Giant, rotating disks of ice (up to 50 feet wide!) that form in slow-moving rivers, carving themselves into near-perfect circles.

A chunk of ice breaks off and floats downstream. Currents push the ice into a gentle rotation, grinding its edges against surrounding ice or rocks. Centripetal force sculpts it into a circle (like a spinning pottery wheel).

In freezing temps, the disk grows as water freezes onto its edges. Can persist for weeks if temperatures stay below freezing.

Just below 0°C (32°F) cold enough to freeze, but not so cold the river solidifies. Slow-moving currents (e.g., bends in rivers like the Sheyenne River, North Dakota). Critical， without spin, it’s just a floating ice chunk.

Shows how viscosity + rotation create geometric perfection. Few documented cases (most in Scandinavia, North America).

To see them
Sweden’s Lule River (1987, 2007)
Michigan’s Pine River (2019)
Maine’s Presumpscot River (2020)
Look for them at dawn， frost makes them glow!

Sources
NASA Earth Observatory