Pressure Ridges

A classic pressure ridge. This one served as the border fence between the US and Canada in 2005.

Pressure ridges are compression ruptures that typically form as long cracks on ice sheets a mile or bigger in size.  They typically come in when the ice is 2-4” thick, often as ice warms and expands as the cold spell that caused the ice to catch comes to an end.  They generally run between points of land across the lake or across bay mouths.  They also frequently form along the shore. 

 Pressure ridges are a very popular way to get in trouble.  Roughly half of lake ice-vehicle fatalities in Vermont are associated with ridges.  The ice in and near a ridge consists of broken and often poorly attached pieces.  Plates held under water can erode significantly or even melt away completely.  They come in a variety of types and they are constantly evolving.   

 Ice sheets typically form during cold spells.  As the ice warms at the end of the cold spell thermal expansion puts compression stress in the ice as it pushes into the shoreline around the lake.  When the stress exceeds the compressive strength of the ice it ruptures.  Points of land create stress risers that make it more likely that the rupture will occur there.  They come in two major types.

  • Overlapped ridges are the most common.  They form abruptly, often with a loud bang as the ice ruptures.  One side of the rupture climbs over the other breaking into loose plates from 5 to 30 feet wide.

  

A small over/under ridge on 3-4" ice. Note puddles showing where the overlap switches sides.

 

  • Folded ridges come in two variations: downfolded and upfolded.   Downfolded ridges occur when the plates break and buckle downward leaving a deep puddle with ice underneath.  Upfolded ridges are similar but the center is folded up and smaller puddles often occur on each side.    They  form much more slowly than overlapped ridges.  The puddle over the folded plates lengthens at a rate of a few to a few tens of feet an hour when the ice sheet is warming and growth stops when the heating stops (usually because the sun goes down).    On the fast side we once saw one that progressed at walking speed until it got about 100 feet from an existing ridge. At that point the last 100 feet came in with an impressive bang. Downfolded ridges are more common than upfoldded (although the reverse is believed to be the case in Sweden, at least on new ice)

  Classic downfolded ridge (left half of picture). Photo by Jeff Brown

Folded ridges often come apart within a few hours after they form.  The plates float to the surface making a particularly treacherous condition (a loose plate ridge).  The plates on over-under ridges are typically better bound to the ice sheet so crossing them is generally less dicey.  Folded ridges often occur at the end of a much longer over-under ridge that terminates away from shore.  If you walk to the end of the folded section (often a couple hundred yards or less) it will change into a tectonic crack which can generally be crossed easily.  Folded plates are pushed into warmer water so they will melt over time.  Early in the season, before the water is fully cooled this happens faster.

Eroded underwater plate of a downfolded ridge: Shelburne Pond, VT , December

General suggestions for crossing ridges: 

  • Don’t do it in vehicles
  • If you are on foot, have a buddy or two and everybody with throw ropes, and ice claws
  • Have someone in the group with experience at this. 
  • Look for a reasonable place to cross, most often the best prospects are where the ridge overlap switches sides   
  • Often many of these ridge switches are less than  ideal.   Keep looking until you find a reasonable looking place to cross.  Use a test pole to check for thin ice and loose plates.  There is often thin or no ice in the center of the ridge.  At a good crossing this will only be a few inches wide.
  • Before crossing use your pole to stab for thin spots and push hard for several seconds to test plate stability.
  • A Big step (generally sideways on skates) is often the best way to make the manoeuvre.
  • Once across move promptly away from the crack in case the plate you are standing on is not as well attached as you thought it was.
  • Be patient and be prepared to go back to shore if you can't find an acceptable crossing
  • An iceboat can often be used as a bridge.
  • Sailing across ridges is always a dumb idea.  Sooner or later one of the runners will get caught and you will break the boat and maybe yourself. 
  • Keep in mind that a ridge that was reasonable to cross in the morning may be a mess a couple hours later.