Planet in a Pebble

This review appeared in the Times Literary Supplement on 7 January 2011

Jan Zalasiewicz, The Planet in a Pebble: A Journey into Earth’s Deep History, 234pp., Oxford University Press, £16.99

Early in our planet’s history, around four and a half billion years ago, a Mars-sized neighbour named Theia crashed into us at forty thousand kilometres per hour. As the two planets atomised from the violent impact, most of Theia melted into the Earth, its iron core sinking into ours to form the medium-sized body we call home. As some of the outer materials of both planets hurtled away from the impact site, they, too, began to condense into a new, smaller body that would go on to orbit the re-formed Earth: our Moon. “It is a fine story”, as Jan Zalasiewicz observes, though it is only one of many fine stories that can be told through the apparently ordinary pebble from which this extraordinary book proceeds.

It is a brilliantly simple idea: pick a pebble at random from, say, a beach in Wales, and see what its constituent elements can tell us about the history of the Earth. A slate-grey pebble, criss-crossed with lines of whitish quartz, it is composed mostly of oxygen, silicon and aluminium, along with traces of nearly a hundred rarer elements, including strontium, vanadium and rubidium. What exactly are these elements, where did they come from, and how did they come to be compressed into the “enormous atomic vault” of our palm-sized pebble? To answer these questions, Zalasiewicz takes us on a dazzling voyage of telluric discovery, from exploding stars whose storms of neutrons seeded our planet with mineral grains, to dense layers of solid methane that lurk beneath the oceans to this day, and through which the proto-pebble would have migrated slowly during its billion-year journey underground.

Today, almost everything that happened to the pebble and its particles can be retrieved in the laboratory, and Zalasiewicz’s explanations of how geologists have learned to unravel such complex geochemical matrices are just as gripping as his detailed accounts of the pebble’s eventful history. Take zircon, for example, a high-density accessory mineral that often turns up in trace quantities in rocks such as granites and quartzes. Zircon crystals may not be much to look at, but they have a set of unique chemical properties that allow geologists to reconstruct the long-vanished landscapes in which they formed. Even better, zircons act as eerily accurate atomic clocks that can tell us when as well as where they first appeared, granting earth scientists access to a virtual “time machine”, as Zalasiewicz describes it, “one that can traverse an entire planet from its beginnings” nearly four and half billion years ago.

Because such dizzying depths of time are all but impossible to grasp (I, for one, have no real sense of what “four and a half billion years ago” actually means), they offer a challenge to anyone setting out to write a work of popular geology. Zalasiewicz’s solution is to divide these great swathes of time into distinct topographical regions through which our pebble blithely wanders like the hero of some picaresque novel. Thus, 300 million years ago, during the Carboniferous period, the surface of the Earth began to teem with life, with heavily armoured fish colonising the waters above the long-buried pebble. Later on, at the end of the Cretaceous, around 65 million years ago, the giant meteorite that is likely to have killed off the dinosaurs would have caused the entire Earth “to ring like a bell . . . the pebble-form, while still firmly part of its underground rock stratum would have trembled too as the impact waves passed through.”

Treating the pebble as a Zelig-like protagonist helps to build a surprising narrative tension, and towards the end of the book, as the pebble-stratum is about to break the surface and start its new existence as a weather-beaten slab of rock, the story begins to get into its stride as a geological page-turner. Waves crash against the shoreline, picking up the exposed sheet of slate and hurling it against the cliff face. “The now-nearly-pebble breaks off, as a sharp-edged shard of the slab. It has all the features of its own narrative, that we have followed until now,” writes Zalasiewicz. Once the sea takes over proceedings, the time-frame shortens dramatically, and after only a few years of incoming tides, with the tens of thousands of collisions they provoke, the pebble and its neighbours have lost their jagged edges, and now take their places in a shingle bank where the forces of erosion are already grinding them back to their constituent parts. Every tide reduces a pebble to a detectable degree, and on particularly exposed stretches of shoreline, an individual pebble can lose nearly half its mass over the course of a single season, while cosmic rays relentlessly bombard its surface, breaking down silicon and oxygen atoms into fragments of radioactive debris. “Night and day”, notes Zalasiewicz, “the pebble is disintegrating.” Not many books can make you care about a dull-looking fragment of rock, but one of the many achievements of this enjoyable excursion into the deep geological history of our planet is a momentary sadness at the thought that Zalasiewicz’s ancient protagonist – his “capsule of stories” – will soon have been worn away to nothing.

Richard Hamblyn


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