Pressure Ridges

The pressure ridges with Castle Rock and Mt. Erebus in the background

I was able to get out to the pressure ridges during my first two weeks in McMurdo. I liked them so much that when the Rec department started looking for volunteers to lead pressure ridge tours, I signed up right away. Alister, the guide from my first tour, also ran the training for the volunteer guides. There were 11 of us in my class, and another 10 who got trained in later. We started with about an hour in the classroom, about the pressure ridges and some of the dangers we should be in the look out for when we take groups out.

The parking meter for Americans at Scott Base

So, what are the pressure ridges? In a lot of ways they are very similar to the mountain ranges that are created when plate tectonics shift. They are formed when the ice shelf (glacial ice, in this case the McMurdo Ice Shelf) collides with the sea ice that forms along the shore. The glacial ice begins to crush the much thinner sea ice, which causes ripples and waves to form in the ice.

A crude drawing of the pressure ridges. 

Eventually these waves buckle and crack. If the waves crack on the top portion, or crest, then entire slabs of ice can rise from the surface.

Some interesting ripples on the underside of an ice slab

If the bottom portion, or trough, cracks, it forms a “melt pool”. Melt pool is a misnomer, since, at least at this point in the season, nothing is melting; it’s just sea water flowing through the crack in the ice to form a pool.

Melt pool surrounded by ridges

And another view

Many times, the Wendell seals, who live on fast ice (as opposed to the Crab Eaters and Leopard seals, who live out in the pack ice) will use the cracks as breath holes, rather than having to make their own with their teeth. Seems like a good plan to me. Once a male seal takes over one of these holes, they will defend it from other males, since who ever controls the hole, controls the females in the area. Right now the seals are in pupping season, so in another month or two we should have seal pups hanging out on the ice.