#slickenside

[artemistorm]

Silly Game Time: What was the most recent internet search you're comfortable with sharing?

Slickensides! Slickensides are rock races that are polished smooth by rubbing against another rock at a fault line (source of earthquakes). We found one on our field trip (and I looked them up online to see what other look like).

This polished surface was so smooth and hard that it felt like porcelain. It's just crazy to think that this was caused by rocks rubbing against rocks.

[earthstory]

Golden Fault This sample is a piece of one of the gold-bearing mountains in California. The gold was likely deposited as a hydrothermal vein – hot waters associated with the growing volcanic mountains flowed through the ground, dissolved components like gold and the vein quartz you can see sticking out the side, and then deposited those dissolved elements where there were cracks.

If you look carefully at this sample, there is a fabric of small grooved lines running down from the top right hand corner of this sample. These are described as slickensides – grooves that are created as rocks are moving and grinding past each other in a fault zone. Fault zones are good places for fluid flow and mineralization since they’re open cracks that run substantial distances through rocks. Whether this gold was deposited during fault motion and the slickenlines represent a pattern in the rock controlling the gold growth, or the fault kept moving after the gold grew and the gold was ground into…this is truly a golden fault. This sample weighs just about half a kilogram and is about 10 centimeters wide. It is available for auction later this month from Bonhams. -JBB Image credit: https://www.bonhams.com/auctions/26082/lot/164/?category=list&length=214&page=1

[pagosapig]

Slickenside

When I first started collecting rocks along the banks of the Ohio River I thought I had found a goldmine of petrified wood. I collected many small pieces and proudly displayed them on my shelf. I went out on a usual evening walk at the river when I found what looked to be the fossilized stump of a massive tree! What a treat! I thought I had discovered something amazing (and I did, but not for the reasons I thought). I marked the location on my map and headed home to do some research before sharing my find. That was the night I learned what a slickenside was. Basically it just the friction lines created when certain rocks break (if you want to sound smart say something along the lines of “it is the striations caused by faulting”). So they may not be fossils, but they are nonetheless beautiful in their own right.  A gorgeous creation of kinetic movement. To this day one sits proudly on my shelf. It reminds me of a zealous rock hound looking for beauty in everything he saw. I want to always be like him. 

[stealth-science]

I went hiking at Coyote Ridge and found a slickenside. Which was weird because I literally only found one. Just this one.

[earthstory]

Slickensides and slickenfibers

This photo taken in a limestone quarry near Brno, Czech Republic and annotated by photographer Petr Hykš, shows one of the features faults can produce on rocks. When two rocks on opposite sides of a fault move against each other, rough spots on each side will grind into the other side, leaving streaks called slickensides. Basically, they’re the geologic equivalent of scratches – run something abrasive like sandpaper over a weaker surface in one direction and you’ll produce similar lines. The lines on this surface of a rock produced by fault motion are called slickensides – their motion direction is illustrated by the yellow arrows.

Often when geologists find slickensides out in the field they can run their hands along them and figure out exactly which way the fault moved – if the hand moves in the same direction as the fault motion the rock will be smooth while if the hand moves opposite the direction of fault motion the rock will be much rougher. This roughness can be in part simply from how the rock was ground, but it also can occur because of new mineral growth.

See the white streaks marked with the red color? Those are new minerals grown as this fault creeped. Every time there was a tiny bit of motion on the fault it opened a little bit of space for water to flow and that water gradually grew new carbonate minerals in that spot, creating new minerals growing in one direction called slickenfibers and represented here by the bands of white minerals. In this fault, you can therefore tell that the opposite side of this fault was moving up relative to this rock because the minerals grew gradually away from the steps marked by the red lines. Of course, that detail would be more useful if the rock were still in place rather than at the bottom because then you could even tell if this fault were a normal fault or a reverse fault, but then the picture might not be as pretty.

-JBB

Image credit: Petr Hykš https://flic.kr/p/QrRAcs

More reading: http://bit.ly/2jNw1xt http://bit.ly/2itCz2z