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March 31, 2023
Negative inversion of a sandbox model thrust fault
Inversion of normal faults during compression is a popular topic in structural geology, but thrust or reverse faults can also be inverted by extension. Like inverting normal faults, favorable fault dips are an important part of this process, particularly when it is reproduced in a sandbox model like the one shown here. A detail of the thrust inversion setup that may impact the slightly lower dip of the initial normal fault segment is the presence of the weak microbead decollement layer on the thrust ramp. During shortening, the microbead layer is carried up the ramp. The microbeads have very low internal friction and are exceedingly unstable on the ramp at its high dip. As soon as compression is released and the layer pack can “relax,” collapse within the steeply dipping microbead horizon begins immediately.
March 22, 2023
Simple lateral spread landslide models made with glass microbeads and shaking
An illustrative, and entirely dry, lateral spread model can be made using glass microbeads beneath a cohesive sandpack, which I make by combining sand and flour. During shaking, the microbeads behave like a viscous fluid and deform the overlying sandpack. Lateral spread models made in this way are entirely conceptual and illustrative, but they look cool and do reproduce details of ground deformation above a seismically liquefying horizon. Deformation to the cohesive sandpack above the microbeads is visually interesting, with complex arrays of scarps and rotated blocks, as seen below.
December 18, 2022
Another track left by huge sandstone boulders visible with lidar, Big South Fork National River, Kentucky
An interesting aspect of the boulders is that very few have been known to slide or roll into place since folks started recording such things, and they very, very rarely show visible tracks or paths downslope in lidar-derived imagery. The question of how the boulders got to their resting place is legitimate (more on this below), but sometimes, lidar serves up a nice answer, as in the case of the two huge (115 ft or 35 m long) McCreary County, Kentucky, boulders shown below. The boulders and their track are highlighted in the lower image for comparison to the bare lidar.
December 2, 2022
What’s under that anticline? Fold-thrust belt interpretation ideas from geologic sandbox models
Complex structures like this are common in Earth’s sedimentary fold-thrust belts and are tough to fully interpret without seismic surveys or drilling through them, but field- and concept-based information can be gathered to at least give some idea of what might be beneath an anticline like this one. I offer up a few of my own thoughts here, and there are undoubtedly many other possible strategies.
November 1, 2022
Blowout landslides, part 2: Material movement, and did anything actually “blow out?”
So, what actually happens when these slides occur? I have not personally witnessed one, but I think that a look at details of failure surface shape and the behavior of saturated soil during failure can be used to figure out why blowouts appear to “blow out” instead of just slide. Lack of disturbance of the slopes below blowouts was remarked upon by both Eisenlohr (1952) and Hack and Goodlett (1960), with Hack and Goodlett going to the length of determining just how small of a sapling tree could survive a blowout strike in their study area.
October 29, 2022
“Blowout” landslides and the lidar signature of extreme Appalachian rainfall events
On the night of June 27, 1995, the Albemarle County, Virginia, mountainside shown below received an exceptional amount of rainfall. No one knows how much, but a nearby rain gage recorded ~ 11 inches (28 cm) of rainfall with only 2 hours…the rain event continued for several more hours. Unsurprisingly, a tremendous number of landslides resulted. The slides are clearly visible in this lidar hillshade image, and those marked with yellow arrows are of particular interest in the context of the storm’s outrageous precipitation rate and total, which likely reached 30 inches (76 cm).
October 28, 2022
What lives in frozen soil for 25,000 years?
In October 2022, Josephine Galipon visited Alaska to see if she could tease out genetic information from gray cylinders of permafrost — silty soil that has been frozen for at least two years, but in this case thousands.
October 23, 2022
Home insulation from wood and fungus
Robbin Garber-Slaght is a Fairbanks engineer who works for the National Renewable Energy Lab’s Cold Climate Housing Research Center. She notes that Alaskans pay more than double the national average to keep their homes warm during the winter and also pay a lot for sheets of foam insulation, which travels a long way to get here by truck and boat. She is teaming with Phillipe Amstislavski to develop insulation boards made of wood fiber bound by mycelium, the root-like tendrils of fungus.
Sandbox models with high-displacement thrust faults compared to features of some Canadian Rockies sections
Sandbox models don’t always produce the geometry the modeler wants, but with properly scaled materials, a “failed” model run can still produce worthy analog structures. I recently came up short on attempts to model some details of the southern Appalachian Valley and Ridge, instead producing structures reminiscent of some well-known Canadian Rockies sections. Were the model a real thrust belt, drilling through the first anticline of the upper thrust sheet, through the thrust, and into the upturned footwall beds might be interesting, whether you’re into exploration or carbon storage. A hypothetical well is shown here…
October 15, 2022
The man who knew moose like no other
Vic Van Ballenberghe died on Sept. 22, 2022, at the age of 78. The man who knew moose better than perhaps anyone else on Earth had stood amid their knobby legs for many springs and falls in Interior Alaska. I got to join him in the field once, 11 years ago. Here is my story from that day…
