You are browsing the archive for The Geo Model Blog Archives - The Field.
August 4, 2023
The geology lab cross-cutting relationships exercise as a sandbox model
If you have studied geology in recent decades, you have probably encountered a cross-cutting relationships exercise, particularly in a lab class. I thought it would be interesting to turn this ubiquitous classroom activity into a real-life analog model. Igneous intrusion is a bit beyond my scope, but deformation, erosion/unconformities, and subsequent deformation are easily achieved in a 3-D “table top” model with overlapping, movable base plates. All of the events that affected the model can be observed in the thumbnail image…can you identify them?
May 22, 2023
Lidar highlights impressive landslide density on the southern Appalachian Blue Ridge Escarpment, western North Carolina
With a high quality (0.5-meter resolution) lidar dataset, it’s hard to examine much of the Blue Ridge Escarpment without seeing a few landslide features. Some slopes have more than others, however, and I think the McDowell County slope shown below probably has the greatest density of slides I have seen in the last 3 years of mapping. The most likely explanation is an extreme precipitation event. This area was heavily impacted in both 1916 and 1940 by extreme rainfall events, and the clustering of slides and their lack of damage to mid-20th century road grades would fit a connection to either of the storms nicely. These McDowell County slopes also resemble slopes in the headwaters of the Moorman’s River west of Charlottesville, Virginia, which experienced numerous slides due to an exceptional thunderstorm event in 1995.
April 29, 2023
A sandbox model of China’s “other” Rainbow Mountains that fits in the palm of your hand
While these colorful landforms get most of the hype, China hosts another, arguably more impressive, zone of colorful, tilted sedimentary layer mountains–the Keping fold-and-thrust belt. Located at the northern edge of the Tarim Basin, the Keping showcases elongated ranges of pink, green, purple, and tan mountains that stretch for 10’s of kilometers and are readily seen from Google Earth. I made this sandbox model in a couple of hours to shrink and fast-forward the development of the Keping fold-thrust belt. The layer sequence was specifically chosen to match the stratigraphy seen on the Keping mountain ridges.
April 25, 2023
The Blacks Beach landslide: Why did part of the beach rise as the cliffs slid down?
The January 20, 2023, landslide at Blacks Beach near La Jolla, California, dramatically uplifted a small portion of the beach during the slide’s movement. The beach uplift produced an understandable reaction from onlookers, and many of the early comments on YouTube videos of the slide focused on why the uplift occurred. In this case, the uplift is directly related to what is below the land surface–weak mudstone layers that provide a nice slide plane underneath the base of the slope and the beach. I made these models to illustrate this slope failure scenario, and Blacks Beach provides a nice opportunity to relate scaled models to a real-world slide that happened to be filmed.
April 17, 2023
Sandbox model anticlines formed above normal faults during extension
Early model faults and associated folds (H. Caddell and B. Willis, for example) were produced by compressional deformation of a variety of layered materials, and an intuitive association certainly exists between buckle folding and squeezing/shortening a sequence of layers. Extensional deformation, however, can also produce folding if the dips of normal faults change with depth. These folds are “forced folds,” meaning they are effectively collapse structures that develop as a mass of material changes shape to match an underlying fault surface.
April 16, 2023
Lidar reveals details of four impressive landslides in the Appalachian Mountains of southwest Virginia
I spent about 10 years studying and then teaching at Virginia Tech, and I can confidently say that it’s hard to find a better to place than Blacksburg to live and work as a geologist. The majority of my research and field teaching took place in southwest Virginia, so it’s always exciting when new lidar imagery comes along to offer a new perspective on familiar landscapes. The four large landslides shown here are not really noticeable without lidar help–I have passed close to (or across) all of them many times, and I never noticed them based on field expression alone.
April 1, 2023
Sandbox model debris flows meet Appalachian lidar imagery in Virginia and North Carolina
The speed and mobility of debris flows make them particularly dangerous among landslides. Debris flows move so quickly because they are fluidized masses of saturated soil, boulders and rock fragments, wood, and water, which causes them to erode or scour their flow path and accumulate more material than initially failed on the slope. I wondered if it might be possible to use granular analog materials to create similar eroded tracks in a sand model. Using a sand-flour mixture to make a cohesive slope was a V-shaped channel sculpted into it, I triggered slides of glass microbeads at the channel heads. The microbeads tend to fluidize when they are moving and colliding with one another, producing a dry analog for a natural saturated flow with lots of rocky debris.
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.
