'Candyland' stone forests form using deceptively simple physics

The stunning, razor-sharp spires of stone forests can form in deceptively simple conditions, a sugary new experiment finds. 

Using sticks of candy, researchers discovered that cylindrical shapes can naturally sharpen into points in still water as they dissolve — no complicated flow required. This phenomenon could explain why sharp stone pinnacles are often found where easily-dissolvable limestone rock predominates. Examples include the Stone Forest, or Shilin, of Kunming, China, the jagged pinnacles of Tsingy de Bemaraha National Park in Madagascar, and the Pinnacles of Gunung Mulu in Malaysia. 

"We found the simplest recipe for how to make one of these pinnacles," said Leif Ristroph, an experimental physicist and mathematician at New York University who led the study, published Sept. 8 in the journal Proceedings of the National Academy of Sciences. 

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Simple sugars

The recipe was simple indeed. Ristroph and his team cooked up hard candy — like a lollipop — in the shape of a cylinder with a domed top. They stuck the candy upright in a tank of water and simply let it dissolve. 

One might imagine that the candy would simply shrink away, staying more or less the same shape. But that's not what happened. Instead, the candy gradually sharpened into a point as it dissolved. These points could become quite sharp, Ristroph told Live Science — at least 10 times thinner than a human hair. 

"These are very fine structures that come out, and they kind of make themselves," Ristroph said. 

The next step was to do the math to figure out why this sharpening effect occurred. As the candy dissolves, Ristroph said, the water directly next to the sugar column becomes laden with sugar. This makes it heavier than the surrounding water. This sugar-laden water thus sinks downward, almost like a skin sloughing off the candy. 

This sinking means that the dissolving candy essentially creates its own flow. Fresh water flows in from the sides, only to become laden with sugar itself and sink. This flow is what sharpens the candy into a point. 

Sugar to stone

Limestone and other dissolvable rock is more complex than simple sugar, Ristroph said, and there are likely other factors that help shape the stone forests found around the world. Rock chemistry, loose sediment and winds likely play a role. But the stone forests largely form while submerged under water, and the simplicity of the candy experiment helps explain the basic process, Ristroph said. 

"Our choice of materials here, as pure water and pure candy, it is purposefully clean so we can understand it in terms of the fundamentals," Ristroph said. 

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The experiment can be done at home, Ristroph said. He and his team have also explored the shapes that appear when spheres of candy dissolve, and the results are often complex, he said. "This is really a beautiful kitchen experiment," he said. 

Understanding shapes in nature is important because shapes reveal the history of an object, Ristroph added. A spacecraft flying over an unexplored planet, for example, might send back images of landforms as the only clue to the history of the planet's surface. River-like features on Mars were the first hint that the Red Planet once hosted water, he said. 

"That shape tells you something about the events that are behind it, the environmental conditions, the fluid flows," Ristroph said. "It's all written onto that shape." 

Originally published on Live Science.