On the Sea Ice
Although planes land on the ice and people trek out on it every day, it is dangerous to be out on the sea ice if you don't know how to identify cracks. Before we spent a lot of time out on the ice, Joni and I went to a sea ice training class to learn how to identify danger.
The Hagglund
As with all trainings I've encountered so far, we took an interesting vehicle to get out onto the ice. We drove out to the sea ice in a Hagglund. Every vehicle has a different tire or track length. The longer the track length, the safer it is to travel over a sea ice crack. The rule of thumb is that the ice crack should be one third the width of the tire track length or less to safely travel over it.
Michelle Brown sits in the driver seat of a Hagglund during Sea Ice Training.
A full view of the Hagglund, showing the two cabs.
Finding an Ice Crack
We drove out on the sea ice looking for evidence of cracking. Although the sea ice seems still and lifeless, it is constantly moving. Sometimes the ice moves towards each other or land and buckles upward, similar to how plates create mountains. Tides can also affect the sea ice, creating large cracks. We learned that there are different types of cracks:
compressing cracks: cracks that are being pushed together
contracting cracks: cracks that are being pulled apart
tidal cracks: cracks caused by the rising and falling tides
proximal cracks: those that come off of a larger crack
straight edge cracks: cracks that follow a straight path
working cracks: active cracks with different vertical levels
We soon found a proximal crack coming off of a large crack called the Big John Crack, which extends out from Hut Point into McMurdo Sound. We stopped to investigate the crack and see if it was safe to pass.
Corey shows us a proximal sea ice crack that runs off of the Big John Crack at Sea Ice training.
Drilling to know the depth
When you come across a crack in the ice that is unsafe to pass (i.e., it doesn't follow the one-third of your tire/track length rule), you need to measure the depth of the ice surrounding it.
First we dug a trench to get to the ice layer since there was snow covering the ice. We then drilled holes sequentially crossing the crack. We used hand-powered drills to make it easier. We drilled until we hit the seawater below. Then we sent down a measuring tape with a cylindrical weight attached to it. The cylindrical weight caught at the bottom of the ice hole, allowing us to measure the depth of the ice.
Michelle Brown uses a motorized drill to make a hole in the sea ice.
Corey shows us how to use a measuring tape and weighted cylinder to measure the depth of the ice.
Michelle Brown uses the measuring tape to record the depth of the sea ice.
We repeated this (drilling holes and measuring the ice depths) until we crossed over the ice crack and had a good idea of how deep all the ice before and after it was. Corey, our instructor, recorded the depths and the distances between each drilled hole in his notebook. Although the ice was 47cm thick near the crack, it was much thicker as it went outwards. If the ice crack is small, and the surrounding ice is about 75 cm or thicker near the crack, it is safe for Pisten Bullies and similar vehicles to cross it.
Corey shows us how to use a measuring tape and weighted cylinder to measure the depth of the ice.
After drilling and recording various sea ice depths, Corey shows a profile of the sea ice using the data.
It's a lot of work to drill holes and measure their depths every time you see evidence of an ice crack. A shortcut for measuring ice thickness is to use the length of the drill to estimate ice thickness. The length of the drill is about one meter. Since 75 cm or so is a safe thickness, if the drill goes 3/4ths of the way into the ice, then you can assume it is safe.
Securing a tent on the ice?
The last thing we learned about in Sea Ice training was how to put up a tent on the ice. It's a lot harder than in the snow or dirt, since you can't just dig a hole to put your tent stakes into. Instead we drilled two holes into the ice at angles so that the holes connected. Then we fished string through the holes so that the string had a strong anchor under the ice and could attach to the bottom of the tent. As hard as we pulled, the string wouldn't budge! A great solution in case we ever get stuck out on the ice and need to set up a tent
Michelle proves her tent would be sturdy if it was connected to the strings looped through the ice, a technique learned at Sea Ice Training.
Rope clearly loops through the ice as Joni pulls on them.
Questions
Why do you think there are such different vehicles in Antarctica? Can you think of other places that have unique vehicles?
What skills do you need to have to cross a crack in the ice safely?
Math Connections
- Look at the picture that shows the depths of the ice at different points.
a. What is the mean depth of the ice?
b. What is the mode of the data?
c. What is the median of the data?