All about Ice: Part III
Why do cracks form in the ice?
Have you ever noticed the long cracks that form in the ice on Lake Windermere? Some of them are short and thin, others are very long and can sometimes be up to an inch wide. Why do these cracks form, and are they dangerous to walk, drive, or skate over?
In the last article, we learned how ice grows. It starts when a water molecule becomes super-cooled and nucleates to form an ice molecule, growing steadily outwards until a thin layer of surface ice is formed. Then, ice grows downwards and gets thicker, forming an ice sheet as more molecules are added to the bottom surface. In this article, we will explore what happens when this ice sheet undergoes pressures and tensions, and why cracks often form and propagate throughout the ice surface.
Stress causes deformation
All cracks in ice are caused by stress in the ice. The word “stress” doesn’t mean emotional stress, such as you might feel when an exam is coming up or when you’re having a bad day. In scientific terms, the word “stress” describes when particles exert an internal force on neighbouring particles. You can imagine stress like standing in a crowded room, shoulder to shoulder with other people, and someone begins to lean slightly to the left; everyone around that person will also be pushed to the left, and the way people are standing in the room will shift accordingly.
Physicists talk about stress in terms of a force being applied over an area. Stress in ice can be locally distributed (small area) or broadly distributed (large area) across an ice sheet.
The stress we speak of comes mostly from temperature changes in the ice sheet, wind, and waves – these actions exert a force on the ice, which causes its internal molecules to shift in order to accommodate the stress.
Stress can be translated as tension (stretching force) or compression (compressing force). Materials often have a threshold, or breaking point, for stress. This is the point at which the forces acting on the ice overcome the breaking point and cause the ice to crack.
Very new ice generally does not have many cracks because it has experienced relatively little stress; but by the time it is a few inches thick and the sheet has been through a couple of warm-cold cycles, cracks are all over the place.
Deformation causes cracking
That tension and compression, when exerted long enough over the right density and thickness of ice, causes the ice to crack in certain ways. Natural cracking occurs during the entire season, but is most frequent during the spring and early winter. Not all cracks will affect the strength of the ice, however some do decrease the ice strength by varying degrees.
As ice warms and cools, like during a sunny winter day, it expands and shrinks. This causes tension and compression in the ice sheet, making the ice weaker and resulting in cracking.
There are many different types of cracks:
Dry cracks – as the name suggests; a crack with no water present
- Cracks do not penetrate all the way through the ice.
- Make a loud “boom” or “ping” when they occur (the cause for ice “singing” seen in some popular Youtube videos!)
Wet cracks – a crack filled with water
- Usually signals a crack that has penetrated the ice, allowing water to rise up through the crack.
- Not to be confused with a wide, dry crack that has filled with rain water or other surface water.
- Can be a very dangerous crack if it turns into a pressure ridge.
- A wet crack roughly cuts the normal strength of ice in half.
Pressure ridges or elastic compression areas – areas where compression forces push sheets of ice together
- Pressure ridges are compression ruptures that signify areas of weak and tricky-to-cross ice.
- Typically form when large temperature fluctuations occur.
- Pressure ridges generally run between points of land across the lake, or across bay mouths. They also frequently form along the shore when the lake edge is too steep to push the ice sheet up onto the beach.
- These are especially common places for vehicles to go through the ice.
Load-induced cracks – caused by a heavy weight applied to the surface of the ice (e.g., a vehicle)
- A load on the surface exceeds the strength of the ice
- Radial cracks extend straight outward from the load.
- These indicate the ice is supporting about half it’s maximum weight, and the load should be moved to avoid further disruption of the ice.
- Circumferential cracks form in circular shape around the load, between radial cracks (like a spiderweb).
- This is a very serious sign that the load may be about to break through.
- Combination –> when circumferential cracks connect two radial cracks, triangular ice wedges are formed that may plunge into the water without notice. This is extremely dangerous, and you must leave the area immediately.
- Radial cracks extend straight outward from the load.
Expansion cracks – caused by rapidly warming temperatures
- Ice expands and moves upwards in locations that have thinner ice cover.
- Can form pressure ridges in conjunction with contraction cracks.
Contraction cracks – caused by rapidly cooling temperatures
- Ice contracts, and tension causes ice sheet to crack and separate.
- Typically 1/2 inch to 1 inch wide, relatively straight. Can often run parallel to one another.
- If a contraction crack fills with water then re-freezes, there is the possibility of a pressure ridge forming.
Wind cracks – caused by effects of wind on wet cracks
- High winds (above 55km/h) can form ridges when blowing over open cracks that contain water.
- These ridges usually extend in a parallel or perpendicular direction to the shoreline
Water level fluctuation cracks – caused by a rise or drop in water level
- A change in water level causes ice sheet to move from where it’s anchored to shore.
- This results in a significant amount of wet cracks, which can be dangerous and form pressure ridges due to processes described above.
Dynamic wave cracks – caused by wave action underneath the ice
- Vehicles travelling quickly over thin ice can cause waves under the ice sheet.
- In areas of thinner ice, waves may crack and rupture the ice.
- A slow moving vehicle deflects ice slowly, which allows waves to pass the vehicle. This provides time for the ice’s crystal lattice to re-adjust and it can maintain almost all of its initial strength, thus resulting in fewer cracks.
Staying safe around cracks in the ice
For the most part, many of the cracks we encounter on Lake Windermere will not be dangerous to our safety as we skate, ride, or walk across them.
If you notice a pressure ridge or other wide, wet crack while out on the ice, it might be better to avoid that area and go around it if you can.
Along the shorelines, ice can sometimes become hollow if the water recedes beneath it – these areas may crack underfoot if the ice is not thick enough to support your weight; but generally, the area underneath the hollow ice will be just dry beach. Ankle or leg injuries could still occur, so be cautious when walking near the lake’s frozen edge.
As you know, the thinner the ice is the more prone it is to crack under significant weight. Only go out on the ice if you can be sure it is thick enough to support several times your weight.
Vehicles are involved in more than half of all ice-related fatalities. When driving a vehicle on the ice, your risks are compounded and the dangers of breaking through the ice, especially at a point where dangerous pressure ridges or circumferential cracks begin to form, increases.
Whenever possible, only drive a vehicle on the ice if you absolutely must, and make sure you know the route you will be travelling. Travel at slow speeds whenever possible (<15 km/h), to reduce the amount of waves you generate and help keep the ice at good strength.
That’s it for this year’s All About Ice series! We’ll be back next winter with more fun facts about lake ice and tips for staying safe and keeping the lake clean throughout the winter. See you next fall!