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Ice Grain Structure

by Bob Dill - March 1990 - Revised January 1991

How ice forms in the Fall has a lot to do with how it behaves in an advanced thaw condition in the Spring. The following article reviews how water ice forms into one of two types of ice that are very different from each other.

There are two basic types of ice: water ice and snow ice. Water ice is ice that forms under the surface of the ice sheet by water solidifying into the crystal structure of the bottom of the ice sheet. It has a grain structure that is uniform through the ice thickness. It contains relatively little dissolved air.

Snow ice is any ice that forms from snow or slush freezing on the top surface. It has a grain structure comprised of the ice granules from the original slush. It contains a lot of air trapped between the ice gains.

This article primarily concerns water ice. In most cases the snow ice will melt off the underlying water ice after an extended thaw because it strongly absorbs sunlight.

Melting Factors

Ice melts in two ways, on the surface and internally. Surface melting is caused by warm air, especially with wind and to a lesser extent, rain. In December 1990 we had 4" plate of ice on a" pond near Burlington. 24 hours of 47-50 degree temperatures, 15-30 mph wind and less than an inch of rain melted 2 inches of the 4" plate and completely melted 2 + inches of ice on several" warm water holes. In Syracuse, Indiana at about the same time, they had similar ice exposed to the same temperatures with 4 inches of rain. They had very little ice loss.

Puddles of water on the ice, whether from warm wind or rain. They drain through any crack or hole in the ice. The water erodes little holes into a runner sized one. There are some excellent pictures of this in Think Ice. The size of the holes is dependent on how much water drains through them and how warm the water is. They can form to runner size in less than 24 hours in 12 inch thick ice. This often happens in mid-season thaws.

Internal melting of thick ice is usually a more gradual process, requiring several days of sunny, above freezing days and warm nights. Of course, if the ice is only a few inches thick and it is 50+ degrees and sunny, things can happen fast!

Ice Grain Types

Water ice freezes into crystal families called grains. The melting takes place between the grain boundaries of the ice. The melting point of water is slightly lower at the grain boundary because the crystal structure is less perfect there.

This first shows up in black ice as sheets and strings of bubbles in the ice along the grain boundaries. The ice becomes gray from the bubbles after a while. After an extended period of heating by the sun the grain boundaries will melt all the way to the bottom of the ice, forming many tiny drain holes. This is evident when all the puddles disappear from the ice surface of thawing ice.

When water ice is exposed to prolonged sun at above freezing temperatures it thaws into either honeycombed ice or, if the ice grains are larger, into interlocked grains. In the latter stages of the thawing process there is a big difference in the weight bearing ability of these two grain structures.

Well over a foot of honeycombed ice may not support a man or a DN. A piece of large grain ice only three inches thick with the same thaw history will support a man. It can sag as much as 6 inches before breaking.

The grain structure of the ice is determined by the conditions of the ice when it freezes. When ice freezes an ice crystal starts growing at a nucleation site on the surface.

Large Grain Ice

A nucleation site is something the cold water molecules will accept as a seed crystal. It can be a small ice crystal, a snow flake, or a piece of dirt the water accepts for an ice crystal. If the surface freezes in still conditions with no snow falling. relatively few nucleation sites are available. The ice crystals will form long, flat spears (they are dendrites, look like Christmas trees and can be seen in any still water in the process of freezing). The spears grow until they run into one another. They then fill in the intervening surface with dentritic branches.

If it is not disturbed, the first thin skin of ice determines the grain structure for the ice that forms as the sheet thickens. Some of the spears grow rotated in the water surface. The ice grains that develop from the rotated spears will grow at an angle the surface. The grains mechanically lock together because the grain boundaries are relatively large planes which have a lot of interlocking between the grains.

Small Grain Ice

Small Grain ice (Honeycomb ice) starts with a lot more nucleation sites. They can be snow or frazzle. Even a very light snow provides plenty of nucleation sites to form a small grain structure. If you look closely at ice forming in still conditions with a little snow falling on its surface, you can see the small grain structure developing. Frazzle is broken ice spear slush that forms in water disturbed by waves. In cold, wavy conditions frazzle can form a thick slush on the surface.

The ice that forms with a lot of nucleation sites has grain boundaries typically less than an inch apart. In this case the grains constrain each other so much they grow almost Straight down. This results in the long narrow grain structure typical of honeycombed ice. The relative weakness of small grain ice is a result of the grain boundaries being interlocked on a much smaller scale. This small scale interlocking melts out quickly when the ice melts.

Ice forming on still water with no snow falling will have the largest grain size. As the water is more disturbed the grain size will become smaller. If even a very light snow is falling, small grain ice is almost certain.

Safety on Thawed Ice

If you find yourself walking across well thawed spring ice, looking at the surface of the ice may tell you which ice is strongest. Using a probe (ice chisel or the like) to test the ice before you walk on it is always a good idea on weak or unknown ice.

Small Grain ice is typically dark with a small grain structure made up mostly of 3 to 6 sided polygons. The average dimension of these polygons is generally less than 1 1/4 inches. It chips out into small columnar pieces then hit with the probe. It may be possible to drive the probe through a foot or more if this ice. If the ice feels like it is settling a little when you walk on it, it is!. If you stomp on it you can sometime punch a hole through it. A good time for a dry suit and a belay rope.

A Large Grain Ice surface distinctly shows a much larger and more irregular grain structure. The typical grain dimensions range from 3 inches to as much as 2 feet. This ice tends to have a grayer color in advanced thaw condition. It chips into chunks when poked hard. In an advanced thaw condition, it is much stronger than small grain ice.

It is common to have areas of different grain types mixed together. The size of the sheets of one ice type vary from entire lakes or bays to a patchwork of mixed pieces a few feet in size. Broken up plates of one type often fill in with the other type.

Cobblestone Ice

When a thick ice is thawed enough to melt the grain boundaries all the way through the sheet, an unusual and very rough ice surface can result. The ice melts more at the grain boundaries than on the grains between the boundaries, After refreezing, this can result in a regular but very rough, polished surface. The dimension of the roughness is proportional to the grain size. In large grain ice the result is much like a cobble stone surface. This probably accounted for the extremely rough conditions at World Championship in Arsunda Sweden last year.