The thrust belt in my driveway

It's snowed a lot in the past two weeks. Somewhere around two feet of snow fell between the last day of classes and December 26. Good for ski areas, but a lot of work to shovel. Fortunately, structural geology can make all kinds of difficult labor fascinating.

When you push a shovel through a relatively thin layer of snow, it piles up next to the shovel in a wedge shape. Push the shovel along, and the wedge gets longer and thicker, but it maintains the same shape: a triangular wedge, with a consistent angle at its front end.

Photo: a wedge of folded and faulted snow in my driveway, 12/22/08.


You see similar shapes in belts of thrust-faulted rocks from mountain belts all around the world, from the Himalayas to the Canadian Rockies to the Appalachians to Taiwan:

The cross-section above is from a classic paper by Davis, Suppe, and Dahlgren (1983) that explains what's going on mechanically in these wedges. The shape of the wedge is governed by a balance of horizontal forces: the push from behind the wedge; gravity, which would tend to flatten out the wedge (by moving material from the higher back part of the wedge to the front); and the frictional resistance to sliding, which tends to keep the front of the wedge from sliding along, causing the wedge to be steeper. The other big assumption is that the wedge is always just about to fail: its internal strength exactly balances the stresses that compress it. This means that the nature of the material also makes a difference - cohesive sheets of material behave differently from loose sand.

There's a lot of underlying math behind the explanation of wedge mechanics, but the cool thing about it is that it all boils down to a pretty simple concept. If the friction at the base of the wedge and the mechanical behavior of the rock (or snow) stays the same, the wedge should maintain the same shape. It can get bigger, but the front of the wedge should keep the same angle. That simple geometry has a lot of predictive power. It tells what should happen in a mountain belt that's eroded by a lot of rain or glaciers, compared to one that's in a rain shadow. It's been used as one explanation for the exhumation of high-pressure metamorphic rocks. And it solved a long-standing problem for structural geology: how could huge masses of rocks slide along a nearly flat plane for immense distances, as had been observed in places like the Canadian Rockies.

And it turns snow shoveling into an analog modeling experiment. Take a driveway made of smooth concrete. Drive a truck over it, and pack down snow in parallel ridges. Then let it snow another couple inches before shoveling the driveway. The results of the experiment look like this:


The wedge started out small. It's hard to see the exact structure forming - are there thrust faults beneath the folds on that surface? But it formed a nice taper, and slid along fairly easily. (Note the highly rigorous descriptions of basal friction from this experiment.)

I pushed the shovel a bit more, and the wedge got longer and thicker, but kept the same shape:


But then I reached the old tire track, and it suddenly got really hard to push that shovel. And the wedge looked like this:


The taper of the wedge suddenly got steeper, at least until I got past the tire track. But by that time I was experiencing significant edge effects, both from the top of the shovel and the snowbank beside the driveway, so the experiment ended.

The different snow storms led to somewhat different wedge shapes. Over Thanksgiving, we had a couple inches of wet snow. It formed a cohesive layer, and I could see the individual fault blocks. (I even had some tear faults separating thrust faults with different offset! Unfortunately, I didn't have the camera yet.) Before AGU, we had an inch or so of powder, which compacted somewhat when I shoveled it. And on Christmas night, we had about five inches of wet, heavy snow, and I stopped doing experiments because moving it was a lot of work, and besides, it kept falling over the top of my shovel.

My students still think that snow is for skiing, not for experimenting with structural geology. But if you've got to shovel, you might as well have geeky fun at the same time.

Reference: Davis, D., Suppe, J., and Dahlgren, F. A., 1983, Mechanics of fold-and-thrust belts and accretionary wedges: Journal of Geophysical Research, v. 88, n. B2, p. 1153-1172.

Photo note: I adjusted the levels on all the snow photos, because I haven't figured out how to take good photos of snow with my new camera yet.