Though they're rarely photographed, ice boulders like the ones found by a Michigan lakeshore visitor this weekend are not as uncommon a phenomenon as one would think, according to Thomas Niziol, winter weather expert with The Weather Channel.
They have been reported in places as widely scattered as Antarctica and the Arctic regions, and they have been found in many parts of the Great Lakes including the Keweenaw Peninsula, the western basin of Lake Erie and Lake Ontario.
An example of the phenomenon known as ball ice, they generally consist of a combination of slush and frazil ice -- the first stage of sea ice, which usually consists of loose, needle-shaped ice crystals in water that resemble slush. They also can be found in shapes like pancakes or flattened discs in somewhat less turbulent water to spherical shapes in more turbulent waters.
Wave action turns over clumps of slush time and time again, rolling them into these spherical shapes. If the rolling of the ice spheres occurs where there is sand suspended in the water, concentric bands of sand end up within these spheres.
How do they wash ashore? By wave action, and in the case of the Lake Michigan occurrence (shown in the video above), it is likely that strong winds and waves acted to deposit these sprheres on the shore line.
They have also been reported to form long lines of spheres out in the open waters of the Great Lakes and great sheets of spheres when they freeze together under colder conditions.
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Pleasant Prairie, Wis.
iWitness contributor Brian Kelly sent this picture from Pleasant Prairie, Wis. (iWitness/Brian Kelly)