Ice disk

Ice disks are floating plates of ice that appear nearly circular and may turn slowly on cold rivers or lakes. Reports describe examples from small experimental-scale disks to natural formations tens of metres across. They are uncommon because their appearance depends on a particular combination of ice, water movement and temperature. The name is also written “ice disc”; researchers commonly use the plural “ice disks.”

The processes involved are not fully settled and may differ between natural settings and controlled experiments. River bends and eddies can help shape or keep a disk moving, while changes in water temperature and melting can also generate flows beneath floating ice. In 2016, laboratory experiments reported that a downward plume of cooled, denser water beneath a melting disk generated a vortex, with viscous entrainment contributing to rotation. A 2023 follow-up study found that disks on nominally still freshwater behaved unpredictably and argued that residual water motions could trigger rotation, with meltwater flow amplifying it. These findings caution against treating a single mechanism as a complete explanation for every natural ice disk.

Laboratory researchers have used flow-measurement methods, including particle image velocimetry, to examine water moving beneath melting ice disks. Field observations are harder to interpret because the current, riverbed shape, temperature and ice conditions vary together. A large disk reported in the Presumpscot River in Westbrook, Maine, in January 2019 was estimated at roughly 100 yards across; a University of Colorado at Boulder glaciologist interviewed by PBS described the need for near-freezing conditions and a suitable river bend with a large eddy. Such individual observations document the phenomenon but do not establish one universal cause.