November 19, 2019
A different take on the model volcano, the “most cliché science experiment” you can do (at least that’s what the internet says)
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.
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.
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.
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.
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.
-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.
-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!
-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.
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!
This post was originally published on The Geo Models blog.