Pebble Ice

Dr. James R. Carter, Professor Emeritus

Geography-Geology-Environment Department

Illinois State University, Normal IL 61790-4400

In the process of Ice Segregation cold water moves through a medium toward the presence of ice, freezes and adds to the ice. When this occurs at the surface of soil, it produces Needle Ice . When it occurs on plant stems it produces Ice Flowers . When it occurs on pieces of dead wood the resulting ice is called Hair Ice .  Recently it has been found to occur on small rocks and pieces of brick and pottery, all the size of pebbles. (4 - 64 mm / 0.016 - 2.5 in.) Thus, I propose to call this form of ice Pebble Ice.

In all cases these growths of ice occur at the Earth’s surface when moisture in the medium is liquid and the air temperature is below freezing. Normally such conditions occur over night in late fall, winter and early spring. As long as there is a balance of energy flows keeping the water in the pebble from freezing and keeping the surface of the pebble below freezing ice can continue to grow on the pebble to create rods and ribbons of ice, sometimes far larger than the host pebble.

Ice Segregation  is known to occur in subsurface environments and can be a significant geologic force in areas with permafrost.

Pebble Ice in Nature

Figure 1 -- I saw my first example of pebble ice in 2005 when I found a straight rod of ice about 5 cm (2 in.) long in an area where I found ice flowers and needle ice. I had no idea what that rod of ice was or how it was formed. I took some photos, picked it up, but did not probe to see what was at the base of the rod of ice. I now know that this rod was an example of pebble ice. Had I dug down I would have found the ice was growing from a pebble.

Then I received photos from colleagues in Alabama and Missouri.

Figure 2 -- Three examples of Pebble Ice captured by Jared Wilson of Missouri. The lengths of these examples are more than 3 cm (1 inch) long. In the photo on the right some residual snow is evident. The pebble in this photo is probably less than 1 cm across.

Figure 3 -- Three examples of ice growing from pebbles in the yard of Jerry Green of Alabama. The lengths of this ice are more than 1 inch long (2.5 cm) and he notes the rod of ice on the right is 2 inches tall (5 cm). Note in the image on the left the rod of ice has lifted a larger pebble. These pebbles are about 1 cm across.

Figure 4 -- Two more photos from Jerry Green of Alabama. In the photo on the left a twig with some ice on it was lifted as the ice grew up from the pebble. In the photo on the right we again see that as the ice started to grow it picked up another pebble and lifted it.

For more images of such ice from Jerry Green see:

Ice Flowers, Crystals and Ribbons - Gofish (smugmug.com)

My colleagues in Alabama and Missouri sent me pebbles on which they had seen ice grow. I put those pebbles in my yard where I had seen needle ice. Indeed, ice did grow on some of those pebbles, but not as dramatic as shown in their photos. But it demonstrated the process that produced needle ice also produces growths of ice on small rocks.

Figure 5 -- A pebble was placed in soil in which needle ice was known to form. That small rock did produce Pebble Ice, standing above the needle ice. By 3 PM warming turned the soil to mud and partially melted the pebble ice. The next morning there was new pebble ice pushing up the ice cap from the day before and new needle ice. That afternoon by 4 PM warming did away with the needle ice and melted part of the pebble ice cap. In the photo on the right the boundary between the ice produced the first day is distinguishable from the ice produced the second day.

Recently I found that there are two postings of images of ice on pebbles on the Story of Snow web pages. On the first page the author tells of seeing columns of ice standing apart from needle ice in a rice field. He noted the cap of ice easily and cleanly separated from the pebble. This is obviously Pebble Ice. On a second page written in 2013 the author shows photos of ice growing from asphalt and offers an explanation of the processes underlying such growths of ice and suggests the name Pebble Caps for such ice formations.

Growing Pebble Ice in a Freezer

Rather than wait for cold mornings to produce such ice, I set out to replicate the process in a refrigerator freezer. I used a small ice chest that fits in the freezer. I put a small light bulb in a clear plastic bucket in the bottom to generate heat. I overlaid a sheet of aluminum to disperse the heat and then set another plastic bucket with wet sand over my heat source - the light bulb. I packed insulation between the walls of the chest and the plastic buckets. I then adjusted the amount of light with a rheostat to find the right combination of temperatures that might grow ice on the pebbles placed on the wet sand/soil. Sometimes it worked very well.

Figure 6 -- On the left is the picnic ice chest which I put into the freezer of a refrigerator.  Note the small plastic bucket filled with wet sand and five pebbles that extend above the sand surface.  The three pebbles on the left have ice on them.  On the right is the light bulb that is the heat source.

Observations on Growing Ice in a Freezer

Growing ice in my refrigerator has permitted me to try many different pebbles under different conditions. I have had ice grow on a few pebbles and pieces of clay pots and brick. After many trials and thousands of photos, I show what I have discovered in the selected images below.

Advantages:

  You can do it 12 months of the year

  You can experiment by varying only one factor at a time

  It is quite inexpensive but gives considerable insights

  Being inside gives access to electricity, lighting, water and other equipment.

Disadvantages:

  The height of the water in the bucket of sand can fall below the base of a pebble and I have found no way to add water during a growth session.  When this happens, growth stops for the lack of water.  In nature soil moisture is drawn from a larger area and may include some flowage.

The top of the freezer limited the height to which the ice could grow, on occasion.

  The setup does not permit setting precise standards for temperature and humidity, so you work with what you get. 

Other observations:

  There appears to be no difference between using tap water and distilled water

  It seems to be impossible to replicate the growths of ice on the same pebbles in the same position, suggesting some randomness in the process. 

On the nature of the pebbles which grow ice

Using pebbles provided me by my colleagues in Missouri and Alabama, plus what I have found in many places in my travels, I have come to understand something about which pebbles will grow ice. Early in my experience I found ice growing from a small, flat red pebble (Figure 7) in my yard which turned out to be a weathered piece of brick (about 2 cm long, 0.8 in). I then added pieces of brick and pieces of broken pots to the collection of pebbles I experimented with.

Because such growths are produced by ice segregation the pebble must be permeable enough to permit water to move through it but not so porous that water will drain from pores.  Such pebbles when dry will wick-up moisture displaying capillarity.

To identify pebbles that might have such capillarity I weighed dry pebbles, then wetted them on a sponge for 15 seconds and then weighed them again.  If the pebble gained 0.02 mg or more then it was likely to produce ice through ice segregation.  Most pebbles tested gained no noticeable weight and did not grow ice. 

A petrologist noted that another test is to place the pebble against your tongue.  If it will wick-up water through capillarity it will stick to your tongue.  It works.

Good Examples of Pebble Ice Grown in the Freezer

Figure 8 -- Above are three photos of the same growth of ice which is quite voluminous. Note that some of the sand is picked up as the ice grows. I do not remember details about the pebble.

The pebble below from Jerry Green measures 5 X 3.5 X 1 cm (2 X 1.4 X 0.4 in)

Figure 9 - When this pebble was inserted vertical and buried rather deep, it produced a substantial band of ice near the top (upper left). Note how it sweeps off to the sides and blends with the ice in the back. Because in this case it was placed deep into the soil there is a larger supply of water that could be brought to the surface of the pebble. Even then ice does not extend all the way to the top of the pebble. By contrast when the pebble is shallow in the sand it can tap into less water and thus, we see less ice (upper right).

When placed flat on the sand a thin sheet of ice covered the entire surface (lower left). In the lower right image, the ice formed part way up the pebble and not at the sand surface. In this case none of the other pebbles in the setup had any ice, indicating the freezing plane was not at the sand surface but some 2 - 3 cm above the sand.

These four images show the water in the pebble takes the shortest route to the freezing surface. Thus, in the upper left image the diagonal route to the ice is shorter than going to the top of the rock. In most cases the ice grows from the sides of the pebbles.

The Pebble Ice Environment

This diagram shows the basic environment in which pebble ice forms. The sand is porous and contains water that will drain away under the influence of gravity. The pebble contains water that is held in place by capillary attraction. Once Ice Segregation starts, water moves from within the pebble to the freezing front, which is on the surface of the pebble. As new ice is formed on the pebble surface it pushes out the existing ice. As water moves to the freezing front additional water is wicked up through the pebble, lowering the level of water in the sand.

If the freezing plane is a little above the sand, pebble ice may form higher up on the pebble. This process will continue until the water level in the sand drops below the pebble, or the energy flows become unbalanced and freezing extends into the pebble and sand and everything freezes, or the freezing plane extends above the pebble and nothing freezes.

On the Process of Trying to Grow Ice

The diagram above shows conditions at the time the ice is growing on the pebble. When the cooler is placed in the freezer the temperature of the sand, pebbles and water are normally at about 20 degrees C (68 degrees F). The base of the sand may be warmer because the light bulb has been generating heat while the unit has been out of the freezer. Once the unit is placed back in the freezer everything has to cool to reach an equilibrium as shown in the diagram. Normally, I leave the setup in the freezer for 18 or more hours, many times for multiple days.

Before putting pebbles in the wet sand, I soak them in water. I put the wet pebbles in the sand and then remove any standing water from the top of the sand. Normally I then put the setup into the freezer.

To see how long the pebbles will retain any internal water, I have let them sit in the sand at room temperature for up to 22 hours to air-dry. Even when the top of the sand was dry, I sometimes got good ice on the more productive pebbles.

To initiate Ice Segregation there must be an ice crystal on the surface. The freezer has frost built up in it and normally there is some frost deposited on the sides of the plastic bucket holding the sand. So, I assume frost provides the ice crystals needed to start the process. However, there are times when no ice forms on one productive pebble while good ice forms on a productive pebble 2 cm away. Is that because a crystal of ice did not deposit on the pebble in the right place?

I would like to know much more about this process. It is a slow process to grow ice, but I have been at it long enough to get many examples that tell me about the basic process.

More Examples of Good Pebble Ice

Below are photos of ice grown in the freezer. The shapes of these ice formations vary considerably. In the diagram above the top of the pebble extends well above the freezing front, explaining why most ice will form along the sides of the pebble near the freezing plane.

Figure 10 -- In most cases the ice on pebbles in the freezer setup are fairly white, probably showing a mixture of air with the ice.  But, on occasion the ice is clear, as in these examples.  In the photo on the left it is hard to distinguish the ice because it is so clear.  You can see the underlying sand grains through this ice.  In the pebble on the right the ice seems to be emerging through a crack and not from the broad surface. 

Figure 11 -- At least two pebbles have produced ice of this form. On the left a small collar of ice emerged all the way around the base of the pebble and nothing from the top, leaving a hole in the center. The middle and right photos show two perspectives of the same growth of ice. In the middle photo the ice grew on the outside of the pebble forming a tube extending up about 3 cm. The photo on the right is looking down into the tube and showing the pebble surface with no ice. Note how many sand grains were lifted here.

In many examples there are growth ridges shown in the ice. To keep a relatively constant temperature a refrigerator cycles on and off. Measurements show the temperature in the freezer varies by about 1.2 degrees C over a 40-minute cycle. Although this is a fairly small temperature variation, in some cases it is reflected in the pattern of ice.

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Figure 12 -- In the two examples on the left the ice grew up as wings in these small pebbles. In the middle is a thin piece of brick where we see more ice emerging from the bottom than higher up, giving a rotation around the top of the piece of brick. In the photo on the right the growth ridges are fairly close together in the piece of clear ice.

The pebbles below show wings of ice growing out of the side of larger pebbles.

Figure 13 -- On the left are two large wings growing up on a large piece of chert called Wave. One wing shows distinct growth ridges. It appears a smaller third wing has grown toward the camera. In the image on the right there are small growth ridges but more obvious are the striations parallel to the direction of the growth of the ice.

I set up many different experiments over the years. I stacked up pebbles to see if ice would move through one pebble to lift another. Indeed, it did in some cases. Below are examples using Wave, one of the pebbles provided by Jared Wilson. Size 4x3x2.5 cm (1.6x1.2x1 in)

Figure 14 -- On the left ice grows out from the sides of the large piece of the chert named Wave. In the middle photo we see ice that emerged from the sides and top of Wave and another pebble that lifted a relatively large pebble. On the right a large volume of ice from the side of Wave lifted an overlying pebble and rotated it up through about 90 degrees. In these examples it appears more ice grew from the side of the lower pebble when another pebble was placed on top of it. This suggests that the presence of the overlying object influences the temperature of the underlying pebble.

The pebble I named Round-black from Alabama has been a very productive in growing ice. Below are three examples of the many that have been produced with this rock 4.5x2x2 cm (1.8x0.8x0.8 in)

Figure 15 -- This pebble is quite consistent in producing substantive displays of ice, probably because it is relatively long and seems to have the ability to draw water up through its length. As such it extends deeper into the wet sand than do most pebbles. The pebble is not of uniform composition and some parts of the surface do not support the growth of ice. Once a small piece broke off along cleavage lines. The large ribbon in the center photo is about 15 cm (6 in) long.

Figure 16 -- When I found the pebble above it was a half dome with a flat bottom - left. It has been a good producer of ice. In the center photo it shows that the ice grew a large rod of ice and then the freezing face descended into the pebble and split the pebble parallel to the bottom. The ice continued to grow pushing the upper part of the pebble away. After some melting the image on the right shows the top and bottom portions of the pebble and the ice in the middle.

The pebble below is a broken rock I found. 4x4x2 cm (1.6x1.6x0.8 in) This pebble was included because it is so common looking, yet still produced ice.

Figure 17 -- The relatively large pebble with a fractured concavity on one side shows the best growth of ice on the more exposed side. The ice is thickest at the sand interface and thins out moving up the rock. This is consistent with the idea that the water takes the shortest path to the freezing front.

As noted, I found a few pieces of brick and pottery that produce ice.

Figure 18 -- On the left is ice on a piece of brick inserted vertical into the sand. The other two photos show ice on a triangular piece of pottery, measuring 4x5x5x1 cm (1.6x2x2x0.4 in). The center photo shows that ice grew out in many directions from the piece of pot, while the photo on the right shows the growth of wings similar to those in other photos.

In my experiments I sometimes used a mix of soil with a large clay content as the base, rather than sand. The results below show what I got one time. Many times, I got a mess and nothing worth photographing.

Figure 19 -- In these examples water did not move through the soil and rocks to grow ice on top of the pebbles, but rather the ice emerged from the soil to lift the pebbles. In nature this is called Needle Ice . This needle ice shows that the ice carries soil as it emerges and thus is not very attractive. Also note the large cavity in the photo on the right, showing how the ice emerged rather uniformly from the soil and then a few strands of ice became dominant and pushed the structure up while breaking the connection of some of the ice with the soil. This is consistent with what I have seen in Needle Ice in nature.

After observing the results of hundreds of attempts to growth ice through the media of pebbles in a freezer in a refrigerator I have gained an understanding of the process. The physical process is Ice Segregation and is consistent with the products of ice segregation as seen in Ice Flowers on plant stems, Hair Ice on dead wood, Needle Ice in soil and in layers of rock and deposits in the subsurface.

The key here is to have pebbles and rocks that have the appropriate texture and porosity to transmit water to the surface to feed the growth of ice. Then these pebbles must be positioned so that they are wetted from below while being exposed to freezing temperatures on the surface.

Fig. 20 - In east Tennessee I found this mix of Needle Ice and Pebble Ice. The curved hook in the center is a good example of Pebble Ice and to the right is a good example of Needle Ice. There are 3 growths of Pebble Ice in this photo and many pebbles that have no ice on them. There are many clumps of Needle Ice and at left center the clump of Needle Ice has been tipped over by about 90 degrees.

Remember to look down on frosty morning, take photos and share them with a larger public. I give thanks to the Jerry Green and Jared Wilson for capturing such photos and sharing them with us.

Reflections on a World of Ice Formations

Fifteen years ago, I knew about icicles and patches of ice on streets and walkways, and that was about it. Now when it is above freezing part of the day and below freezing part of the day, I look for ice in many places, and often find it.  I have learned much by observing these many forms of ice. I hope you will join me in the search for ice in its many forms.

We know that Pebble Ice is not a form of frost, which comes about when the air becomes saturated and water vapor is deposited on a surface as an ice crystal.  If the air temperatures are above freezing, we get dew, but when below freezing water vapor is deposited as frost.  And, frost is required to initiate Ice Segregation.  For this reason, people may give the name frost to any formation of ice on anything at the surface.  There is no authority or process to regulate names plus it is fun to let our imagination run wild and assign names for what we see. But, do not get confused. There are at least three different types of features in nature people that use ‘frost’ as the appellation in the name.

Thank goodness for the Internet and digital cameras for they let us exchange information about these attractive ice formations.  Please look for interesting ice when the freeze/thaw processes are underway.   For additional perspectives on ice see  http://www.jrcarter.net/ice/

Feel free to contact me at  [email protected]   to share your photos of ice of this nature from your outings or freezer experiments.