Earth’s oldest ocean crust formed 340 million years ago

The world’s oldest oceanic crust – around 340 million years old – lies at the bottom of the eastern Mediterranean Sea, according to a geologist in Israel.

Roi Granot at the Ben-Gurion University of the Negev, Beersheba analysed patterns of Earth’s magnetic field locked in submerged rocks and, working backwards, calculated a patch to have formed when the supercontinent Pangaea broke apart during the Palaeozoic era.

His work was published in Nature Geoscience.

Earth’s crust is a constantly recycling and evolving. As plates slip beneath others into the mantle of hot rock below, new crust is formed at oceanic ridges – volcanic strips that spew vast amounts of magma onto the surface of the crust.

But this means that most oceanic crust today is less than 200 million years old. Continental crust, on the other hand, is less dense and “floats” on the mantle. This means parts of it push into billion-year-old territory.

Could chunks of ancient oceanic crust remain today? Geologists have suspected so – a 2014 study that traced the movements of the continents over hundreds of millions of years predicted some may still be around.

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The crust is thought to have once been part of the southern Tethys Ocean which formed when the Pangaea supercontinent tore apart a little.

The problem was the site today is the so-called Herodotus Basin – the north eastern edge of the African plate sits under the eastern Mediterranean Sea – and it is buried under more than 10 kilometres of silt. 

Digging through the sediment to check how old that crust is, and even if it’s oceanic crust at all and not just submerged continental crust, would be an expensive, time-consuming exercise.

So Granot took advantage of the Earth’s changing magnetic field to find answers.

As hot rock pours from volcanic rifts, it cools. While the rock is still soft, magnetic compounds align with the planet’s magnetic field at the time, then set in place.

Over millions of years, Earth’s magnetic field has wandered around. This creates magnetic strips in the crust – a little like a barcode where each stripe is a time stamp.

Between 2012 and 2014, Granot collected data from a sensor that measured these magnetic stripes, called a magnetometer, which was towed behind a boat.

Some 7,000 kilometres of sea floor were covered – and they allowed Granot to create a map of the eastern Mediterranean Sea floor.

He saw 250-kilometre-long striations in the Herodotus Basin consistent with volcanic oceanic ridges. And by tracing the skewed patterns in the stripes, he calculated the seafloor to have formed roughly 340 million years ago (plus or minus 25 million years).

If the basin is a remnant of the Tethys Ocean, this means the ocean formed around 100 million years earlier than previously thought.