Frozen or On-The-Rocks? Anchor Ice is Both!

For the past week, we have been diving near the McMurdo Jetty. Our dive hole and hut are located very close to shores of Ross Island, and during our 15ft, 3 min safety stop – we enjoy playing underneath the sea ice close to shore in amongst the wonderous anchor ice.

This view of McMurdo Jetty and Observation Hill was taken from beside our dive hut (“hut 19”) on the sea ice. As you can see, our dive site is actually very close to shore. Photo credit: Rowan McLachlan

Achor ice forms in the spring/early summer here in Antarctica. Supercooled water flows out from underneath the ice shelves and moves North. Although this “supercooled” water is only 0.1-0.2°C [or 32°F] colder than the surrounding seawater (the annual mean water temperature in McMurdo Sound is -1.87°C or 28.6°F) the temperature difference is enough to stimulate ice crystal growth.

Diver Rob Robbins swims amongst the anchor ice at the Jetty dive site, McMurdo, Antarctica. Photo credits: Rowan McLachlan

Beautiful, shimmering crystals form in the water column and larger crystals start to grow on the seafloor. Eventually, a giant blanket of thick interlocking crystals known as “anchor ice” forms on the shallow seafloor, growing up to 2 feet thick in some areas.

A Sterechinus urchin moves across the anchor ice in McMurdo sound. It better be quick though, or else the anchor ice may grow over him! Photo credit: Rowan McLachlan

Swimming next to this anchor ice is a phenomenal experience. As you gently graze the anchor ice, the crystals break off and float up into the water column. With light reflected in all directions, you feel like you are inside a magical kaleidoscope or surrounded by fairy dust.

Anchor ice floating in the water column. Photo credit: Rowan McLachlan

The experience of being beneath sea ice is almost indescribable. The colors of blue that pass through the cracks in the snow and ice are so beautiful – it almost reminds me of the milky way!

Photo credit: Rowan McLachlan
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