Rock Deformation for Moonlighting Chemists - The Freelance Files MMCCLXVIII

Extreme example of ductile deformation

It's not unusual – common, even – to find a freelancer who has made a valiant attempt to transcribe¹ useful information that could conceivably be seen as tangentially related to said freelancer's area of expertise. Then again, many of them seem to have a rather loose definition of "tangentially related." One such writer is Madhu, of DifferenceBetween.com; here making her fourth appearance on the DotD podium. Madhu claims to be a chemistry student, but also appears to be the site's self-appointed geology expert based on her previous appearances in the categories of mineralogy and sedimentary petrology. Today, our madhu scientist returns to regale us on structural geology and tectonics with her post, "Difference Between Ductile and Brittle Deformation."

Madhu decided that her answer should be about earth science, which is fair (and right up one AN staffer's alley). We'll give Madhu her due for a fairly cogent regurgitation of what she called "Diffusion creep," which all but metamorphic petrologists simply call "recrystallization" (based on her off-topic dive into chemistry). Madhu had some pretty serious problems with the rest of her article, however, starting with one of her earliest paragraphs in which she tried to "inform" her readers that,

"Ductile deformation in Earth science is the production of large, open folds in the sediments or rocks in front of an advancing glacier which can develop into overfolds. This can cause the sediments or rocks to begin to undergo internal thrusting due to continued ice advance..."

Our staff geologist simply looked at those two sentences and said, "WTF? Glaciers??!! OK, well, there is such a thing as glaciotectonics but it is quite rare and it is not the cause of the vast majority of ductile deformation of rocks. No, that would be tectonic activity!"

Unfortunately, Madhu skipped all the large-scale features she mentioned in that first paragraph in her rush to get to some fine details. In the process, she glossed over folding and faulting (the macroscopic, visible evidence of, respectively, ductile and brittle deformation) to get to – wait for it – the most technical details of deformation of crystalline solids, including "mechanical twinning/kinking, grain boundary sliding, and rigid body rotation." Really, Madhu, that's not what the average eighth-grader (your presumed audience) would want to know. More to the point, rocks aren't crystalline solids. Nope, your average middle-schooler studying earth science wants to know that both brittle and ductile deformation take place in a three-dimensional stress field, and the form of that stress field (along with rock composition) is where ductile and brittle deformation differentiate. A simple explanation follows:

Where the vertical stress, or confining pressure, is relatively low (and rocks are relatively cool), deformation is more likely to be brittle: faulting. Where confining pressure is relatively high (and so is temperature), the result is more likely to be ductile deformation: folding. If confining pressure is reduced or horizontal stress increased, folding can give way to faulting. In other words, depth of burial is a major factor, along with the physical properties of the rock bodies. 

That's complex enough in and of itself without going into the sort of PhD-level discussions of microscopic deformation Madhu attempted.

Last, Madhu intoned that,

"The key difference between ductile and brittle deformation is that ductile deformation occurs at low strain rates, whereas brittle deformation occurs at high strain rates."

While nominally true, that's not the key difference between ductile and brittle deformation. The key difference is that, while both forms of deformation are irreversible, brittle deformation entails rupturing of the body while ductile deformation involves stretching and/or bending. Those are in the definitions of the terms our Dumbass of the Day was throwing around.

¹ By "transcribe," we really mean "attempt to avoid getting caught plagiarizing."

SI - STRUCTURAL GEOLOGY