Out in the Woods

Needle ice pushing up soil particles

Photo Credit: Thomas Bresson-Wikipedia

Needle Ice

By Kevin McKeon, Maine Master Naturalist

On our trail walks during early winter, we often see small areas of ice crystals poking above the ground, looking like groups of tiny icicles. We also hear them crunch underfoot. These are ice crystals formed just beneath the soil surface called needle ice. James Carter, former professor of geology and geography at Illinois State University, describes its formation in the following way: 

“On cold nights at the beginning of winter, when temperatures just barely sink below freezing, the ground will stay slightly warmer than the air above. That means that any water in the ground… will remain liquid. In certain soils, though, water that’s in the ground gets sucked upward rather than sinking down. This is a result of capillary action: the adhesion of water molecules to the walls of a very narrow tube will cause the liquid to be drawn upward despite the pull of gravity.”

Let’s dive a bit into this explanation to more fully appreciate this phenomenon. Understanding capillary action may be a good place to begin. It’s defined as liquid flowing through small spaces without the benefit of an external force. The Latin word capillaris means “of or resembling hair.” We use capillary to describe relatively narrow, tubular structures, like the capillary veins in our skin. But the blood flowing through our veins is not capillary action because it’s pumped by an external force…our hearts.

Needle ice is made from water, and one of water’s interesting properties is that its molecules want to adhere to themselves, creating surface tension, like when rain beads form on your car. Water molecules are also a bit sticky and want to stick to a lot of other things. These two properties, cohesion and adherence, allow capillary action, like when the water inside a drinking straw rises a bit above the water level in the drinking glass. Water likes the inside of the straw enough to crawl up into it a bit. Albert Einstein’s first published paper in 1901,“Conclusions drawn from the phenomena of capillarity” was about capillary action

Going back to Professor Carter’s description of needle ice formation, he states that water, capillaries, and certain temperature conditions are required. The granular composition…texture, of certain soils allows the formation of pores which can form capillaries. Soil texture also affects its water holding characteristics. Temperature, the third item in needle ice formation, requires a level of temperature difference called gradience.

In our swimming holes, water is cooler the deeper we dive. These temperature gradients exist in the soil, too. Temperatures in soil take longer to change than in the air. As winter’s cold arrives and the cold air forms thin ice films on Deering Pond, the soil along the trail around it remains a bit warmer and the water in it remains liquid. As the soil slowly gets colder from the surface downward, the water also slowly freezes.

Now for the needle ice. During the times of temperature gradience when the upper soil layer is 32 degrees or lower and the lower layer is above that, the upper layer water is freezing but the lower layer water is not. Through capillary action, water in the soil is drawn upward through the pores until it hits the upper layer where it freezes. It continues to push more water upwards until that water freezes onto the newly formed ice crystal, pushing the whole crystal upward. Thus, water crystals grow, forming long curls 2 to 5 inches long. Sometimes these crystals will fuse, creating ice chunks of varying sizes.

These magical needle ice formations cease to exist when the deeper soil layer freezes. The cool thing about all this is that both the temperature gradience and water-borne soil are most common on soil that’s not insulated by small woody debris and leaf litter. Such soil is created by foot traffic along our trails. So, on your brisk early winter walks, ponder a bit at the needle ice you find along the way and maybe step over it so others may marvel at Natures wonders.

Editor’s note: Did you see something unusual last time you were out in the woods? Were you puzzled or surprised by something you saw? Ask our “Out in the Woods” columnist Kevin McKeon. He’ll be happy to investigate and try to answer your questions. Email him directly at: kpm@metrocast.net