#ductile

[qvicksilversass]

*ੈ✩BUCKY'S SMILE

/Thunderbolts* 2025

[relatetorocks]

[earthstory]

Ductile Folding This beautiful outcrop was photographed on the island of Crete by Dr. Marli Miller. The rocks are part of the Plattenkalk series of sediments that form the backbone of that island; they include fine-grained limestones deposited in the ancient Tethys seaway alternating with more siliceous layers that include terrestrial sediments and sometimes buried sponges. These rocks have been uplifted and clearly folded as part of the collision between Africa and Eurasia that is uplifting islands in the Mediterranean.

These rocks have been folded into a pair of folds – one anticline, where the rocks bend downwards, and then a syncline where the rocks are bent back to a more normal angle. Finding these 2 folds together is common as they start off as flat-lying layers and this pattern brings them back to flat-lying, just with a kink in-between that shortens the layers to accommodate the strain. There are several important details to see in this image. First of all, on the left hand side of this image, we see normal stratigraphy, with the oldest layers at the bottom. The same is observed at the right – if you look at the lowermost layer and move upwards, you progressively go through younger units. However, if you start at the bottom of the kink and move upwards, you will find that you actually move through older units. These units have been folded so intensely at this point that these layers have been overturned; older layers are found on top of younger units. The relationship is clear in this case because we can see the full fold, but overturned units are often difficult to characterize out in the field since the order of layers we get used to gets reversed. Next, look at the thickness of a single layer to the left, outside of the fold, and then again at the axis of the fold and inside the kinked band. You can clearly see that in this case, the layer thicknesses have changed. Sometimes it is possible for rock layers to just bend, but if the layers are weak enough and the rocks are folded intensely, the layers themselves can be squished, causing their thicknesses to change. Geologists would call this “ductile” behavior since the layers aren’t just bending, the sediments in them are actually flowing in response to the bending strain. Some of the white layers are a little more resistant to this ductile flow, and you can see them being stretched and pinched into little sausage-like shapes that geologists call boudins. -JBB Image credit: Dr. Marli Miller (CC Licensed) https://geologypics.com/

[xponentialdesign]

Tin is generated via the long s-process in low-to-medium mass stars (with masses of 0.6 to 10 times that of Sun), and finally by beta decay of the heavy isotopes of indium. 

It is estimated that, at current consumption rates and technologies, the Earth will run out of mine-able tin in 40 years. Tin extraction and use can be dated to the beginnings of the Bronze Age around 3000 BC In 2006, about half of all tin produced was used in solder.

[arkassy]

[relatetorocks]

[earthstory]

Underneath a fault

Here’s the kind of question you might not have thought about if you’re not a geologist; what does the bottom of a fault look like?

Small faults can actually just end, but huge, continent-scale faults like the San Andreas Fault in California or the Anatolian Fault in Turkey have hundreds of kilometers of movement between the rocks on either side. What does the bottom of a faultlike that look like? Strike-slip faults tend to only go down about 10 or 15 kilometers deep. Megathrust faults at subduction zones can go deeper, but for now let's focus on a strike-slip fault. In that type of fault, the rocks are sheared, sliding past each other. Shallow, close to the surface, the rocks are actually able to break leading to earthquakes, but at depths of over 10 kilometers, it gets so hot that the rocks don’t break, they flow.

Rocks at the bottom of a fault are still sheared, but they are ductile. The minerals are able to change shape through one of several mechanisms depending on the temperature. The stresses of the fault reorient the minerals into new patterns, creating a rock like this called a mylonite with long strings of minerals created by the shear.

This rock specifically is an S-C mylonite. See how there are light and dark mineral bands with an angle between them? Those bands form as the shear stress causes the minerals to twist and regrow. Minerals with planar shapes like micas form bands in one direction in-between the white minerals that are rolled or otherwise twisted into the other bands. The angle between these two directions is defined by how the minerals respond to the stress and how much deformation has occurred after the bands formed. As strain continues to increase, the minerals rotate even more and new bands form, creating a process of shear band growth and twisting that can be used to reconstruct which direction a fault was moving.

This rock formed more than 10 kilometers deep in Earth’s crust, buried deep beneath a fault. Ever wanted to know what the bottom of a fault looks like? This is the bottom of an ancient fault, now exposed at the surface.

-JBB

Image credit: https://flic.kr/p/9WqmtS Read more: http://65.54.113.26/Publication/40487400 www.geo.arizona.edu/geo3xx/geo304/Foliations_Lineations.ppt

[relatetorocks]

Folding features resulting from compression.