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Saturday, March 7th, 2009
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Earth: An Intimate History

by Richard Fortey

Plate Tectonics from the Precambrian to Pliny to the Present

A review by Doug Brown

UK paleontologist Richard Fortey has previously written about his particular field in the wonderfully accessible books Trilobite!: Eyewitness to Evolution and Life: An Unauthorised Biography. In the US, the latter title was Life: A Natural History of the First Four Billion Years of Life on Earth, because publishers have a perverse need to rename books when they cross the Atlantic. Trilobite! (I love the exclamation point, which they removed from the US edition) was about Fortey's particular critter of study, and Life broadened out to encompass all of paleontology. With Earth, Fortey has expanded further to cover plate tectonics.

Tectonics, a.k.a. continental drift, is an idea that has been around for some time. Fortey covers the history of the idea, as well as its slow acceptance among geologists. It wasn't until the 1960s, when paleomagnetic evidence from sea floors, combined with detailed sea-floor maps, finally brought plate tectonics into the mainstream. As Fortey explains, it wasn't just dogmatism that kept geologists from seeing what today looks obvious; until the 1960s, there also was no credible mechanism for the process. Once seismic studies of the crust and upper mantle started to provide a picture of what was going on deep under our feet, the picture came into focus. Fortey is very kind to early incorrect notions about the world; as he repeatedly notes, people were working with what little evidence they had, and explaining things in the context of their times. How many things that we think we know are based on fragmentary evidence? To Fortey, part of being a scientist means being humble about what you think you know. Good advice.

For those unfamiliar, here's tectonics in a nutshell. Molten magma from the upper mantle oozes up and forms the basalt sea floor, usually along long rifts. The newly formed oceanic crust spreads away from these rifts, and more sea floor forms. Continents are made of less dense material, so where spreading oceanic crust meets continental crust, the sea floor is subducted underneath the continent. This not only causes earthquakes as the plates rub past each other, but as the oceanic crust sinks it remelts along with the water and sediments it has accumulated, causing volcanoes. As this new molten material contains water and volatile gases, volcanoes on continental edges are more likely to be explosively eruptive, hence the popularly called "ring of fire" around the Pacific basin. The sinking of the plates is what causes them to spread at the rifts; they are being pulled apart, not pushed. In a few places we find what are called hot spots, where a lone magmatic plume feeds volcanoes in the middle of plates. The Hawaiian island chain is the classic example of this, and Fortey spends some time there covering what is special about it. Where continental plates run into each other, mountain chains rise up. The Appalachians are a remnant of when the North American plate and the African plate kissed and parted, and the Himalayas are still being pushed up by India's slow-motion collision with Asia.

The book is set up as a tour around the globe, moving from place to place where tectonic activity is well illustrated. Plants and animals are also included, as throughout history their distribution has been affected by tectonics as well. Fortey rarely misses an opportunity to show how people have been affected by geology, both in where and how they live. My only complaint is a little one, but still one deserving of a waggled finger. Near the end of the book Fortey states that glass is a slowly flowing semisolid, as evidenced by slumping glass panes in cathedrals. This is a well-debunked myth. Glass panes used to be made by spinning molten glass out into a disc, and then cutting the disc into panes. The panes were not of uniform thickness like modern glass -- as a result of the spinning, the outer edges were thicker. These were usually placed into frames with the thicker edge down. Thus, the glass hasn't slumped; it always looked like that. No other glass artifacts from (even older) history show any signs of flowing or oozing. Glass is an amorphous solid, but it is a solid.

Aside from that one half-page, Earth is a well-researched and enlightening read. Like Bill Bryson (who gives a glowing blurb on the front cover of the UK edition), Fortey can frame complex topics in easy-to-understand ways, and his travelogue writing makes you want to go see places for yourself. Earth is, to use the cliché, profusely illustrated, both with lucid drawings and photographs (color and black and white). Fortey's humility and warnings about not thinking we've got it all figured out run like a precious mineral vein throughout the book (hey, it's a geology review -- waddya gonna do). You'll also learn in a very Connections-esque chapter how the dollar is directly related to mining in the Czech Republic in the 1500s. I'll stop myself from the painfully obvious pun "this book rocks," and just say Earth is a good book for explaining why the earth is made of what it's made of, why various rocks (and critters) are found in various places, and why the continents look how they do.


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