If you have a few weather or science-obsessed Facebook friends, you’ve probably seen videos of so-called “ice circles” circulating online in the last 10 years or so. What are ice circles? They’re those slowly rotating, almost perfectly circular, sometimes giant disks of ice that look like frozen lily pads. Scientists have been studying their mysterious rotations since at least the late 19th century—check out this 1895 mention in Scientific American—but it took until 2016 for a team of Belgian physicists to figure them out.
Even after their breakthrough discovery in March 2016, there’s still some misinformation out there about ice circles, published in supposedly reputable places. This list of ice circle facts will hopefully put that to rest. How do ice circles form, for real? Read on to find out.
As an ice circle rotates, it smoothly erodes the so-called “border ice,” sometimes with a blade-like contact point that moves with the circle. The ice circle, in other words, doesn’t “roll” on the walls of the hole it created, but instead continually carves its place out in the border ice. It’s almost like a needle on a record: research indicates, in fact, that some ice circles cause a low-frequency sound as they spin, as long as the “needle” is still making contact.
Further examination reveals that these ice circles or ice disks may actually be more accurately called ice bowls. The small ice circles in the above video, for example, while not nearly as dramatic as their big siblings, are helpful for demonstrating the “true” shape of many ice circles. Researchers in 1987 discovered that a large ice circle on the Pite River in Sweden was slightly bowl-shaped on closer inspection. Erosion from rainwater was ruled out, considering that the bowl-shaped indention would have had to be there in the first place to collect the level of rainwater that it did.
So now that we know a little more about the nature of ice circles, how did scientists actually crack the case? In true Science Fair fashion, they used magnets and popsicles. It’s true: a so-called “tabletop” experiment helped a team at the University of Liége in Belgium to discover the truth behind the rotating ice circle phenomenon. They made their own homemade petri dish popsicles to use as tiny ice circles with a weak nickel magnet inside to keep the ice circles from floating around too much. This helped the team to rule out the eddy hypothesis.
As the petri dish popsicles melted, they began to spin – even in calm waters. This led the team to realize that the ice circles are, in fact, spinning because they are melting – no whirlpools necessary. When the water melts, it doesn’t just flow off the sides; like a flushed toilet, the water actually spins when it sinks. This spinning water underneath the ice circles then spins them around, causing them to erode the border ice and carve out their neat little shape in the wild.