When plate tectonics was adopted in the 1960s and early '70s, researchers quickly mapped out plate movements.  It seemed that plates moved as rigid caps about a pole on the Earth's surface.  But since then, a lot of evidence has accumulated suggesting that plates are not, in fact, totally rigid.  In fact, we can see them flex in response to stresses that are imposed on them.  Such stresses can arise on plate boundaries, such as when two plates collide and one plate flexes down to subduct under the other.  For example, we see a flexural bulge in Northern India where the Indian plate bends down under the Eurasian plate.  Similar bulges are seen at subduction zones where the oceanic lithosphere flexes up before it bends down into a trench, such as off the eastern coast of Japan.  Stresses can also be imposed in plate interiors when the plate is subjected to a load, such as a volcano or a sedimentary basin.  An example of sediment loading occurs in river deltas, such as that of the Ganges in the Bay of Bengal.

Our guest today pioneered an ingenious method of determining the flexural strength of oceanic plates.  The method uses the flexural sag of plates in response to the weight of seamounts, most of which were emplaced on their surfaces by mid-ocean eruptions.  His results suggest that less than half of an oceanic plate actually contributes to its elastic strength.  The rest is brittle (top layer) or ductile on the relevant time scales (bottom layer).Tony Watts is Professor of Marine Geology and Geophysics at the University of Oxford and a Fellow of the Royal Society.

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