Structural Geology

Planet Earth’s many landforms can be breathtaking to behold. Plate tectonics has given us a basic framework to explain their formation, but there is far more to this story than that. I recently mentioned wanting to learn more about geology, having shunned the subject in favour of biology at university. So, fascinated by photos of folded rocks that look like so many layered cakes that had an accident in a bakery, and freshly armed with some basic knowledge of geology after my recent reading of Essentials of Geology, Haakon Fossen’s Structural Geology seemed like a good starting point to deepen my knowledge further.

If you know how to read them, whole landforms, individual rocky outcrops, and even slices of a rock examined under a microscope can reveal a detailed story of how they have formed. Rock deformation, then, is what the subdiscipline of structural geology is all about (this excludes structures produced by sedimentation or magmatic processes, though, once produced, these can later be deformed). The first edition of this book was published in 2010, but Fossen has not rested on his laurels. Incorporating feedback from readers, and adding an online module with animations, the second edition of this advanced-level textbook followed in 2016. The book is specifically intended for use in coursework and includes chapter summaries and review questions. But, as I found out, the book can also be tackled outside of a classroom setting.

Fossen starts off with four chapters on the basics of strain (changes in length or volume of rocks) and stress (how the forces involved acted). These are probably the most technical, introducing basic matrix algebra and the graphical methods geologists have developed to analyse strain and stress and represent these three-dimensional processes on paper in a graph or diagram. Rheology, the mechanical properties of materials and how they flow under pressure and temperature, is one other basic cornerstone that is introduced.

Fossen then breaks down his approach following the two main forms of deformation. Near the surface rocks behave in a brittle fashion, breaking and shattering as the Earth moves, and Fossen here discusses fractures, joints, veins and faults. At depth, under high pressure and temperature, rocks are more plastic and deform slowly by flowing. A short introduction to the basics of crystallography is needed here, after which Fossen treats structures such as folds, foliations, lineations, boudins, and shear zones. This catalogue of structures is then placed in the context of tectonics, explaining how these show up when plates collide, separate, or move past each other. Salt in rock form behaves rather differently from most other rocks, so Fossen dedicates one chapter to salt tectonics (for a fuller treatment see Salt Tectonics). Finally, he looks at restoration: how can you infer what rocks looked like before deformation?

The above paragraph is full of jargon, but don’t let that faze you. Read through Structural Geology, and the terminology gradually will be demystified (even though I found it useful to have a copy of the Dictionary of Geology & Earth Sciences at hand). Before long I found myself casually reading about fracture mechanisms in the brittle regime and realising that these words meant something to me. The glossary helps, but it is especially the huge number of photos and diagrams that clarify much. Trying to describe movements and dynamic processes with words and static pictures will only get you so far, though. This is where the online module shines, as it contains a huge number of Flash animations. I can try to describe in words how a listric fault moves, or how boudins form, or I can show you.

Structural Geology is more than just a technical catalogue of deformation processes, though. Obviously, there is the obligatory nod to the practical relevance of this knowledge for structural engineers and the fossil fuel and mining industries. What I found far more fascinating is what this book reveals about structural geology as a field of knowledge.

First, Fossen stresses how important it is to make field observations. Geologists can do small-scale experiments with plasticine or putty, or they can artificially deform rocks and minerals under high pressure and temperature over the course of days or weeks. Similarly, there are plenty of options for simulations and numerical models. But all these are replete with simplifications and assumptions, such as idealised homogeneous materials. As in other disciplines, it is the only way to get to grips with the data. But how realistic are these experiments and models, especially given the extremely long time scales over which rock deformation plays out? Fossen reiterates throughout how important it is we continue to check this against real-world situations.

Second – and as an evolutionary biologist I can appreciate this – structural geology is a historical and descriptive discipline. We look at (essentially) static structures that result from long and slow processes and we try to figure out what happened.

Third, each chapter introduces one kind of rock deformation, but in the field you are usually looking at rocks that underwent several deformations, either simultaneously or sequentially. Figuring out what happened and in what order can be a challenge.

Especially points two and three explain how we can have competing models for observations, and why geologists disagree amongst each other which is the correct explanation for the formation of observed geological structures.

The last thing I want to remark on are the production values – it’s a beautiful book! Big, heavy, and with a sturdy binding, it is absolutely chock-a-block with full-colour photos and illustrations. And it comes at a very reasonable price, suggesting that the poor binding and high price of an undergraduate textbook like Essentials of Geology are unnecessary.

Not being a geologist by training, it is hard to make hyperbolic claims about this book being the best. I have not read other books on this topic and am not familiar with the literature. I also have not used this book in the setting of coursework or field excursions. What I can say is that, even if some of the technical details might have gone over my head, Structural Geology does a tremendous job at explaining the subject, even to someone with a limited background in geology. The book has received rave reviews elsewhere and it is easy to see why – high production values, a well-structured and well-written text, and a supplementary online module with useful animations make this book a solid choice if you need to or want to read up on this topic.