I noticed stripes in the ice at Big Lake last week and recalled Karl Ramsdell’s incredible photos of ice rings in Maine:
I returned to Big Lake with a drone and was thrilled to see similar patterns.
The Glide: Ice Rings and Stars
*** I just learned about Tony Perelli‘s 2007 “Adventure Skaters Guild of Alaska” newsletter: The Glide. I’m borrowing that name for ice-related posts! ***
Ice rings are lightly documented in the outdoor skating world. Most ice rings have an ice star at the center, and these are a bit better understood.
Ice stars: Water flows up through snow-covered ice and then spills out, saturating and melting the snow cover and leaving dark ‘fingers.’ Additional water traces the existing fingers unless they bifurcate at slush/freeze pile-ups. Ice stars can exploit debris in the ice (like a branch sticking through) but otherwise, their locations seem random. You can read more in this article.
Ice rings: The most popular theory is that the weight of snow on thin ice depresses the ice and promotes water flow up through holes. The additional water/slush weight on the thin ice causes more sagging and initiates a circumferential (ring) crack. Water can seep up through the crack, saturate the snow, and trigger the next crack in a cascading expanding series of rings.
The best reference I can fine is at the Lakeice site, but it is a difficult read (here, and more photos). There might be some insight in this chapter, but I’m not ready to spend $30 for access.
I don’t have a better explanation, but these details bother me.
- Was the ice actually thin?
- Ice under a point load shows radial cracks (like spokes) first, not circumferential (rings).
- Lake stars are observed to have wet radii of 1.5 to 4 meters. Is that really enough water weight to initiate cracking?
- What controls the location of the rings? There are theories about upwelling due to temperature and density differences, but … would that really result in a point location? I think of upwelling as a larger-scale process.
I’ll take a stab at addressing each of these questions below, but first, take a look at some frames from the drone footage:
1. Was the ice actually thin?
I reconstructed the weather history at Big Lake to figure out how much snow and ice were involved. The weather history suggests 2-9 inches (5-23 cm) of snow sitting on less than 1 inch (2.5 cm) of black (congelation) ice.
Temperatures are listed as high/low, ºF. Notes on the sources are at the end of this section.
Nov. 1 – 6: Highs in the mid-30s, lows in the high 20s, overcast. Light snow on Nov. 6th. Some ice in the far east.
Nov. 7: 36/24. First discernible ice cover (patchy) at the eastern end of the lake (not the area with rings).
Nov. 8: 33/25, snow. More ice growth in the east, but the west remains open.
Nov. 9: 33/28, overcast, snow
Nov. 10: 30/25, thin clouds, snow
Nov. 11: 28/18, thin clouds
Nov. 12: 23/04, thin clouds. The area of interest (ring photos) is still ice free. But single digit temperatures (and a clearing sky?) are a great recipe for fast ice growth.
Here’s where things get interesting:
Nov. 13: 26/08, 8 inches (20 cm) of snow before noon. Most of the lake is covered with ice and snow. Note the gray zones in the Nov. 13 image where snow has been saturated from cracks and upwellings.
It seems pretty likely that the ice rings formed at this time. The next clear imagery, Nov. 18th, indicates continued wetting of the surface snow, as well as new ice growth and snow cover, but the original ice textures persist.
Calculating ice thickness
Ice growth varies as a function of air temperature, wind, and solar radiation. I used Ajne’s ice growth prediction model to estimate ice growth within the 24-hour period between the satellite image captures Nov. 12 and 13. Using the nearest recorded temperatures, no wind (the wind sensor was not working), and assigning 14 hours of 50% cloud cover, then 10 hours of 0% cloud cover, Ajne’s equation suggests up to 1 inch (25 mm) of black ice growth. Based on my observations this time of year, that seems like a plausible upper estimate.
As of Dec. 3, the ice thickness was approx. 8 cm of black ice under 20 cm of white ice. I assume that the very cold temperatures (and clear skies) Nov. 18-20 caused some downward (black) growth despite the thickness of white ice.
The rest of the weather history doesn’t seem as important, but here it is:
Nov. 14: 20/01, thin clouds
Nov. 15: 33/20, 5 inches (13 cm) of snow and rain.
Nov. 16: 37/29, overcast, snow (and rain?)
Nov. 17: 28/14, thin clouds, snow
Nov. 18: 14/-14, clear. One pod of open water remaining.
Nov. 19: -3/-19, clear. Final pod has frozen over.
Nov. 20: 27/-19, clear. 40 mph wind gusts from the east strip the snow off of the ice.
Notes about the weather history:
- Temperatures are listed as daily high/low, in degrees F.
- Temperatures from Nov. 1-8 are from a RAWS station very near the lake. That station went offline Nov. 8. Later temperatures are from a DOT station 8 miles away.
- “Overcast,” etc., is based on MODIS imagery.
- Snow is extrapolated from the nearest three SNOTEL stations.
- Satellite imagery is from EO Browser and EOS Landviewer.
2. Circumferential cracks (rings) instead of radial?
The initial cracks in ice due to a point load radiate out from the load rather than wrap around it. So, I’m confused why the weight of saturated snow would cause circumferential cracks.
Here’s a great example of radial followed by circumferential cracking:
Fracture fatigue
One way to get circumferential cracks is with fracture fatigue. Fracture fatigue is caused by cyclical loading and unloading. Industrial and transport industries put a lot of effort into this research because fracture fatigue causes high-consequence equipment failure.
The multiple origins and even the ‘ratchet mark’ in the image above are similar to the ice rings. BUT … these fracture fatigue patterns are created in a very different stress regime: within the fracture plane of a cylinder under load, tension, rotation, or a mix.
Is it possible we are seeing something similar with ice rings? What are the possible sources of cyclic loading and unloading? Wind? Thermal upwelling?
Are wet lake stars heavy enough to initiate ring cracks?
This would probably an easy calculation for someone, but not me. A good place to start might be research in response to the rapid collapse of part of the Antarctic ice sheet. Models suggest that the weight of a lake can flex the sheet and cause cracking at the km scale. Could the same process happen at the cm scale?
What controls the location of the center of the rings?
I don’t understand why the lake stars / centers of ice rings are located where they are. Thermal cells and upwellings seem like they should be on a much larger scale that the m-scale small lake star holes that we see.
Upwelling?
I took a look at the bathymetry in case it revealed any clues.
The location of the ice ring photos is indicated by the small crosshairs in the center of this image. It appears to be a ‘busy’ area … near a NW-SE trough and around the corner from an underwater cliff.
It might be significant that the shallower waters to the east were covered by ice while there was open water to the west (as indicated by satellite imagery). But if anything, I’d expect the warmer water under the ice (insulated) to sink relative to the open water exposed to the cold air (cold water is less dense than warm water up to 4 ºC).
This isn’t a driving force for upwelling under the ice shelf, which is what I’d expect to form lake stars and rings.
The opposite relationship is visible at the location of the photo with twin lake stars, which also has ice rings nearby (not photographed).
In this case, the ice covered deeper waters to the west. The shallower water to the east remained open for another week. Again, it seems like the warmer water under the ice should have sank. What am I missing?
Other ideas?
A lot of people chimed in with ideas on FB, IG, and by text. I couldn’t keep track of the various conversations. Please share any other ideas or resources in the comments below.
But wait … there is more!
Luc Mehl is a certified ice-rescue instructor based in Anchorage, Alaska. Luc studied geology and geophysics at MIT but still has no idea now these features formed. If you like this stuff, you would probably like Wild Ice!, an online course about finding and safely traveling over ice in remote or untested locations.