2 of 4
While geologists were mapping the continents and comparing notes, geophysicists were learning about heat in the earth and developing models for how mountains rise and fall, leading to a comprehensive understanding of how the earth works. The synthesis of continental and marine geology with the geophysics of the inner earth came after World War II. Maps of the seafloor revealed the submarine boundaries of lithospheric plates that merged with mountain belts and fault zones on the continents. It turned out that continents are like lunch trays on a conveyor belt. Magnetic properties of the ocean crust revealed patterns of seafloor spreading away from mid-ocean ridges (where the conveyor belt begins). Mountain ranges, explosive volcanoes, and deep earthquakes mark the locations of plate convergence, even creating deep ocean trenches where ocean crust is pushed back into the earth (where the conveyor belt ends). You get Pangaea at the cafeteria when trays approaching the dishwasher collide into a mangled mess.
Nield explores sociological factors in the gradual acceptance of plate tectonics, suggesting that science is a kind of supercontinent. Just as plates are slowly shifted by a variety of forces, paradigm shifts in science can be preceded by decades of give and take between scientists. Wegener's idea was accepted earlier and more widely in Europe than in America. Frankly, American scientists were put off by Wegener's rather bombastic pronouncements (or at least the tone employed by his English translator). Nield suggests that European science was driven by inductive methods (allowing explanations to emerge from the data set), while American scientists practiced a democratic form of deduction called "multiple working hypotheses" (collection of data to deliberately test or reject numerous possible explanations). By American standards, Wegener and his cohort were jumping the gun. Also, Nield explains that scientific advances can stall or get sidetracked if scientific models become confused with reality. U.S. geographic surveyors were committed to a model of crust properties that was useful for correcting local gravity variations but carried the assumption that rocks do not flow over long periods of time. Europeans embraced a more dynamic model of the inner earth, making the idea of moving continents quite tenable. (While Nield doesn't disparage or completely neglect the role of U.S. scientists in the development of plate tectonics, I feel he ignores some important contributions from the New World to the emerging synthesis after World War II.)
The final third of the book is devoted to discoveries and theories about the earth before Pangaea. Nield uses the analogy of palimpsest texts to describe how geologists use ingenious geochemical methods to unravel the history of the earth's oldest rocks. The work has led to the "discovery" of supercontinents that preceded Pangaea: Ur some three billion years ago and Rodinia about one billion years ago. The influence of these Precambrian tectonic revolutions on the history of life is emerging from the supercontinent of science. Ur—with its marginal shallow seas—was a template for cyanobacteria growing layer upon slimy layer on the seafloor, converting carbon dioxide in the atmosphere into oxygen. Rodinia was an equatorial supercontinent, whose position and fragmentation may have promoted global cooling to the point of converting earth into a temporary snowball (or slushball, depending on the scientist). The appearance of multicellular life with three-layer bodies during the time of Rodinia's icy fragmentation about 650 million years ago implies some connection. "On the supercontinent of science, everything must fit together," the author reflects.
share this page
RSS Feed
