The Tectonic Plates are Moving!

What has plate tectonics ever done for us? Not having studied geology, I have a basic understanding of the movement of earth's continents, but this book made me appreciate just how much of current geology it underpins. Marine geophysicist Roy Livermore, who retired from the British Antarctic Survey in 2006 after a 20-year career, convincingly shows here that the discovery and acceptance of plate tectonics was a turning point in geology, on par with Darwin's formulation of evolution by natural selection. To paraphrase evolutionary biologist Theodosius Dobzhansky: nothing in geology makes sense except in the light of plate tectonics.

Livermore has divided this book into two parts, beginning with the first generation of scientists exposed to plate tectonics. I was surprised he didn't start with Alfred Wegener, whose 1912 theory of continental drift is the intellectual progenitor of today's plate tectonics (recognition for it evaded him during his lifetime, see the splendid biography Alfred Wegener: Science, Exploration, and the Theory of Continental Drift). Instead, Livermore starts in the 1960s with the discovery of the zebra-skin-like pattern of past magnetic polarity stored in spreading seabed (see my review of The Spinning Magnet). The chain of events that led scientists to link this observation to others and suggest that the earth's plates were moving, and the stubborn resistance by especially the US geological to this idea has been told elsewhere in brief (see Four Revolutions in the Earth Sciences) and at gruelling length (Frankel's four-volume opus The Continental Drift Controversy), but Livermore here provides an excellent 180-page executive summary.

The real value of this book, however, lies in the second half, which takes the reader through all the subsequent developments in the 1980s and onwards. This part gives a wonderfully balanced overview of all sorts of controversies and new insights that complicated the picture developed so far. Plate tectonics turned out to not only destroy and create oceanic plates – continental crust could also be subducted and returned to the planet's surface. Then there is continued disagreement over whether the supercontinent Pangaea that existed between approximately 320-175 million years ago was simply the latest iteration in a very long-term cycle of supercontinent formation and breakup. Nield popularised this idea in Supercontinent, but it is not accepted by all geologists.

The US military makes repeated appearances in Livermore's story, and geologists have often benefited from technologies developed during the Cold War. Development of satellite technologies assisted in mapping the seabed, revealing the wonderfully complicated world of underwater subduction zones and mid-ocean ridges.

Then there is the influence of plate tectonics on global climate through the long-term geochemical cycles described in The Oceans. Carbon dioxide is added to the atmosphere via volcanic eruptions and at spreading mid-ocean ridges (both obviously require plate tectonics). Removal of carbon dioxide when rocks erode over time and get washed into seafloor sediments that ultimately gets recycled into the Earth's interior when oceanic crust is subducted. How is that for a neat little long-term thermostat? Additionally, the continents waltzing around and the formation of land bridges (see my recent review of Land Bridges) influence oceanic and atmospheric circulation, and thus climate, directly.

Finally, the current frontier of knowledge where all the action is: geophysics. What happens to the pieces of crust once they are subducted into the Earth's interior? Do they descent all the way to the core to form evocatively called "slab graveyards", from whence they rise up in the form of plumes as in a giant lava lamp? Or do they hover close under the planet's surface in a separated convection layer? (A fiercely contested subject, see Plates vs Plumes – Livermore sides with the idea of plumes). What of these mysterious entities at the boundary between the Earth's core and the mantle called Tuzo and Jason? Torsvik & Cock described them in Earth History and Palaeogeography as plume generating zones responsible for the majority of the large volcanic eruptions linked to previous mass extinctions (see Brannen's The Ends of the World). Have they been fixed in place over deep time? Are there really only two of them? And what of interactions and heat exchange between the Earth's molten core and the mantle? What does this mean for the Earth's magnetic field? When did plate tectonics start? Has it gone through different phases?

Since we can drill and dig just a few kilometres into the Earth's crust, the answers to all these questions are far out of our reach. It has only been in recent decades with the refinement of visualisation techniques such as seismic tomography and the development of complex computer models that we have been able to gather data and theorise on what happens in the Earth's interior. I came away from this last section with a renewed respect for, and interest in, geophysics.

So, what has plate tectonics ever done for us? From providing water to fill our oceans, hydrothermal vents where life probably first evolved, a carbon cycle to control long-term climate, to a geodynamo generating a magnetic field that prevents our protective atmosphere from being obliterated by the charged particles the sun hurls our way... plate tectonics has provided us with a planet that has been relatively stable for billions of years, providing just the right conditions for the evolution of complex life (see also The Goldilocks Planet). Having surveyed neighbouring planets, astrobiologists (they who study the possibility of life on other planets) have realised that plate tectonics will be a prerequisite for a habitable planet (for readable introductions, see How to Build a Habitable Planet and Lucky Planet).

The Tectonic Plates are Moving! is a rock-solid read: the pacing of the book is great, the irreverent jokes and anecdotes genuinely amusing, the overview of different schools of thought balanced, and the explanations lucid. Most of the jargon used is introduced and clarified, though I struggled a bit with all the names for rock and mineral types (there is no glossary included). I hope to remedy that with a basic geology textbook I have finally bought. There is a good number of helpful illustrations included, some of which would have been better had they been reproduced in a colour plate section.

Neither a dull textbook nor an overly technical read, Livermore strikes just the right balance and manages to deliver a compelling book on the importance of plate tectonics and the many exciting developments in past and current research.