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When you think ‘ice sheet’, think ‘sticky water’

Let’s pick up our discussion from last time and talk about viscosity. When you think viscosity, think stickiness. Viscosity is a measure of a fluid’s resistance to flow. Here’s what google has to say:

The special thing about ice is that it’s viscosity is high enough that it doesn’t instantly flow off the land into the water, yet low enough that it does eventually flow down slope. Honey, is a good analogy: If you pour honey onto a surface, it starts as a mound and slowly spreads out.

However, if you tip over a glass of water it quickly spreads all over the table.

Why? Because the molecules in honey (and in ice) experience more internal friction, which opposes flow. In glaciology, we measure viscosity as the shearing stress that causes flow divided by the rate of shear strain (or deformation), that results. I think here’s the main things to take away. 1) ice is flowing and 2) the flow speed is dependent on the viscosity of ice. Viscosity of ice isn’t one fixed number, it actually varies some with the temperature of ice. Here’s a plot. 

Colgan et al 2015

However, if you think back to that velocity map I showed in my last blog, you’ll remember that the flow of ice is like streams in some places and it’s not uniform. That’s because there are many important factors governing the flow of ice sheets and viscosity is just one sticky piece of the bigger picture!

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