Researchers discover Archean continental rocks on India’s Southwest Ridge

Between the Earth’s rigid tectonic plates above and its convective mantle below lies a warm, soft layer known as the asthenosphere. On mid-ocean ridges, the rise of the warm asthenosphere to the surface of the seabed forms new oceanic crust.

Over time, oceanic crust becomes older, colder, and denser, and is eventually recycled back into the mantle at subduction zones. For this reason, oceanic crust is generally less than 200 million years old.

A joint research team, led by Professor Liu Chuanzhou from the Institute of Geology and Geophysics (IGG) of the Chinese Academy of Sciences, discovered abyssal peridotites dredged from the seafloor along the southwest ridge of India which are the oldest ocean rocks ever discovered. Further study revealed that they originated from the neighboring African continent.

This work has been published in Scientists progress June 1.

These newly discovered rocks from the Indian Ocean are as old as 2.7 billion years and date back to a period geologists call “the Archaean”, which comes from the Greek word for “ancient”. By analogy, the presence of these ancient rocks on a young ocean ridge is like finding your mother in your child’s kindergarten class. Luckily, these eerily ancient mid-ocean ridge rocks offer a clue to their mysterious origins.

Moreover, these ancient rocks have highly refractory compositions; in geochemistry, refractory refers to a lack of fusible (i.e. easily melted) components in the mantle. Therefore, they look less like ocean materials and more like rocks from continents. The only possible explanation for this is that these Archean rocks from the Indian Ocean mantle had been dislodged and transported through the asthenosphere 2,000 km to the mid-ocean ridge where they were discovered.

To test whether this hypothetical recycling mechanism could actually do the trick, computer simulations were conducted. The modeling results suggest that the recycling process was very efficient. Up to 20% of the continent could disappear in less than 100 million years, from a geological point of view, in the blink of an eye.

Old continental roots appearing at the bottom of the ocean clearly indicate that oceanic plates are more complicated than previously thought. Therefore, to decipher the evolution of the Earth, more attention should be paid to the ocean floor, which covers three-fifths of the Earth’s surface.

The study was carried out in collaboration with the Woods Hole Oceanographic Institution and the Max-Planck Institüt fur Chemie.