We all have a pretty good idea of what a volcanic eruption looks like, but they are only the surface expression of a much larger and longer underground process that is hidden from view. The internal workings of a volcano, its plumbing if you will, are studied by the relatively new scientific discipline of volcanotectonics. Icelandic volcanologist Agust Gudmundsson has been researching and teaching this topic for two decades and here delivers the field’s first textbook. In preparation, I beefed up my knowledge base by first reviewing a introductory volcanology textbook, but it almost was not necessary—Volcanotectonics turned out to be exceptionally instructive and accessible.
As an academic field, volcanotectonics draws on the disciplines of structural geology, tectonics, geophysics, and, more generally, classical physics, specifically rock-, fracture-, and fluid mechanics. In the field, it draws on studies of rock outcrops, geodesy (the study of the shape of the earth and its gravitational field, here specifically the monitoring of ground deformation), and, importantly, seismology (the study of earthquakes). Additionally, there is a healthy dollop of modelling involved. That is quite the list. Plus, the book is a hefty tome of almost 600 pages. Feeling intimidated already? No need.
There are five underground structures that you will become intimately familiar with reading this book, all of them made of magma: chambers, reservoirs, dikes, sheets, and sills. The first two are, as the names imply, the underground storage parts. Reservoirs are large and deep-seated (typically 10–25 km below ground), chambers are smaller and more shallow (commonly 1–5 km below ground). The second three are the parts connecting reservoirs, chambers, and the volcanoes at the surface. Gudmundsson here uses the word dike to encompass both vertical dikes and inclined sheets. Both of these cut across rock layers. Sills, on the other hand, do not: they form where magma meets a rock layer it cannot penetrate and then spreads out laterally, parallel to it. Sills are thus emplaced horizontally, or close to it.
“Very roughly, it looks something like this: volcanoes are fed by either shallow magma chambers that are fed by deeper and larger reservoirs, or sometimes directly by reservoirs, through a complex, branching network of dikes, sheets, and sills. “
Having explained the basic anatomy, Gudmundsson proceeds with important physics concepts such as stress, strain, and elasticity, including various laws and constants to do with mechanics. It also offers one of the clearest explanations I have seen for when and why to use different modelling approaches, in this case analytical, numerical, and analogue ones. The mathematical details are purposefully kept relatively brief, with readers referred to the technical literature and books such as Rock Fractures in Geological Processes and Modeling Volcanic Processes for more details.
All this setup is a necessary warming-up exercise to understand volcanotectonic processes. Very roughly, it looks something like this: volcanoes are fed by either shallow magma chambers that are fed by deeper and larger reservoirs, or sometimes directly by reservoirs, through a complex, branching network of dikes, sheets, and sills. Many of these are dead ends: dikes can become deflected into sills if they hit rock layers they cannot penetrate and stop there (or, with luck continue upwards again if they hit on weak spots), or they can run out of steam, reaching equal pressure with their surroundings and end up as arrested dikes. Those that make it to the surface are called feeder-dikes and result in volcanic eruptions.
Gudmundsson argues that chambers originate from the rapid (geologically speaking) emplacement of sills. As long as new dikes from the reservoir at the crust–mantle boundary feed more magma into the growing sill before it cools down and solidifies, it has a chance to develop into a magma chamber, passing through an intermediate stage called a laccolith. The shape these chambers take is rarely a perfect sphere, more often a flat ellipsoid. All this subterranean movement, meanwhile, is frequently accompanied by small, local earthquakes occurring in swarms, and small but measurable ground deformation, both of which are important for monitoring and forecasting eruptions.
“What makes the book all the more convincing and attractive is that it relies on more than just theory, calculations, and modelling exercises. Gudmundsson adds results from field studies on recent eruptions and fossil volcanoes worldwide and includes numerous photos.”
For magma to force its way through the surrounding rock requires sufficient pressure to overcome local tensile strain and stress. Interestingly, the process is analogous to the artificial hydraulic fracturing, or fracking, for shale oil and gas, which has allowed the testing and confirming of ideas. Large sections of the book go into the calculations and equations you need to quantify how much pressure is needed, how this is affected by local stress and strain, and how modelling assumptions make a difference. Simplifying assumptions of a homogeneous and isotropic (i.e. uniform in all orientations) rock crust make for easy calculations but sometimes unrealistic results. In reality, our subterranean world is not quite like that: Earth’s crust is heterogeneous and anisotropic, with many layers stacked on top of each other, stiff ones alternating with softer ones. Dedicated chapters explore what this means for the often erratic paths that dikes follow as magma moves through the crust, and for the size of eruptions. Gudmundsson also outlines how particularly catastrophic eruptions (for instance Tambora, Krakatoa, and Toba) resulted from caldera collapses. Here, various dikes join up to form a ring-fault that isolates a block of rock that can subside into the magma chamber, squeezing it largely empty in the process. A final chapter applies the material to forecasting and (possibly maybe) prevention of eruptions.
There are obviously many more subtleties and complexities to it than this brief sketch, and I learned an incredible amount, suggesting that some of the unknowns Ellen Prager highlighted in her book Dangerous Earth are being filled in. What makes the book all the more convincing and attractive is that it relies on more than just theory, calculations, and modelling exercises. Gudmundsson adds results from field studies on recent eruptions and fossil volcanoes worldwide and includes numerous photos. Dikes and solidified chambers, now called plutons, come to the surface over the long stretch of time due to erosion, while quarries and road-cuts offer another window into the underground.
“The real selling points of Volcanotectonics are its superb structure and accessibility. […] earth science students […] can pick this up without trepidation”
The real selling points of Volcanotectonics are its superb structure and accessibility. Clearly formulated aims start each chapter, while summaries and lists of all the symbols used end each one. Student exercises are accompanied by worked examples that go through equations stepwise and explain symbols and units. Some degree of repetition and cross-referencing between chapters ensures important concepts are reiterated. Mathematical operators you are unlikely to have seen before are extensively introduced. And jargon is consistently and repeatedly explained. For example, the fact that strike- and dip-dimensions of dikes correspond (roughly) with their horizontal and vertical dimensions. By constantly repeating this, I noticed that halfway through the book I had absorbed their meaning. Repetition works. Arguably the only thing that students might miss is a glossary.
The result is that even someone like myself (a biologist interested in geology), whose reading on the subject is limited to an undergraduate geology textbook and introductory graduate texts on structural geology and volcanology, never felt out of my depth. Volcanotectonics should be a hit with earth science students and they can pick this up without trepidation—Gudmundsson’s many years as a teacher shine through. Significant questions and challenges remain, for example regarding submarine volcanism near mid-ocean ridges, but Volcanotectonics brings together a vast body of work and represents a significant advancement in our understanding.
Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.
Other recommended books mentioned in this review: