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Not Continental Drift but Plate Tectonics
(Released May 2010)

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  by Adam T. Mansur  


Key Citations




Radioactivity and Isostasy


Radioactive decay describes the spontaneous breakdown of unstable (or radiogenic) atomic nuclei. When an atom decays, it ejects part of its nucleus and a burst of energy. The discovery of radioactivy in 1895 forced a reinvestigation of the thermal history of the Earth. Geologists like John Joly and Arthur Holmes recognized that decay of radiogenic isotopes of potassium, thorium, and uranium would serve as an important source of heat in the Earth's interior.11 Radioactivity undermined geological theories of contraction, because the calculations of the cooling Earth at the root of these theories assumed no active heat source within the Earth. With radioactivity, this assumption was no longer valid. The Earth would be cooling more slowly than was previously believed and might even be heating up. Either possibility posed a problem for contraction theory, which already strained to account for the amount of shortening seen in the Alps. The existence of radioactivity required that the Earth was both older and more dynamic than previously thought. It would prove key to the oncoming debates about continental drift.

Equally problematic for existing theories was the development of the principle of isostasy. Isostasy describes the gravitational stability of a section of the Earth's crust. The crust behaves much like an ice cube in a glass of water: Its elevation is supported by the thickness and density of the material comprising it. Thick (or less dense) blocks attain higher elevations than thinner (or denser) blocks.

isostasy diagram
Isostasy in the Earth's crust. Identical elevation profiles can be explained by differences in (A) the density of the crust, where high elevations occur where rocks are relatively buoyant, or (b) the thickness of the crust, where high elevations are supported by thick roots (like an ice cube).

The first explicit observation of isostasy dates to British India in the 1840s.12 The British Surveyor-General there noted discrepancies in maps of the Himalayas prepared by triangulation and astronomical measurement. Believing that the extra mass of the mountain range may have been deflecting his surveyors' plumb-bobs and skewing their results, he commissioned John Pratt to assess the effect of the mountains on the deflection of a plumb-bob. Pratt's results were surprising. He found that the measured deflection of the plumb-bob was smaller than that predicted based on the size and density of the Himalayas.13 This finding implied that the material comprising the crust below the Himalayas was thicker or less dense than that underlying the surrounding countryside. In either case, the high elevation of the mountain range was compensated by the reduced mass in the crustal column beneath the mountain. In the first decade of the twentieth century, John Hayford and William Bowie of the United States Coast and Geodetic Survey organized the first large-scale study of isostasy in the Earth's crust, work widely regarded as confirming the geological principle of isostasy.14

Isostasy had crucial implications for geological theory. It implied a fundamental difference between continental and oceanic crust: The continents sat at higher elevations because they were comprised of material distinct from that comprising the oceanic crust. It was unclear at the time whether the continents and mountain ranges were less dense or thicker or both; the exact reason was to an extent immaterial. Rather, the key point was that this difference existed. Continental and oceanic crust could no longer be considered interchangeable. Moreover, the continents were stable: Suess's sinking continents were verboten, as were the continental land bridges invoked to account for fossil homologies on Dana's permanent continents.15

Both radioactivity and isostasy undermined contraction theory and the ad hoc mechanisms that geologists had used to explain fossil homologies. Suess's contraction theory was essentially finished; Dana's theory of permanent continents persisted, but absent thermal contraction and land bridges, was hard-pressed to account for the origin of either mountain belts or fossil homologies. The need for new explanations was evident and growing.

This was the environment in which Alfred Wegener first published his theory of continental drift. He completed the first edition of his book The Origin of Continents and Oceans in 1915, twenty years after the discovery of radioactivity and on the heels of Hayford's and Bowie's landmark work on isostasy.

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