Pressure Ridges, Cracks and Slush on Ice: What to Know

The short answer

Pressure ridges, cracks and slush all mean the ice is not stable there. Stay off them.

A pressure ridge is a long ridge of ice that forms along a crack. The Minnesota DNR says pressure ridges are always dangerous. Ice can buckle at a ridge, and you can also find open water along one. Never drive or ride over a ridge. Follow it at a safe distance until you find a safe place to cross, or turn back.

Cracks come in two kinds. A dry crack does not go through the ice. A wet crack does, and it has water in it. For a wet crack, an Army Corps of Engineers ice page says to double the required minimum thickness. It also says to cross cracks as close to a right angle as you can, not along them.

Slush is a warning sign. The DNR says it adds weight to the ice and its effects can be unpredictable. When you measure slush-covered ice, count only the clear ice, not the slush or snow on top.

Booming and cracking sounds are usually normal. The ice is expanding and contracting as the temperature changes.

No ice is ever 100% safe. Check it yourself.

What causes pressure ridges and cracks

Ice is always moving, even when it looks locked up.

The Minnesota DNR explains it this way. Cracks form because the top and bottom of the ice sheet contract at different rates. Water gets into the cracks and freezes. The sheet expands a little. When air temperature rises, the extra expansion pushes hard against the shore. Alternate warming and cooling keeps adding push.

Ice also cracks because its edges are sometimes firmly attached to the shoreline. The DNR says cracking is especially common in years when the ice has no insulating snow cover.

Minnesota Sea Grant gives a sense of scale. In northern Minnesota in December and February, the average daily temperature range is about 20°F, and it can reach 40°F. For a round lake about a mile across, a 20°F warm-up would expand the ice by about four feet. A 40°F warm-up would expand it by almost 10 feet.

Wind adds to it. Minnesota DNR coverage says pressure ridges are caused by temperature changes and strong winds. They can show up even where the ice is very thick. The DNR also says they can form in a matter of hours.

How big can they get? The DNR says ridges range from a few inches to over 10 feet high. On shore, the same force piles soil and rock into mounds called ice ridges, ice pushes or ramparts. The DNR says those can be as high as five feet or more.

Wet cracks and dry cracks

Check every crack before you commit to it.

• Dry crack. The Army Corps page says a dry crack does not go through the ice sheet and is not a problem.
• Wet crack. The same page says to multiply the required minimum thickness by 2.

That page says to cross cracks as close to perpendicular as you can, not parallel to them. Its load table is for clear, sound ice with no flowing water underneath, so treat it as a rough guide and check the ice yourself.

For reference, the DNR minimums for new, clear ice are 5 to 7 inches for a snowmobile or small ATV, 7 to 8 inches for a side-by-side, and 20 inches or more for a heavy-duty truck with a wheelhouse shelter. Those are for good ice. They are a floor, not a promise.

Why ice booms and cracks

Think of a frozen lake as a very large drumhead. Minnesota Sea Grant uses that picture. The ice warms and cools with sun and air. As it cools it shrinks away from the shore. As it warms it pushes back.

The DNR says booming and cracking ice is not necessarily dangerous. It only means the ice is expanding and contracting. Winter ice is quieter than early ice. It is thicker, heavier and often snow covered, and those things dampen the noise.

The sound is not the hazard. The crack, the ridge and the water are.

How to deal with a ridge or crack

The honest answer: avoid it.

• Do not cross a pressure ridge. The DNR says crossing is very dangerous.
• If a ridge blocks your way back to shore, go along it until you find a safe spot. Do not force a crossing.
• Expect open water along a ridge. It can be hidden by snow.
• Wear a flotation device and carry ice picks.
• Do not trust old tracks in the snow. Check the thickness for yourself.

If you are out at night, talk to local experts first and use extreme caution.

On shore, ridges push mud, rock and soil up the bank. The Minnesota DNR has a permit rule for removing them. An individual permit is not needed if the ridge formed within the last year and the work meets set limits. Check with your own state first. This guide does not cover the details.

Slush: the quiet hazard

Snow looks harmless. On ice it is not.

The DNR says the insulating effect of snow slows down freezing. The extra weight also reduces how much weight the ice can support. Minnesota Sea Grant adds that snow drifting onto thinner ice slows its thickening through insulation.

Water that reaches the surface wets the snow. When that mix freezes it makes snow ice, also called white ice. Sea Grant describes snow ice as forming when snow is wetted by outside water or melt. The DNR says white ice is only about half as strong as new, clear ice. Double the thickness guidelines on it.

Driving and riding in slush

Slush is a travel problem as well as a strength problem.

• Weight on slushy ice is risky. The DNR says slush adds weight and its effects can be unpredictable.
• In a December 2022 DNR report after a heavy, wet snowstorm, the DNR said that in some areas, especially where ice was newly forming, conditions could not support shelters and vehicles and led to break-throughs.
• Heavier machines add more load. Trucks and wheelhouses need the most clear ice.

Check the clear ice under the slush before you go on.

If you break through

The Minnesota DNR lists these steps:

1. Do not remove your winter clothing. Heavy garments trap air for warmth and flotation.
2. Turn toward the direction you came. That ice likely held you.
3. Place your hands and arms on the unbroken surface.
4. Kick your feet and dig in your ice picks to work your way back onto solid ice.
5. Lie flat once you are out, and roll away from the hole.
6. Get to a warm, dry, sheltered place and re-warm right away. Seek medical help for moderate to severe hypothermia.

Tell someone your plan before you go. Wisconsin DNR also asks you to wear a flotation device and check ice with a spud bar as you go.

No ice is ever 100% safe. Check it yourself.

FAQ

Is it safe to cross a pressure ridge?

No. The DNR says crossing a ridge is very dangerous. Ice can buckle there and open water can sit along it. Follow the ridge to a safe spot or go back.

Are cracking and booming sounds dangerous?

Usually not. The DNR says it only means the ice is expanding and contracting as the temperature changes. Still check the ice where you are.

What is the difference between a wet crack and a dry crack?

A dry crack does not go through the ice. A wet crack does, and shows water. For a wet crack, double the thickness you need for your vehicle.

Why is slush bad for ice?

Snow insulates the ice and slows freezing. The weight also cuts what the ice can hold. The DNR says slush adds weight and its effects can be unpredictable. White ice, which forms when snow melts and refreezes, is only about half as strong as new, clear ice.

How thick should ice be for a snowmobile?

For new, clear ice, the DNR lists 5 to 7 inches. Double that for white ice. No ice is ever 100% safe. Check it yourself.