Every other Friday on Morning Edition NHPR’s Sam Evans-Brown tracks down answers to questions about the environment and outdoors for our listeners in a segment we call “Ask Sam.”
Matt from Concord asks: We were out hiking the other day and we saw these ice crystals coming up out of the ground and pushing moss up off the ground. They look like these long thin needle-like things. What causes that? Thank you!
It has come to my attention while digging into this question that not everybody has seen this phenomenon, despite the fact that it’s all over the place in New Hampshire. If you’re ever walking across a lawn or soccer field and you feel that crunchy sensation under your feet (not like, frozen grass crunchy, but weirdly frozen dirt crunchy) there’s a good chance you are smashing some of these little ice-formations that are hidden just under the grass.
This stuff is, not surprisingly, called Needle Ice, and to make it here’s the recipe.
- Water. Or more specifically moist soil. And not only that, when you start this process, the water needs to liquid; the soil can’t be totally frozen yet.
- Cold. But not too much cold, because needle ice forms when the temperature of the air drops below freezing, but the water is still flowing underneath.
- Loam. All of these ingredients are necessary for needle ice, but this one is the where the real magic happens. You need soil where the particles have just the right spacing. If it’s clay the particles are too small, if it’s sand they’re too big. Loam is the sweet spot, it’s a 40-40-20 mix of sand, silt and clay.
“There’s something about the pore spacing that produces just the right size… smaller than a droplet,” says Jim Carter, a professor emeritus from Illinois State University and ice enthusiast, “Ice forms and ice continues to grow right at the surface... pushes the old ice out and away.”
Because loamy soil has just the right amount of space between the particles, something called ice segregation happens. Instead of all the water coming together and just forming a big chunk, or each quantum of ice freezing in its own little dirt chamber, you get tiny ice structures that grow from the bottom, kind of like Play-Doh extruded out of a toy.
The needles grow the way they do because of the magic of capillary action. Without getting too deep into the physics, this is the thing you learn about in high school biology that says that water can climb up skinny tubes like the ones inside plants. It’s the force that makes it so that paper towels and your tear ducts work. It’s this force that moves water through the unfrozen, perfectly spaced pores in loamy soil, up toward the surface where cold air is waiting.
Once a bit of ice forms at the surface, and the water beneath flows up via capillary action, it makes contact with the ice and forms another layer on the bottom, pushing the previous layer up. (Really, could there be too many of these gifs.)
So what’s with the moss? If the needle starts to form a little bit below the surface, that Play-Doh extruder will push anything on top of it up into the air: moss, a little bit of dirt, or what have you.
Here’s the next layer of cool, though. Needle ice is fairly common because loam is a pretty common, but this same process can result with other types of weird ice structures. Some dead wood that’s full of fungus can produce diaphanous hair ice; there are plants that when they freeze the water squeezes out and form even more delicate structure, ghostly ice flowers; and some small pebbles can even grow icicles that almost look like an ice stalagmite, which Carter has dubbed pebble ice.
When I asked Professor Carter what good it does us to know about this phenomenon, he replied, “it probably does about as much as people doing a bunch of flowers in their yard every year to see some pretty colors out there. What does mowing the grass do for us?”
In other words, the world is a nifty place, and sometimes things are just nifty.
That’s Sam Evans-Brown, host of NHPR’s Outside/In which you can subscribe to where-ever you get your podcasts, If you’d like to submit a question you can record it as a voice memo on your smartphone and send it to email@example.com, OR call the hotline, 1-844-GO-OTTER.