Beautiful. Check it out.

Labels: art, fluid dynamics

I shot these two videos this weekend from the Billy Goat Trail. They both show the surface of the Potomac River, decorated with little blobs of foam. As the river flows, the blobs of foam record the flow and deform in distinct patterns. I am reminded of the processes that must have occurred in the very rocks I was standing on to take these videos. (See the previous posts on boudinage, folding, and texture in migmatites.) You can see foliation developing, shear zones, folding, and even boudinage. The blobs of foam are acting like more competent geological units (feldspar or garnet porphyroclasts, for instance), while the intervening water is less competent (easily flows out of the way, like quartz or calcite under sufficient pressure).



This one really shows boudinage well. Track the big blob that gets "fed" into the shear zone a few seconds into the video. As deformation proceeds, it separates into three augen-shaped chunks that then move apart along the plane of foliation (which is itself deformed).




A note of caution: these foam blobs are not perfect analogies for the flow of rocks at depth. The dynamics you're observing in these videos are playing out on a two-dimensional surface where water meets air. Because its density is intermediate between the water and the air, the foam stays at this surface, though the water in between the blobs is free to circulate downward into the river if conditions demand it. In real rocks, the deformation would be a three-dimensional phenomenon, and hence a bit more complicated.

Labels: analogies, fluid dynamics, rivers, structure