Every scientist has something that is frequently portrayed incorrectly that grates on them.

One of mine is the idea that tension in the crust relates to far-field forces pulling apart the rock. You see this a lot, but it’s just not right!

Mini thread... https://twitter.com/iris_epo/status/1264697718292779008

The tensile domain occurs when normal stresses are negative (or positive if you’re an engineer, but that’s another topic), which would be away from each other, like in the @IRIS_EPO movie.

But this doesn’t really happen in nature.

In the crust, the stresses are always compressional. Look at the Andersonian fault classification. Stresses always pointing inwards (compression), even for Normal faulting. Normal faulting involves EXTENSION (strain), but not TENSION (stress/force).

So, what about tensile cracks and things like these shale dykes? Surely they involve far-field forces or stresses pulling apart from each other? Nope.

The thing that fracturing/faulting block diagrams don’t portray is that it’s not the far-field stresses that control deformation - it’s the effective stress, (stress minus internal fluid pressure).

Anyone remember the Hubbert & Rubbey Beer Can experiment (here from Davis 1985)?

Tensile failure in the crust typically occurs because of high internal fluid pressures. Tensile fractures form when those pressures (pushing outwards) exceed the far-field forces (pushing inwards) and tensile strength.

That’s how veins like this form.

Now, there’s some v. localised places where tensile failure occurs differently, like outer arcs of folds & inside shear zones.

But, even then, it would be correctly portrayed as little outwards pointing arrows inside the ‘block’ - not big outwards pointing arrows outside.

So, folks, can we please stop putting big outwards pointing arrows on fault and fracture block diagrams? Or at least point out that the arrows are for strain (extension) and NOT stress/force.

So, what minor thing grinds your geological gears?

BTW. I’m not trying to pick on @IRIS_EPO here, as they do absolutely awesome work! Their outreach activities are just fantastic. This is just a minor technical thing that gets me grumpy!