Plates, Plumes, and Paradigms

Summary and Review

Abstracts of all papers are available on the Web

At nearly 900 pages, Plates, Plumes, and Paradigms is the size of a phone book, or a bible. Having followed the research on its subject for the last decade, I am inclined to think of "P3" as a bible that sets forth the next stage of plate tectonics. Every geology library should have it, and every wide-minded geologist and ambitious graduate student should consider it.

Plate tectonics is geology's primary theory of the Earth. A handful of core elements account for its sweeping success in explaining the configuration and dynamics of the lithosphere—the continents and oceans, gross patterns of topography today and the tectonic forces that have dominated the last 2 billion years or so. When these elements of plate tectonics were formalized in the 1960s, among other things they made sudden sense of almost all the world's volcanism.

Except for the leftover "hotspot" volcanoes. These lie within the plates, away from the active edges where other volcanoes cluster. The textbook examples are the Hawaiian island chain in the mid-Pacific and the line of extinct volcanoes that ends in the Yellowstone region in North America. Soon a plausible hypothesis arose: these volcanic lines are the trails made by plumes of hot material rising from fixed spots deep in the Earth's rocky mantle, perhaps from its very base at the edge of the iron core. The hot spots leave trails of volcanoes as the plates move above them, like ink blots and scribbles left on a moving page from a fixed pen. The hotspot or plume hypothesis was born.

The editors of Plates, Plumes, and Paradigms make a good case that the hotspot/plume theorists have spent the last 35 years in a blind alley. They do this through 47 papers that (1) undermine plumes, (2) rethink the possibilities in plates, and (3) attack the mental inertia of the paradigm. After a while one gets the unsettling idea that generations of geoscientists, beguiled by a beautiful but inadequate hypothesis, have taken neither plumes nor plates seriously enough.

Plumes Undermined: The original hotspot theory has morphed over the years. It was first elaborated upon, for instance, to explain another mystery—great bodies of lava known as the oceanic plateaus and the continental flood basalts—with a model involving hot plumes of molten rock. Later still the plumes were allowed to wave and shift in the "mantle wind" and to flow sideways as far as needed. The editors of P3 argue that the constant dodging and weaving of plume theory over the years has left it without power or integrity today. They demonstrate a steady record of failure in plume theory's predictions: hotspots are not hot and not fixed; plume heads are not preceded by large swells; plume trails are not orderly successions; plumes are not seen in the deep mantle; plume heads and plume tails do not match up. And progress in mantle studies has displaced the initial assumptions behind plumes.

Plates Reconsidered: Too many modelers have operated with outdated ideas about plate tectonics and the Earth's structure: that plates are rigid and strong, propelled by convection from below; that the mantle beneath is arranged in neat layers of well-mixed rocks; that heat from below forces the mantle into upwellings.

Plates, Plumes, and Paradigms, edited by Gillian Foulger et al

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A Paradigm Punctured: Paradigms are bodies of accepted evidence, tightly bound together by theory and underlying assumptions, that are shared by communities of scientists. They can keep the community in efficient coordination, giving everyone a common language and program, or they can go awry in groupthink. Newtonian physics is at the one end; the scientific creationism that met its end in the 1900s is at the other. Paradigms can withstand a great deal of contrary evidence and weaknesses in theory until their assumptions must be changed; that's human nature and it can be irrational. Papers by William Glen and by Don Anderson and James Natland are uniquely valuable in tracing the history of plume theory, its hardening into a "quasi-paradigm" and its response to critics (or lack thereof).

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