The Atlantic Ocean is growing – but only for now

Roughly 180 million years ago, the supercontinent Pangea broke up, and the Atlantic Ocean formed in the rift between continents.

The Atlantic has has grown ever since, and continues to do so while the Pacific shrinks, with the Pacific’s volcanic “ring of fire” a hallmark of land closing in on the ocean.

But Atlantic expansion might not be inevitable. A new modelling study suggests that in about 20 million years – soon, in geological terms – the Atlantic will have an encroaching ring of fire of its own.

The study, published in Geology, finds that the Atlantic is being invaded by a subduction zone in Gibraltar, on the edge of the Mediterranean Sea.

Contrary to previous beliefs, the authors state that the Gibraltar zone is still active, and will roar back to life.

Over the scale of billions of years, oceans follow a pattern called the Wilson cycle. A continent breaks up and an ocean forms in the interior, the ocean grows as continents drift apart, and then they reach a turning point – subduction initiation – where the old, heavy plates at the edge of the ocean begin to slide underneath lighter continental plates. Then, the ocean shrinks until the continents at its edges collide.

Map of atlantic ocean showing tectonic plates
Map highlighting the Atlantic subduction zones, the fully developed Lesser Antilles and Scotia arcs on the western side and the incipient Gibraltar arc on the eastern side. From Duarte et al., 2018. Credit: João C. Duarte

But subduction zones don’t form spontaneously: super-strong tectonic plates need to break and bend for subduction to happen. Usually, subduction initiation happens when subduction from another ocean migrates into the growing ocean.

This is subduction invasion, and the researchers say it’s happening at Gibraltar, on the southernmost tip of the Iberian Peninsula.

“There are two other subduction zones on the other side of the Atlantic – the Lesser Antilles, in the Caribbean, and the Scotia Arc, near Antarctica. However, these subduction zones invaded the Atlantic several million years ago,” says first author João Duarte, a researcher at the University of Lisbon, Portugal.

The Mediterranean Sea is a closing basin, with the oceanic plate being subducted under the continental plates that house Africa and Europe. There’s a subduction zone around Gibraltar, but it’s been slowing over the last few million years and many researchers thought it was no longer active.

Satellite image of strait of gibraltar
A satellite view of the Strait of Gibraltar, the narrow opening between Morocco and the Iberian Peninsula where the Mediterranean meets the Atlantic. Credit: NASA, Public domain, via Wikimedia Commons

Newer 3D modelling in this study suggested that wasn’t the case.

“Subduction invasion is inherently a three-dimensional process that requires advanced modelling tools and supercomputers that were not available a few years ago,” says Duarte.

“We can now simulate the formation of the Gibraltar Arc with great detail and also how it may evolve in the deep future.”

Duarte and colleagues think that the Gibraltar subduction zone is merely in a “period of quiescence”, and its activity will ramp back up in about 20 million years. Then, it will invade the Atlantic ocean and begin the shrinking process.

“Studying Gibraltar is an invaluable opportunity because it allows observing the process in its early stages when it is just happening,” says Duarte.

Besides geological interest, the researchers say their findings have implications for earthquakes in the region. Subduction zones are often the cause of the most powerful earthquakes, like the 1755 Lisbon Earthquake.

This earthquake – the strongest ever reported along Europe’s Atlantic coast – also caused a huge tsunami with ripples as far away as the Baltic coast.

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