Threads of ice, as thin and delicate as spun sugar, seemed to be growing out of the ground. All three of us bent close and marveled at the odd sight. One friend plucked a piece of the ice up to examine it; the strands melted in his hand.
We spotted more of the mysterious ice as we continued along the trail. It sprung from the dirt in patches, like grass. And as we hiked, we speculated about the source of the phenomenon. It had rained earlier in the week — maybe that had something to do with it? Did the ice crystallize on some other surface that had since been washed away? Our theories got stranger and stranger, until we left the realm of science entirely and started fantasizing that the ice was debris from a magical battle between the mountain's fairy inhabitants and strip-mining trolls. (I am very weird. Luckily, my friends are, too.)
But I couldn't stop thinking about the ice. Where did it come from? Why have I never seen it before? I asked friends and colleagues about it, but none of them had answers.
And then I realized — explaining stuff like this is literally what I do for a living. Who better to answer this question than Dear Science herself?
So, without further ado:
What on Earth is this stuff?
Here's what science had to say:
Some preliminary online searching lead me to this news release from the European Geosciences Union about a study published last summer. Scientists in Germany and Switzerland had figured out that “hair ice” — which has the fine, silky appearance of candy floss — occurs when liquid water is squeezed out of pores on the surface of rotting wood, turning when it hits freezing air. Fungus on the surface of the wood gives structure to the ice formations, and an organic compound called lignin helps stabilize it.
The hair ice didn't look quite like what I had seen, so I emailed the lead author of the study, physicist Christian Matzler, with a few pictures.
Matzler is retired now, but he kindly took time to look at my photos and identify them.
“What you saw is a relative of hair ice. It is called 'needle ice,'” he wrote back. “The needles are much stronger, harder and thicker than the hairs of hair ice.”
Matzler explained that, unlike hair ice, needle ice grows from pores in fine grained soil or pebbles. But he wasn't sure what exactly causes the structures.
That was okay — Matzler had given me a name for the phenomenon. I thanked him and typed “needle ice” into my search engine, which led me to the website of James Carter. I called him up, and he introduced himself as an emeritus professor of geology and geography at Illinois State University.
“That means I'm old and retired,” he said wryly.
Carter is the kind of source science reporters love to find: funny, insightful, delightfully enthusiastic about the minutiae of his field and full of entertaining anecdotes. He told me about his experiments creating ice formations in his freezer (more on that later) and the best way to find pebbles for growing it on.
“I take a small pebble and put it on my tongue and if it really sticks to my tongue, then it's a good one,” he said. “I'm always going around tasting pebbles, hundreds of pebbles a year. I'd say only 1 in 500 of them works.”
“I guess I should go around sticking rocks in my mouth?” I joked.
“You should!” he replied, completely in earnest.
Carter was also exactly the person I needed to talk to. An atmospheric scientist and geographer, he specializes in phenomena that occur as a result of tiny fluctuations of temperature.
This is what is happening when needle ice forms, he explained. 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 — like the rain that had fallen a few days before my hike — 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. (The U.S. Geological Survey has a solid explanation of why this happens.)
Soil like the kind I saw along the trail in Shenandoah, as well as very particular kinds of pebbles, contains pores that are just wide enough to allow capillary action to occur. Water in the ground is drawn upward through the pores, right until it hits the air. Then it freezes. As more water is drawn up, it freezes at the ice interface (a phenomenon called ice segregation) and pushes the newly formed needle of ice outward, kind of like the hair on one of those Play Doh toys you may have had as a kid.
Carter has also investigated flower ice, another unusual consequence of capillary action, ice segregation and temperatures that are just right. In these cases, liquid water is drawn upward through the thin veins of plants and squeezed out through cracks in the stems, freezing in curtains as it hits the air.
The ice flower phenomenon is probably the most famous of all the ice formations. It was first documented by scientists and mathematician J.F.W. Herschel, who in 1833 wrote about finding ice that “seemed to emanate in a kind of riband- or frill-shaped wavy excrescence . . . presenting a glossy, silky surface.”
Carter estimates that there are about 30 plants in the world that wick up water in just the right way to produce ice flowers. On the east coast, look for Verbesina virginica, or white crownbeard, and Cunila origanoides, also known as American dittany.
You can also try to grow ice structures yourself. Carter has had varying degrees of success growing ice on pebbles in a contraption he built from a cooler, packing peanuts, a light bulb and a lunch meat container full of damp soil.
He sticks the entire experiment in a freezer, and the light bulb keeps the soil warm from below. Over time, the pebbles grow a swatch of ice on their tops, like a bad toupee.
“I’ve learned a whole lot about this in my refrigerator,” Carter said. His website has more detailed directions for anyone who would like to do the same.
I haven't attempted to grow my own ice yet — I have three housemates, and I doubt they'd take kindly to finding a tub full of dirt in our freezer. But I will keep my eyes open on all future winter hikes, searching for ice needles, hairs and flowers, and any other questions that might pop up along the way.