An ice circle (also called ice disc or ice pan) occurs in slow moving water in cold climates. It is thought that ice circles are formed when surface ice gathers in the center of a body of water rather than the edges. A slow moving river current can create a slow turning eddy, which rotates, forming an ice disc. Very slowly the edges are ground down until a gap is formed between the eddy and the surrounding ice.
The gif is from Evan Gregg’s capture of a ice circle in England on January 26th 2013.
(Source: scientistmary, via silkysassafras)