December 19, 2019

Spreading volcano follow-up: Cross sections showing normal faults and thrust faults

Posted by larryohanlon

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

This is a follow-up to the first model volcano post of a couple of weeks ago. I wanted to try another experiment with a cheap plumbing sealant available at Wal Mart as an analog for extremely weak sediment layers deforming under the load of a volcanic pile. The sealant showed promise in the first post, so I dropped another $4.00 to try it again. The GIF below shows the results of about 15 minutes of deformation with fresh sealant straight out of the tube. The summit of the cone collapses into a graben, and the flanks of the cone spread outward, creating compression that generates thrust faults and folds.

GIFs are much more pleasant to make than videos. As the center of the cone drops, watch for small folds to develop outside of the cone’s flanks.

Obviously, this is an illustrative model only, and I can’t comment on the mechanical particulars of the sealant or this particular sand. Conceptually, the overall result here is pretty good, and compressional deformation extends all the way to the outer limits of the buried sealant layer. This is a lateral distance that is a bit more than twice the cone height.


Normal faults are red, with tabs on the down-thrown block. Thrust faults are black, with teeth on the hanging wall. I assume that all visible surface folds are associated with a fault, so fold axes aren’t drawn. Arrows indicate general down-and-out collapse movement.

An interesting aspect of the sealant is that it coats with sand, but the sand does not sink into the sealant. The result is a “patty” of sealant that can be removed from the model, re-shaped slightly, and deformed again. It cures quite slowly, and I constructed a second model to be gelled and sliced.

Side view across the model made for gelling and slicing. The little “foothills” at the base of the cone are compressional anticlines above thrust faults that sole into the sealant layer. Normal fault (extensional) scarps are visible on the cone.

This trial produced a comparable result to the first, although it deformed for a few hours. The numerous normal fault scarps looked cool with low-angle lighting. Note that they run mostly from top to bottom in the image below; this is a result of the rectangular shape of the sealant layer below the sand. A wider sealant base would have permitted radial spreading.

Top view of the model for gelling. Lots of extensional scarps formed on the cone. Compression extended to the outer edges of the sealant layer.

I think the overlapping scarps roughly correspond to this transport pattern. If you drew a big rectangle around all of the “lobes,” you would see the general extent of the sealant layer below the sand.

I had a grand plan of gelling this model by capillary action alone to preserve the scarps. I intended to do this by pouring gelatin solution into a large heap of sand constructed next to the cone, in hopes that the solution would soak its way through the whole model.

The plan was to pour near-boiling gelatin solution in the tall sand pile at right and let it soak into the model. It worked well enough, I guess.

My gelling approach was generally successful, but the height of the cone was an issue. The sealant also remains very gummy and sticky weeks after cure, so cutting it cleanly is challenging. Even so, the normal faults within the cone and the compressional folds along the flanks and beyond are visible in the sections. The yellow and blue layers represent pre-volcano sediments, and the red layers are marker horizons within the “eruptive” sequence that built up on top of the blue and yellow layers.

Black lines show normal faults, which offset the red layers and underlying yellow and blue, which represent pre-volcano sediments. A thrust related anticline can be seen in the yellow and blue layers at the toe of the slope. Outermost compressional deformation is at A, but it doesn’t show well in the layers. I think the undulose appearance of the clear sealant layer is due to deformation, but it does always match patterns in the overlying brittle layers.

Faults are shown in black only here. Normal faults due to collapse and extension offset the red layers. The anticline at A’ is due to compressional folding.

I ended up with cleaner cuts on the side facing away from the camera’s position three images up from here. Note the A’ label below for reference. This overall view gives a good sense of the general style of deformation, where the center of the edifice collapses, pushing the flanks outward and expelling the ductile sealant layer.

A big-picture image. Faults in the center offsetting red are normal faults; outer faults at the toes of the cone’s slopes are thrust faults. Note A’ for reference to above images.

Image sourced here.

The model results look good in comparison to the Nicaraguan volcano cross sections above from this paper. I mentioned Concepción in the last post; Mombacho is another good one. Compressional deformation at the toes of these volcanoes is visible in Google Maps Terrain.

So this is the magic sealant material…As the warning label indicates, it’s foul stuff. Be prepared to have lots of variously substituted benzene rings floating around inside your work area if you should choose to play with this stuff. It does dry clear, and the visual is pretty cool if you get a clean cut through it. Note that it can’t be cut for days (at the very least) after it deforms. I plan to try a poor man’s growth fault setup with it soon.

This post was originally published on The Geo Models blog