Tuesday, January 27, 2009

Ice Dunes

ice dunes
ice dunes on Lake Michigan (photo by J Young)
based on a news article in the Erie (Pennsylvania) Times News, "Inside Ice Dunes," by Cody Switzer, Jan 24, 2009

Frozen edges are a feature seen on almost every shoreline of the Great Lakes. The waves known as "ice dunes," "ice ridges," or "the ice foot" form where the waves of the lake meet the frozen water's edge and clumps of snow. Dan Powell, a ranger at Presque Isle, Pennsylvania, State Park said, "They can form overnight."

A study by M. Leonard Bryan and Melvin G. Marcus of the University of Michigan has published a report on the phenomenon titled "Physical Characteristics of Near-Shore Ice Ridges." When snow falls into the water it does not melt instantly. Some of the flakes attach to ice floating in the water, and some snowflakes are joined together by wave action. The ice is washed toward shore and builds up in layers. Two important requisites of icefoot development are sub-freezing atmospheric temperatures, and open water bodies that, because of their large size, remain ice free well into the season of sub-freezing air temperatures.

Add to that wave spray, which quickly freezes and another layer is added. Add more snowfall and the layers continue to build. Ice dunes of 6 feet or higher are relatively common where there are waves. But where there is little wave action, the ice dunes can remain relatively flat.

The smaller Great Lakes, like Erie and Ontario may freeze more completely than the larger ones. The result is more flat ice dunes. "That's because you don't have the wave action," said Kathleen Ryan, environmental education specialist at the Presque Isle.

ice at water's edge
ice at the lake edge (photo by J Young)
The dunes buffer the shore from battering by winds and storms, and help hold sand in place. But ice dunes can also be dangerous. They can be hollow or riddles with caves because of the way they are formed. Even when they look stable, or even quite thick, a high ice dune could have a thin crust. If one broke through they could fall directly into freezing water at the bottom of a sheer "well." It would be impossible to get out without help.

But they can be beautiful to look at. Ryan encourages people to observe the winter shoreline from safe distances. "I think a lot of people think of sand and sun," she said. "But now it's like you are looking at the Arctic, and it's such an interesting environment."

See "Physical Characteristics of Near-Shore Ice Ridges"
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