Fifty years since the acceptance of the theory of plate tectonics, scientists have finally found evidence of upwelling from deep beneath mid-ocean ridges, which could be pushing the American, European and African plates further apart from each other.
The discovery has just been published in the journal Nature and illuminates the driving mechanisms of plate tectonics. Maltese scientist and University of Malta alumnus, Dr Matthew Agius, was the lead author.
Until now, conventional wisdom is that the spreading of plates is normally driven by distant gravity forces as denser parts of the plates sink back into the Earth. However, the driving force behind the separation of the Atlantic plates has remained a mystery because the Atlantic ocean is not surrounded by dense, sinking plates.
Seismic waves from earthquakes around the world travel deep inside the Earth and are recorded on the PI-LAB seismic network. The data are then analysed to image the structures inside the Earth. The thinner than average mantle transition zone suggests anomalous high temperatures that facilitate material transfer from the lower to the upper mantle that may play a role in driving plate tectonics
In 2016, a scientific experiment was set in the middle of the Atlantic Ocean. 39 seismometers were placed at the bottom of the ocean across the Mid-Atlantic Ridge. Seismologists have imaged the layers inside Earth down to 700 km depth and found evidence of an upsurge of matter in the mantle – the material between the Earth’s crust and its core. Upwellings beneath ridges have been thought to originate from much shallower depths of around 60 km, however, the new observations provide evidence that upwellings are likely to originate from much deeper.
The result is part of the PI-LAB (Passive Imaging of the Lithosphere-Asthenosphere Boundary) experiment and the EURO-LAB (Experiment to Unearth the Rheological Oceanic Lithosphere-Asthenosphere Boundary. It is one of only a few experiments of this scale ever conducted in the oceans.
The 1-year long seismic dataset allowed the team to image variations in the structure of the Earth’s mantle near depths of 410 and 660 km – depths that are associated with abrupt changes in mineral phases. The observed signal is indicative of a deep, sluggish and unexpected upwelling from the deeper mantle. The findings provide a greater understanding of plate tectonics which causes many natural disasters around the world, including earthquakes, tsunamis and volcanic eruptions.
Deployment of one of the ocean-bottom seismic stations in the Atlantic ocean
Lead author, Matthew Agius, a former post-doctoral fellow at the University of Southampton said: “This was a memorable mission that took us a total of 10 weeks at sea. The incredible results shed new light in our understanding of how the Earth interior is connected with plate tectonics, with observations not seen before.”
Dr Agius is a University of Malta graduate with a Bachelor of Science Honours degree acquired in 2004 followed by a Master of Science degree awarded in 2007 under the supervision of Prof. Pauline Galea. He is also the co-founder of the Science Students' Society (S-Cubed). He then pursued further studies in seismology at the Dublin Institute for Advanced Studies and read for a Doctorate of Philosophy from Trinity College Dublin in Ireland. Dr Agius is currently at Università degli studi Roma Tre in Italy on a Marie Skłodowska-Curie Individual Fellowship awarded by the European Commission conducting research on the tectonic systems in the central Mediterranean.
Dr Matthew Agius, next to one of the ocean bottom seismometers onboard the RV Marcus Langseth
The experiment is funded by the European Research Council (ERC) and Natural Environment Research Council (UK) and directed by Dr Catherine Rychert and Dr Nicholas Harmon at the University of Southampton (UK).
The publication, “A thin mantle transition zone beneath the equatorial Mid-Atlantic Ridge” can be found online.
