Breaking rocks

When rocks are broken, they are not necessarily useless. Quite the opposite – geologists are often happy when their rock samples show signs of breaking. This does not mean you should go into our offices and smash all the pieces of rock you see there (we will not be happy then), but if we can relate a crack to the natural process that caused it, we might be able to puzzle out one more piece of a rock’s history. Some geologists become so fascinated with broken rocks that they behave in a way their non-geologist friends deem worthy of a picture: KD1_fig_1 What the geologist in the picture (i.e., me) was trying to photograph are fractures in the pavement next to the Place royale du Peyrou in Montpellier, Southern France. As someone interested in fractures, I found it curious that most of the cracks in this pavement go right through the middle of the large blocks. Let us now try to figure out why the cracks form parallel to the short axis of the blocks. Why did they not choose another direction? When a fracture forms, its orientation depends on the direction of the highest stress, that is the direction in which the block is squeezed or pulled the most. If we, for example, push like this, everything is trying to move away from the squeezed area, and the fracture will form parallel to the direction we push in: KD1_fig_2 The same thing will happen if we pull like that: KD1_fig_3 On first sight, we cannot decide whether there was squeezing in one direction, or pulling in the direction perpendicular to that. We need more information for that, and we will get to that in one of the next posts. Even if we know why the cracks are oriented the way they are, we still do not know why they are in this exact position. Why not a little more to the left or right? To explain this, we need to include the surrounding blocks in our argument: KD1_fig_4 All of this might look trivial and, somewhat embarrassingly, this pavement is not even made of natural rocks, but some kind of man-made pseudo-rock material. Still, it shows how much you can think – and find out – about even the simplest things surrounding you. And, more importantly, we can use the same principles when we study actual rocks. And in these, it gets more complicated very, very quickly. For now, just a little taste of what this may look like in a rock: KD1_fig_5 The black parts (the mineral chromite) once belonged together, but have since been pulled apart. Very conveniently, this type of fracture is called pull-apart fracture. (Sometimes geology is simple like that.) Another mineral (olivine) filled the newly opened space. Because there are a few smaller fragments of chromite in the middle, one might speculate that there was not only one fracturing event, but several: KD1_fig_6 However, several parallel fractures might also have formed simultaneously. Analysing the minerals between the chromite fragments can help us here: If the composition of olivine grains between the two leftmost fragments is different from the composition of the grains between the two fragments on the right, they probably formed at different times. So, even though the rock as a whole looks relatively undeformed and happy as it is, by zooming in and looking at small, microscopic structures we can analyse deformation events that may have happened millions of years ago. One little chromite and its neighbours might be the single survivors able to tell us something about them…

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