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Lecture 3:

Listen to part of a lecture in a geology class. The professor is discussing mass wasting.

Remember, mass wasting is our geological term for movements of land. you know some already like landslides which are dramatic and might get attention in the news. I mean rock moving at almost 300 kilometers per hour would get your attention right. and there are rock falls also you’re probably well acquainted with mass wasting associated with steep hills and mountains. But what about creep?

Well, creep is another type of mass wasting. And it is by a long shot, the more common process, mainly because there are a lot more gentle hills or slopes. The term i’ll use creep is caused by the same things as other mass movements. Gravity is ultimately responsible, and water affects movement. but creep happens slowly.

That is, the soil moves slowly, only a few centimeters to a meter downhill in a year. so how does creep happen? Basically in a freeze thaw cycle, waiter, cold followed by summer heat and so on. let’s take a look at the diagram in your book.

when the Top layer of soil freezes in winter, it expands, so it pushes up and away from the bedrock layer beneath it. then when the ground warms up in the spring, when it thaws, everything contracts. but those little loose bits of soil on the Top don’t go back to their original position. as the arrows show, they follow just a bit farther down the slope. Thanks to gravity, this movement is almost imperceptible, and it’s not exciting. I mean, you’ll never see it on a tv show about the dangerous forces of nature.

But over the years, you may see curve trees or tilted fences. That’s evidence of creep. creep can happen in warmer areas to do to wet dry cycles where wet conditions cause the soil to expand. And dry conditions cause it to contract. but I’m interested in colder regions, especially regions with permafrost permanently frozen ground. the process, similar to creep in permafrost areas is called solar flexion.

now, in solar flexion, the soil moves uniformly down the slope as one large mass, not as individual particles. why? well, in summer, the Top layer of soil above the permafrost, the active layers we call it, will melt. now, at this point, this active layer is saturated with water and heavier, allowing gravity to do its thing. below the active layer, the ground is still frozen, which is a perfect slide to move on. now in spits bergen, I’m sorry, but I hope you’re not tired of hearing about the arctic in norway. But it’s my area of interest, as you know. anyway, there’s a great example there. It’s on the foot of a slope. look at this picture. these are stumps left over from a 1940 zero structure. even though the bottom of the stumps are in permanently frozen ground, the tops have been pushed over this far. it’s a perfect visual example of this slow process. In the spitz bergen area, there are varying rates of solar flexion.

In some areas, it’s slower than in others. Now, why would that be? well, as you know, mass wasting is primarily a function of slope and gravity aided by water.

Right? well, we can add one more factor. we think that in some areas of spits, bergen strong fast winds blow snow off the slopes, meaning the effective precipitation, the snow that actually stays on the ground is small. so we don’t have as active and active layer of thawed soil. And therefore you get a slower rate of solar flexion. with all this in mind. You know, building on hills isn’t necessarily the best idea, in my opinion, but we do it partly because slopes can be stabilized in various ways. building on permafrost, though, is a different story. you simply shouldn’t build on it, especially on a slope. I said on it directly on Top. You want to use posts to elevate the building. If you have to build it all this way, you don’t over heat the surface. That is, the freezing air can flow under the structure, keeping the permafrost intact. now, if you do heat it up, you’re risking that. And otherwise, normally gradual process speeds up. so instead of solar flexion, the slow regular process, you destabilize the ground. I mean then you get slides or flows instead and all the problems they cause.

go out for 12 hours in Quebec, Canada.