March 09, 2012
Photographer: David K. Lynch; Dave's Web sites: a; b
Summary Author: David K. Lynch
Slickensides are smooth, often polished surfaces of parallel scratches or grooves. They form at fault planes when rocks on either side scrape past each other. Frictional contact carves out striations parallel to the fault motion. This can happen when rock is sheared suddenly during an earthquake or gradually during fault creep. Any kind of rock can show slickensides. The rock shown above is arkosic sandstone from the Pliocene Anaverde Formation -- collected from a roadcut in the San Andreas Fault. In some cases, heat generated during slip can produce unusual chemistry at the interface. Slickensides may originally cover thousands of square meters and form several kilometers deep. When uplifted and exposed, the polish surfaces may be fragmented. Slickenside-like structures also form when glaciers grind over rocks or in expansive soils. Photo taken near Palmdale, California in 2005.
Photo details: Camera Maker: Canon; Camera Model: Canon PowerShot S70; Focal Length: 20.7mm; Aperture: f/5.3; Exposure Time: 0.0040 s (1/250); Exposure Bias: none; Metering Mode: Matrix; White Balance: Auto; Flash Fired: No (enforced); Orientation: Normal; Color Space: sRGB.