ASK-A-GEOLOGIST by Jeff Wynn

Glaciers can flow uphill (!?)

A rather large percentage of the terrain we lay our eyes on in the mid-to-high latitudes around the world has been shaped by glaciers. A major clue to this action is when we see “U-shaped” valleys or scoured “cirques” on the flanks of high peaks— like someone took a giant spoon to the side of a mountain made of Extreme Moosetracks ice cream. These tell a practiced eye that something big and remorseless has ground everything smooth. Most glaciers are not small things, either. I have been in a fjord in Norway that shows scoured edges 3,000 meters up from the current water-line! For American readers that represents a moving mass of ice nearly two miles thick. The huge weight of these glaciers, at their max height 10,000–16,000 years ago but now removed, has caused parts of Scandinavia to uplift — to literally float up, a process called isostacy — by as much as a meter per century. Entire ports have become lifted up and isolated from the sea!

Q: Hi, I live <in> Minnesota which experienced substantial effects of glacial movements during the ice ages. My question is: How were the gravitational forces sufficient to move the glaciers south into the upper Midwest given the current elevations above sea level of Hudson Bay (400 ft), Duluth (1430 ft), and Minneapolis (840 ft). Thanks

• Jim T.

A: You can’t think of glaciers as slow-moving water — they are plastic, with very high viscosity. Precipitation moisture (mostly snow) falls and “pancakes” instead — it compresses, forming glaciers, with the center of the highest precipitation building up steadily, and the edges then being pushed outwards. If you pressed down with a spatula on the center of a still-fluid core of a pancake (before it was cooked through), it would extend or “smoosh out” sideways. This is the same displace-and-inflate behavior that we also see in lava flow-growth in Hawai’i. If the pancake filled the entire griddle or pan, and there was confinement at the edge or lip of the griddle (the analog of the broad increased elevation at Duluth), then the still-plastic material would rise above and flow over this barrier. If the elevation-increase is an isolated mountain, the glacier will instead slowly move around it on both sides — but still lap up locally at its edges, and thoroughly scour smooth those edges.

Franz Josef Glacier, New Zealand. (Wikimedia) Note the smoothed walls of the canyon.

Keep in mind that behind those continent-scale glaciers was just more glacier all the way back to central Russia — so the ice front could only move southward in North America. And believe me, nothing epitomizes the word “inexorable” quite like a glacier on the move.

It’s early in the morning, and I must be still thinking of breakfast to come up with a pancake metaphor.