Testing the strength of tectonic plates

15 September 2017

What do volcanoes and earthquakes have in common? Apart from sharing the bigscreen in classics such as ‘Dante’s Peak', volcanoes and earthquakes both occur at the boundaries of tectonic plates. Now scientists have measured the strength of these tectonic plates and solved a long-standing discrepancy.

Laboratory estimates of tectonic plate strength vary widely and seem to overestimate strength compared to real world observations. This disparity has been the cause of much head scratching among geologists for years. 

Now, University of Oxford researcher Lars Hansen and his colleagues, writing in Science Advances, have used a new technique called ‘nanoindentation’ to solve the problem.

This technique involves pushing a microscopic diamond stylus into a crystal of the mineral olivine, a common component of rock that gives strength to tectonic plates. What Hansen found was that the measured strength of the crystal depended upon the size of the diamond stylus he used: the bigger the tip, the weaker the crystal appeared to be.

Materials scientists are familiar with this effect, but it’s less well known in the Earth Sciences. According to Hansen, this means that rock made up of really big crystals would seem weaker than it really is. Rock with smaller crystal grain sizes, on the other hand, would appear stronger.

"We looked at all these previous datasets and plotted the measured strength versus the sizes of the crystals, and what we realised is there’s a really nice relationship," says Hansen.

The rock the team studied is found deep underground where solid rock meets the molten magma. This is known as the lithospheric mantle. Scientists can get clues as to the crystal sizes of rocks found here from samples brought up by volcanoes.

Future research can now use geologically relevant crystal sizes to get more accurate estimates of tectonic plate strength. In addition, scientists are now using powerful X-rays to examine crystal samples subjected to extreme pressures and temperatures. A technique called X-ray diffraction can be used to look at the spacing of atoms in the crystal samples, which give an indication as to the forces the sample is subject to.

Armed with these modern techniques, scientists can set out to answer the big questions of the field. For example, whether there is a critical crystal size past which larger crystals do not mean weaker rock? In addition there are a plethora of different types of minerals, apart from olivine, that need to be interrogated to get a full picture of the different structural levels of strength all the way through the depth of a plate. It is this kind of approach that will bring us a deeper understanding of the dynamic process at play right beneath our feet.

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