Italian scientists have proposed an explanation for the origin of mysterious ripples on stalactites.
Stalactites in the Choranche caves in the Vercors, France (David Monniaux)
Stalactites grow from cave ceilings not as dull cones but often sporting elegant corrugations. Two researchers from the Polytechnic University of Turin have now explained these mysterious, wavy patterns using standard fluid mechanics.
The new theory, detailed in the journal Physical Review Letters, shows that the horizontal ripples form because spatially periodic patterns arise in the rate of mineral deposits from the water flowing down the stalactite.
Starting from this model, climate scientists might in the future use stalactite surface structure to reconstruct variations in precipitation patterns over tens of thousands of years.
In certain caves, a tiny but steady flow of water from above brings a constant supply of minerals – especially calcite, which is calcium carbonate – to add to slow-building structures such as stalactites and their lowly siblings, stalagmites. Some stalactites build up horizontal, wavy patterns called crenulations. These hills and valleys repeat at regular lengths, usually between five and ten millimeters, and they slowly migrate upward, according to data from stalactite cross sections.
Such structures must arise from the interplay of fluid dynamics – the flow of water that brings fresh material to the mineral growth – and the deposition process. In an idealized, symmetric situation, water moves in layers parallel to a flat rock surface, and the water’s solute concentration varies smoothly from top to bottom, decreasing as the water slides down a stalactite and deposits calcite. At any given point on the rock surface, the rate of deposition depends mainly on the local concentration of solutes. For crenulations to appear, something must cause the fluid motion – and the concentrations of solutes it carries – to vary in a periodic fashion.
“Past models have assumed that the flow remains everywhere parallel to the rock surface and thus were too simplistic,” said Dr Carlo Camporeale, an environmental engineer at the Polytechnic University of Turin.
In the new study, the researchers have developed a model that allows for more complicated fluid flow. They also included equations for the chemical reactions and the resulting rate of calcite deposition.
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Bibliographic information: Camporeale C., Ridolfi L. 2012. Hydrodynamic-Driven Stability Analysis of Morphological Patterns on Stalactites and Implications for Cave Paleoflow Reconstructions. Phys. Rev. Lett. 108; doi: 10.1103/ PhysRevLett.108.238501
