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A walk on the sea ice.

Disclaimer - I am not a sea ice nor an ice shelf expert. This information is a mix of details found from online reading and from people much more qualified in the subject than myself.

Pressure ridges are features formed from sea ice as it buckles and packs together when it collides with another mass. The pressure ridges described in this post are formed when the seasonal sea ice in the McMurdo sound push against the Ross ice shelf and Ross island itself. This three-way junction between the immovable island, the slow-crawling and incredibly massive glacier, and the dynamic and ephemeral sea ice forms beautiful structures as they all churn together - land beats glacier beats sea ice.

The pressure ridges themselves are formed as ice breaks off and stacks up underneath the leading edge of the ice, forming what is called the "keel" below and the "sail" above the ice level, seen in the following diagram of free-floating sea ice.

Credit Lusilier, Wikipedia

When the pressure ridge is grounded to the sea floor, it is given the name "stamukha" from the Russian "стамуха", roughly translating to "stranded hummock".

Credit Lusilier, Wikipedia

It should be noted that pressure ridges are distinct from sea ice cracks, and are discernable by the buildup of ice chunks above and below the working crack. Where free-floating sea ice cracks may drift in either direction or commonly apart, pressure ridges are more convergent over time.

You can see a large sea ice crack in the center of the image, with a pressure ridge nearer to shore as the tides push and pull and churn the ice there.

Pressure ridges forming in sea ice not parallel to land near Hut Point.

The dynamic nature of the pressure ridges causes frequent openings to the sea ice below - a massive hazard for those of us who aren't adapted to swimming in ~28°F waters. However, the seals love it as it provides easy access to the ocean for feeding, and they are frequently spotted hanging around these sea ice cracks.

The presence of these hazards and the day-to-day shifting nature of the pressure ridges means that tours of the pressure ridges are guided and along a flagged route that is frequently checked by Field Support & Training. I managed to get onto the last tour of the season, shortly before the end of the year.

Note that at the peak of the summer, above-freezing temperatures create many tidal pools and cause even more shifting of pressure ridge sails.

Tidal cracks (large and small) are formed as the tides come in and out and shift the ice in yet another way.

As the sea ice blows out in front of McMurdo, the seals migrate to the pressure ridges to laze in the sun on nice days.

Thank you for reading this short-ish post about pressure ridges, and I hope you enjoyed the photographs.

As thanks, enjoy these seals slugging around on the ice.