Thursday, November 12, 2009

The Mountain Has Came Off

Geobloggers far and wide have been posting variations of this footage. When I see the same thing over and over I tend to assume others have seen it elsewhere, but I do have a somewhat different demographic than typical geobloggers. And this is an amazing catch. It is not made clear in the clip, but a geologist had warned the road crew that a large slide was imminent. No people or machines were injured in the filming of this clip.

Hat tip to ReBecca for a nice write-up; see her post for more information on "The Rock Star of the Week," Vanessa Bateman. She also has links to a number of other videos of this slide. The CNN clip has a couple of slow motion segments. Also (I hadn't looked at it when I first posted this), an AMAZING clip of a mudflow!

Let's look at the situation here...Note the layering of the rocks, and note that the layers are not supported at their base (bold yellow arrow, below). You can also see that the weather is wet and rainy. One of the reports ReBecca links to says it has been rainy for weeks... we can pretty well assume the ground is saturated. (The same report refers to these rocks as granite, which they're not, but we'll let that slide... heh, heh)
The action of water in this situation is often referred to as "lubrication," but physically, the more important component is probably a reduction of normal force. Frictional force (resistance to sliding) is physically described as the normal force (in this case, the weight of the rocks pressing down, pulled by earth's gravity) times the coefficient of friction. The water may play a role in reducing the coefficient of friction ("lubrication"), but when it saturates the cracks and pores, it applies hydraulic force both up and down, as shown by the blue arrows below.
The upward force isn't enough to "lift" the rocks, but it can dramatically reduce the normal force. With both the normal force and coefficient of friction reduced, the total frictional force is much, much lower... meaning the resistance to sliding is very low. With no support at the base of the layers, slides are inevitable.
This is a pretty classic rock slide scenario, and one that pretty much any geologist could recognize. Predicting that it's going to happen now, though, is much tougher. Ms. Bateman's call likely saved a number of lives, injuries, and equipment loss. Well done!

And as an aside, one of the things I love about geology is that you need to understand all the other sciences to understand the earth well. Physics, as above, chemistry, biology, astronomy, math... if you're not fairly well grounded in all of them, you're going to miss a lot of the earth's awesomeness.

1 comment:

ReBecca Foster said...

Thanks :D
Isn't that mudflow footage awesome!