Mass Wasting Assignment

Feedback

Essay Question:


Mass wasting may be prevented by decreasing the angle of the slope, so that the actual slope angle is less than the critical slope angle. At this point, gn will be greater than gt, and the slope will be stable. Two ways in which slope angles can be decreased include adding material to the base of the slope (this will also provide support to the base of the slope), or removing material from the top of the slope. Note that this is exactly what nature does when mass wasting occurs! Mass wasting is just nature's way of reestablishing equilibrium on a slope.

It's not just enough, however, to consider slope angle when trying to stabilize a slope. This is because the force of friction also affects downslope movement. From the inequality stated in the original question, we can see that the force of friction acts along with gn to hold material on a slope. If friction is reduced, then the slope may become unstable. So, to make a slope more stable, we must find a way to increase friction. This is usually done by getting rid of the water on or in a slope, because water acts as a lubricant to promote downslope movement. It not only "greases" the slope, but it also acts as a means of facilitating flow within the moving mass. Water is usually removed from a slope in one of two ways. It can either be channeled away on the surface, or it can be drained from within the surface material by placing drain pipes into the surface deposits.

Multiple Choice Question #1

Which one of the following conditions would be most likely for mass wasting to occur?

C is the answer. Note that when layered bedrock is tilted at the same angle as the slope, the layers will serve as a glide plane along which the bedrock can slide. If the sedimentary rock at the top of the tilted pile is a porous sandstone, water can seep down through the bedrock until it encounters an impermeable layer such as shale. At this point, the water will move along the tilted shale surface, greasing that surface and making it slippery. The overlying pile of rock can then slide downslope along the greased shale, especially when support for the slope has been removed from the base by erosion. See the diagram in multiple choice question 3 for a cross section of exactly this situation.

Multiple Choice Question #2

Which of the following variables are important in classifying different types of mass wasting processes?

All of the choices are used in one way or another to classify mass wasting types. Rate of movement and mechanism of movement are particulary important.

The above photo shows the Madison Slide just outside the northwestern corner of Yellowstone National Park. The canyon at the right center of the photo is the route that the Madison River takes to the Madison Valley just beyond (upper right). The photo was taken just after the slide occurred, and before engineers cut a new channel for the river to drain into the valley downstream.

Question #3

What changes in the hillslope and river valley resulted from the slide? The river valley was entirely blocked by the slide deposit, damming the river and creating a lake. The deposit that accumulated at the base of the slope provided support for the new surface, helping to stablilize the slope following the disastrous event. Notice that material was removed from the upper part of the slope. Together with the deposition that occured at the base of the slope, these changes reduced the slope angle. When the slope angle became less than the critical angle for that slope, the slide stopped.

What evidence can you see in the photo that tells you the slide moved with high velocity? The high velocity of the slide is evident by the fact that the material was able to slide upslope on the opposite side of the valley.

The above photo is of the Gros Ventre Slide in northwestern Wyoming, on the west side of Jackson Hole and in the valley of the Gros Ventre River. Note that the situation here is similar to that of the Madison Slide, where the slide mass blocked the course of the river, resulting in the formation of a lake behind the dam of debris. If you look carefully in the cliffs on the hill above the slide, in the left and slightly above the center of the photo, you can see the layers of sedimentary rock that make up the hillside. Note that these layers are tilted to the left, in the same direction in which the slide moved.

The above cross section shows the pre-slide surface for the Gros Ventre Slide, as well as the present position of the landslide debris. It also shows the tilted bedrock referred to in the caption for the above photo. Note that the directions of the cross section are reversed from the directions shown in the photograph.

Question #4

Compare the slope of the preslide surface with the tilt of the bedrock. Then, note that a permeable sandstone (meaning, a sandstone through which water can pass easily) overlies an impermeable shale (meaning that water will not pass through the shale). Now, describe the relationship between the slide mass and the boundary of the permeable and impermeable rock layers. Finally, discribe the factors that were most likely in causing this slide.

The slope of the preslide surface is the same as the tilt of the bedrock. Furthermore, the boundary between the permeable and impermeable rock and the sliding mass is also parallel. A river at the base of the slope had also cut downward through the sandstone layers, removing support for those layers so that they could eventually slide downward .

These factors, taken together, enabled the bedrock and overlying loose regolith or soil to move along the greased surface of the shale once sufficient water had soaked through the sandstone to the impermeable shale layer. Apparently, a rainy season must have occurred just before the slide in order to reduce friction so that the slope could fail.

Above is a view of a mass wasting deposit in the mountains of Idaho.

Question #5

Is this the deposit the result of a slump, slide, or earth/debris flow? Give evidence, visible in the photo, that supports your answer.

The primary lines of evidence from the photo for earth/debris flow include the wrinkled surface of the deposit, and the fact that the deposit spread outward onto the floor of the basin beyond the canyon. The wrinkled surface of the deposit is similar in a way to the ropy or pahoehoe surface of a fluid lava flow.

Apologies for the poor quality of this photo. Mass wasting has occurred along the banks of this river in South Dakota.

Question #6

Was the mass wasting event one of creep, slump, slide, or flow? Give evidence from the photo for your answer.

The process was slump. Note how the material that moved downward is still intact. It has not broken up and moved as individual blocks as in the case of a slide, nor has it produced a wrinkled surface as it might have if flow had occurred.