Stone forests — pointed rock formations resembling trees that populate regions of China, Madagascar, and many other locations worldwide — are as majestic as they are mysterious, created by uncertain forces that give them their shape.
This video shows an experiment in which a dissolving block of candy develops into an array of sharp spikes. The block starts out with internal pores and is entirely immersed under water, where it dissolves and becomes a “candy forest” before collapsing. Credit: NYU’s Applied Mathematics Lab
A team of scientists has now shed new light on how these natural structures are created. Its research, reported in the latest issue of the journal Proceedings of the National Academy of Sciences (PNAS), also offers promise for the manufacturing of sharp-tipped structures, such as the micro-needles and probes needed for scientific research and medical procedures.
“This work reveals a mechanism that explains how these sharply pointed rock spires, a source of wonder for centuries, come to be,” says Leif Ristroph, an associate professor at New York University’s Courant Institute of Mathematical Sciences and one of the paper’s co-authors. “Through a series of simulations and experiments, we show how flowing water carves ultra-sharp spikes in landforms.”
The researchers, who included Michael Shelley, a professor at the Courant Institute, note that the study also illuminates a mechanism that explains the prevalence of sharply pointed rock spires in karst — a topography formed by the dissolution of rocks, such as limestone.
In their study, the scientists simulated the formation of these pinnacles over time through a mathematical model and computer simulations that took into account how dissolving produces flows and how these flows also affect dissolving and thus reshaping of a formation.
To confirm the validity of their simulations, the researchers conducted a series of experiments in NYU’s Applied Mathematics Lab. Here, the scientists replicated the formation of these natural structures by creating sugar-based pinnacles, mimicking soluble rocks that compose karst and similar topographies, and submerging them in tanks of water. Interestingly, no flows had to be imposed, since the dissolving process itself created the flow patterns needed to carve spikes.
This video shows a side-by-side comparison of the shape development of a single dissolving pillar from experiment and simulation. Both methods show that an initially smooth apex is carved into an ultra-sharp spike. Credit: NYU’s Applied Mathematics Lab
The experimental results reflected those of the simulations, thereby supporting the accuracy of the researchers’ model (see “Video2ExperimentSimulation” in the below drive). The authors speculate that these same events happen — albeit far more slowly — when minerals are submerged under water, which later recedes to reveal stone pinnacles and stone forests.
Reference: “Ultra-sharp pinnacles sculpted by natural convective dissolution” by Jinzi Mac Huang, Joshua Tong, Michael Shelley and Leif Ristroph, 8 September 2020, Proceedings of the National Academy of Sciences.
The paper’s other co-authors included Jinzi Mac Huang, an NYU doctoral student at the time of the research, and Joshua Tong, an NYU undergraduate at the time of the study.
The research was supported by a grant from the National Science Foundation (CBET-1805506).