May 22, 2020
By Philip S. Prince, Virginia Tech Active Tectonics and Geomorphology Lab
The model landslides in this post were produced at the same time as the Llusco landslide model I wrote about last year. They were created using a similar setup, but the slide masses behaved very differently during movement. These model slope failures begin as large slumps, but the slide masses largely disaggregated during movement due to the shape of the failure surface and underlying slope. I associate these models with the term “rock avalanche” with hesitation; I do so because of the near complete breakup of the slide masses and different style of movement of the disaggregated material. That said, the debris in the models does not travel very far relative to the size of the failure, and long runout is frequently one of the most salient features of a real rock avalanche.
One of the model landslides retained a few intact blocks within an otherwise disaggregated slide deposit. Many of the blocks were tilted or rotated, but the largest block (lower left of image) showed minimal tilting. A distinct depression formed at the head of the slide; it is clearly visible in the GIFs shown further down.
The second model broke up more completely, and the resulting deposit consisted of stacked sheets of debris. Again, an intact block remained at the head of the slide, and its back rotation produced a pronounced depression at the base of the headscarp.
I think the movement sequence of these models is interesting to watch, as the progression from the initially intact slide mass to the final deposit is easy to see. Both models appear to produce “cascades” of debris as the slide mass moves out and over the lower portions of the hillslope. A conceptual diagram of the model setup shown later in the post puts this behavior into context.
First, the model containing the jumble of intact blocks… in addition to breakup of the slide, the back-rotation of the upper part of the slide mass is worth a look.
After initial movement, the slide mass begins to deform due to the shape of the underlying failure surface and hillslope. The flexure and associated cracking before the mass falls apart can be seen below.
With additional displacement, the toe of the slide completely disintegrates, and small blocks of intact material are shed from the edge of the uppermost, back-rotating block.
The final result is a slide deposit studded with small blocks at a variety of orientations. The uppermost part of the initial slide mass remains intact, but has back-rotated significantly. Prior to failure, the material preserved in the back-rotated area was tilted downslope. After back-rotation, it tilts back into the slope.
Next, the model producing stacked sheets of debris…significant back-rotation is visible here, too.
This model produces a deposit consisting of stacked masses of debris. Look for them in the GIF above. Sheet 3 is particularly visible, as it carries the larger blocks shed from the intact slump block. Generally speaking, material is finest at the toe of the deposit, and becomes more coarse moving upslope towards the intact slump block.
As the slide mass begins to break apart, material appears to cascade out of the failure zone and spill down the slope and onto the flat base of the model. The numbers below show the locations of the different sheets of debris at this point of the sequence.
The larger blocks of sheet 3 can be seen to back-rotate as they detach from the slump block. Sheet 3 consists of these blocks and some material further downslope; the entirety of sheet 3 is blurred in the image below because it is moving as a unit. The toe of sheet 3 is to the left of the number 3.
The final geometry of the slide is shown below. The blue area represents where a sag pond could develop in the depression between the slump block and the headscarp. Both models formed these depressions due to back-rotation of the upper part of the slide mass.
These models might create very interesting deposits if the disaggregated material could travel further downslope. I had hoped to create largely intact slumps here, but things moved a bit too quickly. The models are made from angular sand (crushed dolomite) mixed with a small quantity of white flour to increase cohesion. Zones of weakness are created in the model “mountain” by burying small zones of glass microbeads within the cohesive sand as the model is constructed. The weak areas are intended to represent bedrock layering and/or fracture or joint planes. The actual failure surface is strongly influenced by the weak zones, but does not perfectly match them. The slides are initiated by tapping the baseplate of the model.
I always try to relate analog models to real-world features, but this one turned out to be a bit challenging. I think both models, particularly the “sheets” model, are worth comparing to the Judgment Cliff rock avalanche in St. Thomas Parish, Jamaica, which occurred on or just after June 7, 1692. This feature is good to look at on Google Earth, and useful discussions can be found at this link and this link. The second link connects to materials from a field trip out of Appalachian State University led by Rick Abbott…must have been a good one! The Judgment Cliff slide can be found at 17.9457N 76.6000W.
The geologic map and cross section of the slide deposit shown below are from Rick Abbott’s page.
The high point in the Judgment Cliff deposit (marked “295” above) is apparently the former (pre-slide) hilltop or summit, suggesting the uppermost part of the slide deposit may be more intact than the lower parts of the deposit. The topographic map on the Abbott link shows that there is a closed depression upslope of the deposit at the base of the scarp, but it appears to drain water internally. Note that the sedimentary rock layers in which the slide occurred dip slightly downhill, including the clay/shale/gypsum failure surface. Distribution of materials seen in the slide deposit reflects the initial geometry of the pre-failure slope. It would be interesting (and challenging) to replicate this in an analog model containing varied layers.
A Google search of “sag pond” will produce several references to the 2014 West Salt Creek rock avalanche in Colorado, which caused three fatalities. A USGS report on the event can be found at this link. This slide does host a significant sag pond between its headscarp and a back-rotated slump block, seen from Google Earth below. I don’t necessarily intend to compare the models to the 2014 event, but the sag pond feature and its relation to the back-rotated block are an excellent and easily visible example.
The rock avalanche events that occurred here resulted from reactivation of an existing slide feature. The GIF below compares pre- and post-slide Google Earth imagery. Movement of the slump block during reactivation was itself quite modest compared to the scale of the rock avalanche deposits.
The avalanching debris from this slide traveled nearly 2.5 miles (4 km). The slide can be found at 39.1775 N 107.8532W. and is definitely worth a look.
This post was originally published on The Geo Models blog.