo) in Arizona’s Kartchner Caverns showed very good agreement except at the tips, where the model no longer applies because drop formation and detachment are not included. (M. B. Short et al., Phys. Rev. Lett. 94 018501, 2005.http://dx.doi.org/10.1103/PhysRevLett.94.018501 )