October 2, 2018
By Philip S. Prince, Virginia Division of Geology and Mineral Resources
This post should serve as a brief respite from the typical structural geology material I write. I have always wondered what would happen if the sandpacks I use for structural models were struck by a high-velocity projectile. This turns out to be a fairly easy question to answer if you have sand and a quality pellet gun…
I prepared a few model stratigraphies in the same way I would for a deformation model. The stratigraphies contained mechanically-distinct sands, but the contrasts in mechanical behavior were minor compared to models containing glass microbeads. I used layered stratigraphies to highlight any structural deformation that might be left by the impact (other than the obvious big pit!) and to track the origin and distribution of ejecta.
The “bolide” was a 7.5 gram lead pellet fired from a Crossman Phantom pellet gun. This is a high-powered, rifled pellet gun that should have propelled the pellet at around 300 m/s, just shy of the speed of sound at my elevation. Interestingly, this pellet gun, which is available at Wal-Mart and about anywhere else with hunting supplies, will shoot a lightweight projectile at supersonic velocity. The lead pellets I used weren’t quite that fast. I would be interested to see if a lighter but specifically supersonic projectile would generate a different “micro-astrobleme.” The pellet struck the layer pack at an angle of 45-50 degrees above horizontal. I purposely avoided a near vertical or very low-angle trajectory.
Much of my interest in structural geology and sand modeling stems from the geometric order that controls deformation patterns. I never cease to be amazed that sand (and large rock masses, for that matter) produce such orderly and geometrically-predictable patterns when subjected to deviatoric stress. As it turns out, sand does not disappoint when struck by high-speed projectiles. The model craters generated very interesting patterns of ejecta distribution and deformation along and below the craters. Interestingly, the normal faults that carry slump blocks do not dip steeply enough to be purely gravitational collapse features that occur after crater excavation. They must occur early in the sequence when the sandpack is “swelling” around the impact point, but the ejecta curtain obscures any visual evidence of their formation.
Obviously the craters fail to display many features observed in actual impact structures, particularly a central uplift and any melt-associated characteristics. The pellet projectile does not fragment or vaporize, and its size and velocity are poorly-scaled. Even so, the model craters developed slump blocks along the crater margins, orderly patterns of ejecta sourcing and distribution, and rays of ejecta like those seen in actual crater features. I gelled and cut the models to produce sections through the craters. This is tough to do for round features; it is a much easier operation to produce serial sections through compressional or extensional models. I think the best way to slice a crater model is to cut it into quarters with two orthogonal cuts. The resulting square/rectangular block can then be cut further to look at other parts of the crater.
A summary of salient features:
-Very little ejecta travels “backwards” towards the origin of the projectile, even though the crater is essentially circular in shape.
-A large amount of material is laterally ejected, and stratigraphy is maintained in the flying ejecta curtain!
-The geologic section “splits” at the point of impact along a line orthogonal to the flight path (see above image and the one below). The material behind the line (red through dark blue) rotates backwards towards the projectile origin and ejects laterally. This ejecta is derived from shallow layers. In front of the line (greenish stuff below the red layer), the section overturns and very deep strata are ejected forwards. Rays of ejecta are visible within an overall ejecta blanket.
-Slump blocks containing intact section can be observed along the crater walls. These blocks are buried within coatings of jumbled ejecta that slide downward into the crater. The dips of normal faults below the slump blocks are too shallow to have formed under simple gravitational collapse into the crater after excavation. They must occur in the early stages of impact during “dilation” of the section, but ejecta obscures visual evidence of this.
-One model preserved a “jet” of ejected material in a tight, overturned anticline. I thought this was a really neat feature. The third model provides a good illustration of the source of ejecta and where the deepest excavation of the crater occurred.
Altogether, this turned out to be a very interesting experiment that I will try again with modification. Unfortunately, projectile size controls crater size, and I’m not sure how to produce a bigger crater formed by a comparably high-velocity projectile. I’d also like to try a deformable projectile. If anyone knows how to accelerate a piece of ice or gelled sand to 300 m/s AND have it stay together in flight, you just let me know!
This post was originally published on The Geo Models blog.