How wide is the ocean?

A geological phenomenon is pushing North and South America further apart from Europe and Africa a new study has found, widening the Atlantic ocean up to four centimetres a year.

A team of seismologists, led by the University of Southampton, UK, suggest an upwelling in the mantle – the material composed mostly of silicates between the Earth’s crust and its core – from depths of more than 600 kilometres may be the reason why plates attached to the Americas are moving apart from plates attached to Europe and Africa.

210128 plates graphic lr
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. Credit: University of Southampton

“This was a memorable mission that took us a total of 10 weeks at sea in the middle of the Atlantic Ocean,” says lead author Matthew Agius, currently at Università degli studi Roma Tre, Italy.

“The incredible results shed new light in our understanding of how the Earth interior is connected with plate tectonics, with observations not seen before,” he explains. 

In between the continents lies the Mid-Atlantic Ridge. The mostly underwater mountain range is a site where new plates are formed and the dividing line between tectonic plates moving west (the North American and South American plates) and east (the Eurasian and African plates). Mantle material rises from beneath this ridge to replace the space left by the plates when they move.

The process is generally driven by distant gravity forces, with denser parts of the plates sinking back into the Earth. However, the Atlantic plates’ driving force for separation has remained a mystery because dense sinking plates do not surround the ocean.

To shed light on the mystery, the research team deployed 39 seismometers at the bottom of the Atlantic. Data collected from the seafloor provide the first large-scale, high-resolution imaging of the mantle beneath the Mid-Atlantic range at depths near 410 km and 660 km. They indicated a deep, sluggish, and unexpected upwelling.

Upwellings beneath ridges are typically thought to originate from shallower depths, of around 60 kilometres. 

“This work is exciting, and…refutes long-held assumptions that mid-ocean ridges might play a passive role in plate tectonics,” says Mike Kendall from the University of Oxford, one of the voyage’s chief scientists. “It suggests that in places such as the Mid-Atlantic forces at the ridge play an important role in driving newly formed plates apart.”

The findings, published in the journal Nature, provide a greater understanding of plate tectonics, which can help scientists develop better models and warning systems for natural disasters.

“This is completely unexpected,” says Catherine Rychert, another voyage chief scientist, from the University of Southampton.

Related reading: Plate tectonics: the hidden key to life on Earth

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