By Philip S. Prince, Virginia Tech Active Tectonics and Geomorphology Lab

The internet will tell you that “erupting volcano” is the most cliché and overused school science project known to man. Google it and see for yourself. This is not necessarily bad press for Earth Science, and eruptive processes are definitely dramatic, interesting, and consequential to humans and the Earth system at large.

I ain’t making it up. Geico obviously thinks highly of the study of Earth Science. Model volcanoes are also lampooned in various car commercials and HBO’s series The Wire, and probably elsewhere.

While eruptive demonstrations will always be cool, I think the gravity-driven structural evolution of large volcanoes is equally interesting and consequential and subject to illustration with models. The long-term result of repeated eruptive events is an enormous, steep, and unstable pile of rock that represents a tremendous load on the rocks atop which it grew. I thought it might be possible to produce simple classroom- or lab-expedient demonstrations that illustrate the consequences of a large mass of rock accumulating above a weak base, which is a generalized way to think about the structural context of many large volcanoes.

Image sourced here. The bottom schematic is a good representation of the basic concept. The edifice is a big, heavy pile of rock that grew on top of very weak sediments (mudstone and detritals). The downward force from the load is translated into expulsion of the ductile material and outward movement of the flanks, causing thrust faulting (sharp-toothed lines in top image). You can see a low line of hills related to this thrusting at the foot of Concepción in Google Maps Terrain. Note the scale in this schematic…this example is small compared to many other global examples.

Lab-scale model concepts from the paper above, utilizing silicone putty for ductile material. The upper left image is a reasonable illustration of how the microbead-cored models shown here are constructed.

The massive accumulation of rock that is a large volcano may be internally weakened due to a variety of processes (hydrothermal alteration, for example) or rest on pre-existing weak rocks that cannot bear the load without deforming or failing. There is an extensive literature focused on this aspect of volcanism (see end of post), which includes some outstanding and sophisticated analog models (see end of post). The highly simplified models I show here lack the precise scaling and sophistication of literature examples, but they are an attempt at connecting surface loading above a weak substrate with some type of gravitational failure in a visually-relatable way.

Three thrusts and associated anticlines have formed at the base of this spreading cone, which has a microbead core. Everything here is underlain by a weak layer of microbeads. Figures 9 and 10 here give a general sense of what these small thrusts and folds represent.

Figure 10 from the paper above showing the toe thrust complex on the south side of Kilauea. Note the scale…it’s big.

Half of this cone is underlain by microbeads, and the other half is stronger sand. Once the microbeads are buried and additional load is added, failure occurs. This is a very crude model, but many of its variables can be controlled in the classroom or lab to learn how the system responds. Using granular media is also highly geometric, so angular relationships can be used to predict when failure may occur.

There are lots of good examples of these processes that can actually be seen on Google Maps terrain and Google Earth. I think the models are adequate to aid visualization of process in the map imagery, such as the spreading flanks of Mt. Cameroon shown below. This large volcano has developed on a shale-rich sedimentary basin, which is deforming under the load.

4 km high Mt. Cameroon is an elongated volcanic edifice developed above a shale-rich sedimentary substrate. Anticlinal areas several kilometers from the summit ridge are interpreted to be compressional zones related to spreading of the volcanic pile above the weak substrate. This is worth a look in Google Maps Terrain. Read more here.

Placing an elongated load above a weak substrate in a simple model can produce a pattern similar to Mt. Cameroon and its surroundings. Here, a layer of honey is beneath the sand. Cold honey would be better. See 3:33 in the video.

The video linked below shows a variety of the very basic models I made. As usual, microbeads aren’t as weak as you would like them to be, and the honey I used was not quite viscous enough. Even so, I think these communicate the basic idea and might serve as a base for further refinement using the same easily acquired materials. Note that all of these failure models are load-based; they don’t include progressive tilting or large-scale magma injection as causes of volcanic edifice failure.

 

To generally summarize the contents of the video:

-Depositing strong and slightly cohesive sand atop a starting “core” of weak microbeads will produce deep seated failure and spreading, with thrust faults at the toe of the cone. This works best if the entire model area is underlain by a microbead layer, with the microbead core cone developed at the center. These models “lurch” along (see video) as mass accumulates on the cone and leads to sudden failure of the beads, which can be seen as lowering of the flank or summit accommodated by movement on a toe thrust.

Development of the toe thrust is linked to sudden “collapse” events in the cone. This process can be seen in the video. This model is about as simple as it gets, but the take-away is that the cone grows by spreading at its base, not by “avalanching” of material.

-Angles and slopes have to be unrealistically steep for this to work, but the process is easily contrasted with the growth of a cone lacking the weak core. In the absence of a weak core, the cone spreads by surface “avalanching” only.

-Using some sort of highly viscous material to replicate weak, ductile sedimentary layers beneath the volcanic pile is a very interesting experiment, but can be hard to control. I think cold honey is a good way to go, particularly in the classroom.

Here, room temperature honey allowed a flank of the ridge to “raft” away. The summit lowered into a graben and significant toe thrust ridge developed. I think refrigerated honey would produce a good result with material of sand’s density, and is a good way to control the viscosity/material variable in the lab/classroom. Mt. Etna is another good example of deformation of a volcanic center above weak sedimentary rocks.

-Various sealant products (silicone or otherwise) that can be purchased in grocery or hardware stores can also work and are easily acquired. These seem to produce decent results, but all will evolve some sort of carrier solvent or gas during curing and would not work so well outside of a fume hood!

This model was made above a base of Liquid Nails brand sealant from Wal Mart. It proved difficult to photograph, but the summit collapsed into a graben, and the movement was accommodated by compressional deformation at the toe of the slope. Using a more cohesive material for the cone would improve detail on the scarps.

Concepción volcano in Lake Nicaragua. Subtle hills related to toe thrusting can be seen in Google Maps Terrain. Refer back to the paper linked in the second image in the post.

-Creating an asymmetrical cone with a weak microbead flank will produce an interesting collapse if a cone of stronger material is built over the beads and progressively overloaded. Using a slightly cohesive overlying material will show more scarp detail once collapse occurs. Mixing a bit of flour with sand will increase its cohesion, but too much makes it hard to pour or funnel onto the cone. Just setting this model up is an interesting challenge. Strategy is required to build the cone over the microbeads without producing a small collapse every time any additional sand is added.

A deep-seated failure moves the summit of the cone, which settles slightly into a minor graben. 2:18 in the video.

I think the models that replicate spreading above a ductile sedimentary substrate (and their real-world examples) are absolutely fascinating, and I intend to keep looking for a good “grocery store” substitute for the silicone putty used in the literature examples. Even with granular media alone, enough variables can be controlled (stratigraphy, edifice shape, tilt of “fixed basement,” etc.) to allow for interesting experimentation and varied results.

A few links are included below, some of which have already been used in the text. There is plenty to read about this concept, and many of the papers are very well illustrated. There are dozens more, and these are only listed because of their easy access!

Kilauea

Mt. Cameroon

Mt. Etna

Spreading and magma intrusion

Low-strength layers

Rice summary

This post was originally published on The Geo Models blog.