When taking a stroll outside in the mountains, it is common to encounter outcrops of rock. In many cases these layers may appear warped, distorted, or cut by fractures, assuming an incredible variety of shapes and forms. When looking at these geological structures, it is simply impossible not to wonder about what forces may have produced the complex geometries we see in rocks and what tremendous history rocks may have faced over geological periods, at depths and temperatures very different from those that we find on the surface of the Earth.
Structural geology is about the description of the geometry of geological structures aimed at understanding the process that generated them: folds, faults, fractures, boudins – how they originated, why they formed, and what is their deformation history. Structural geology investigates features that vary in scale and size from deformation structures within a grain of quartz (microstructures), to features visible in outcrops to folds and faults spanning entire mountain chains for hundreds of kilometers. All these structures contain information about the tectonic history of our planet and also about the conditions and mechanisms at which the Earth’s crust and its materials deforms, as written in a book of stone.
The structures preserved in rocks often preserve a fascinating history, albeit often difficult to read, made of past earthquakes, collision of continents, opening and destruction of oceans, emplacement of magma, burial of rocks at high depths, and exhumation of metamorphic rocks back to the surface. We can learn more about these and many other processes by learning how to read structures in rocks. Not only that, structural geology is fundamental for the society: the location of resources such as hydrocarbons, underground water, and ore bodies, is controlled by geological structures (e.g. hydrocarbon ‘traps’), and so is the exploitation of building stones from mountains. The structures in rocks dictate the likeliness that a mountain or coastline has to collapse, and it is impossible to plan human buildings such as roads, tunnels, or dams, or even planning cities without an understanding of the structures present within rocks.
The structural geologist is the only figure that conducts research on the deformation and evolution of rocks and mountains, providing information on the Earth’s tectonic history and on the processes that occur at tens to hundreds of kilometers beneath our feet at conditions inaccessible to making, and it is the only professional figure that can help society and urban planners in predicting the distribution of natural resources within rocks and understanding the hazards associated with building human structures in and on deformed rocks. At such, the role of the structural geologist in society is of paramount importance, more than ever now that we live in an overcrowded world prone to hazards and with vanishing natural resources that should transition to sustainability.
In these pages, I hope to show you how beautiful and fascinating rock structures are, and how important they are for us, by sharing many photographs of rock structures from the field and the microscope. If you want to support me, please feel free to follow me on my socials or support me at the price of a coffee!
Some quick definitions...
A geologic structure is a tridimensional configuration of elements that delineates a specific geometry within deformed rocks. Structure derives from the Latin struere, to build.
Structural geology is the study of structures produced by deformation within rocks. As such structural geology does not investigate the process of development of primary structures, i.e. structures producing during the formation of igneous and sedimentary rocks such as bedding planes or magmatic banding, but deals with the deformation of such primary structures.
Tectonics (from the Greek tektos, builder) study the deformation of the Earth’s lithosphere and the processes producing a specific sets of structures within the Earth’s crust. The difference between structural geology and tectonics is somewhat blurred, but structural geology focuses primarily on classifying and describing structures we can observe in the field and the processes leading to their formation, whereas tectonics deals with the larger scale processes causing the formation of a specific set of structures in a region of the Earth that was subject to deformation (i.e. tectonic regimes).