Lubricating rocks ease tectonics

A theory that even its proposers describe as “counter-intuitive” could hold the key to a better understanding of the massive mechanics that govern plate tectonics.

The basic principle of tectonics is well understood and uncontroversial: propelled by mantle convection, the nine major plates that comprise the Earth’s lithosphere rise or fall according to temperature. 

Where the plates meet – at so-called subduction zones – the edge of the colder of two will slide beneath the warmer one. Thus are continents and mountain ranges and rifts formed.

Just when and how tectonics first emerged on the infant planet, however, has long been the subject of debate. Not the least of the theoretical concerns underpinning the uncertainty is the matter of how to account for what must have been the extraordinary amount of friction generated by two continent-sized slabs of rock sliding across each other.

Now, however, geologists Stephan Sobolev from Germany’s University of Potsdam and Michael Brown from the University of Maryland in the US think they might have found an answer: lots of little rocks.

Writing in the journal Nature, the pair suggest that the process of subduction was eased by the presence of rocks and sediment on the top of the lower plate – the product of surface erosion. In effect, these rocks acted as lubricants, reducing the frictional forces involved.

The idea brings with it the necessary assumption that surface erosion – by wind, ice, rain and heat – was able to occur. In other words, the land must have been above the surface of the ocean, and exposed to the elements.

This, the researchers write, fits with the evidence. The earliest indications of the development of plate tectonics stems from 2.5 to three billion years ago. This coincides with the geological period in which the planet’s proto-continents first rose above the waves. Shortly afterwards (geologically speaking), the first major glacial period, known as the Palaeoproterozoic Huronian era, took place.

Sobolev and Brown also cite the much later “Snowball Earth”, or Neoproterozoic, glaciations that occurred between 0.75 and 0.63 billion years ago.

Perhaps significantly, they say, the period immediately prior to the snowball glaciations is known, in geological circles at least, as the “boring billion”. It was, they write, “a period of reduced plate tectonic activity about 1.75 to 0.75 billion years ago that was probably caused by a shortfall of sediments in trenches”.

Despite the intricate modelling and physical evidence presented in their study, the researchers do not expect their idea to be readily embraced by other geologists. Indeed, they admit, they themselves found it a bit hard to swallow.

“Our hypothesis is counter-intuitive,” says Sobolev. “That was the main problem for us and we expect will be the main problem for the community to accept our ideas.”