Sea Ice

Ice floats because it's less dense than water, ~20% less dense.  Thus, 80% of an ice berg or an ice cube is below the water.  The same is true for sea ice.  Sea ice forms directly from ocean water.  The salts in the ocean reduce the freezing temperature of seawater from 0°C to -1.8°C.  When sea water cools down to -1.8°C, ice crystals start to form.  They can form at the surface or within the water.  If they form within the water, they float to the top.  Since most of the water is not at the surface and the surface of the ocean tends to move, most of the crystals form within the water and float up.  There, they bind together to form the first layer of sea ice.  (If you want to read about the details, try this page: http://nsidc.org/seaice/characteristics/formation.html.)

After a layer of ice forms, it's temperature can go below -1.8°C.  At McMurdo, the temperature has been fairly consistently averaging -20°C, and much of the surface of the sea ice is around -20°C.  However, the bottom of the sea ice is the same temperature as the water, -1.8°C.  Thus, heat is being conducted from the bottom of the sea ice to the surface (like it is from my body through my clothes and sleeping bag as described in an earlier post).  The water loses heat, which causes it to freeze.  This makes the ice sheet grow thicker with time.  It also forms the beautiful crystals that Bekah raved about in her under-ice dive blog.

As the ice sheet grows thicker, more ice sticks out of the water.  It is still only 20%, but 20% of a larger thickness is larger.  We measured the thickness of sea ice on our trip out yesterday.  We do this by drilling a hole through the ice until we hit water.  We used both a brace (you twist the upper part by hand) and a tiny motor.  With a sharp bit at the end of the trace (the part with the spirals), it is easy to drill through the ice by hand.

Then we lower a measuring tape down the hole.  It has a bar and a wire attached to it so that you can make it catch on the bottom of the ice and then release it by pulling the wire.

Most of the ice that formed this last winter is between 1.6 and 2 meters thick.  The ice that formed several years ago is only slightly thicker because once ice gets that thick, not much heat is conducted through the ice so the water below the ice doesn't freeze very quickly.  Also, it becomes thinner as it melts in the summer even if it doesn't go away completely. 

Wind, tides, currents and waves produce stress in sea ice sheets.  When there is enough pressure put on the ice, it breaks into cracks (when the stresses are pulling it apart) or forms ridges (when the stresses are compressing it).  Let's examine what happens when it cracks: there is open water between the two pieces of ice.

The open water is below the top of the sea ice because the sea ice is less dense than water.  Let's say that snow blows into the crack.  You wouldn't necessarily know that the snow was just on top of open water rather than ice.

 

Often, the snow piles up in a line along the crack.  Thus, if you see a long line of snow, there may be a crack under it, and you want to test it before walking or driving over it.  You can do a quick test by plunging an ice axe into the snow to see if you hit hard ice or slushy water.

Now let's look at a crack that just freezes closed rather than becomes filled with snow.  As the water starts to freeze, the level of the new ice is lower than the old ice because the new ice forms at the water surface.  Also, the 20% rule says that thinner ice has a lower top surface than thicker ice:

 

If the ice is thick enough, you can walk or drive on the crack.  The rules for driving over cracks are:  1) The ice on either side of the crack must be at least 30 inches thick (thicker for bulldozers and other heavy vehicles).  If it isn't, you shouldn't be driving on the ice at all.  2) If the crack is less than 1/3 of the length of your vehicle track, you can drive over it safely (most of the time) even if it is open water.  Your track is not going to fall into the crack.  However, if multiple cracks are nearby, you want to pay attention to the geometry of their intersections.  You might break off a triangle piece and end up in the water, which, as I've mentioned, is freezing cold.  This is just not a good idea.  3) If the crack is more than 1/3 of the length of your vehicle track, you should not drive over it unless the ice in the crack is at least 30 inches thick.

So let's look at what happens as the crack ice gets thicker.  It grows thicker at the bottom, but it is also attached to the neighboring sea ice.  It wants to float higher as it gets thicker (the 20% rule), but it can't without breaking away from the ice forming the crack edges.  Two things can happen:  1) the crack could reopen along the edge, allowing the ice in the first crack to float higher; or 2) the ice in the crack could be held down deeper in the water than the 20% rule suggests.  

If the edges of the ice break away, you get complicated cracks with multiple breaks and ice levels in them.  To see if you can drive across them, you have to drill multiple holes, one in each ice level.  If the ice is depressed, water fills the crack when you drill through the ice.  This happened to us.

Cracks commonly fill with snow whether or not they have ice under them because they are lower than the surrounding ice.  This photo, which should be familiar to you now, shows a wide crack filled with 1.3 meters of ice with about 30 cm of snow filling it in.  The contrast of the bare ice and the snow makes the crack obvious.

However, Matt said that the whole area was covered with snow the day before we went out, and it was much harder to see.  The crack shows up as just a difference in the texture of snow.

This time of year, most of the sea ice is fairly safe to drive on.  There are some big cracks, but they tend to freeze over quickly.  However, as we get close to summer, the cracks become a bigger problem and pose greater dangers.  At some point in the late spring, it is too dangerous to drive on the sea ice at all.  Occasionally, but not every year, all the sea ice in front of McMurdo breaks up and floats north to melt.  It's hard to believe that the view out the window today might be open ocean by the time I get back from the field.

Snow blowing across the sea ice.  This is looking in the direction of Mt. Discovery, but all you can see is blowing snow and clouds.