Speed of tectonic plate movement can change the chemistry of ocean rocks

A fossil ocean ridge exposed on Iceland, which formerly formed the boundary of plates deep in the Atlantic ocean. Credit: Dr Marianne Richter

The speed of tectonic plate movement can affect the chemistry of ocean ridge basalts, an international study published today in Scientific Reports, has found.

Researchers from Monash University’s School of Earth, Atmosphere and Environment discovered that the speed at which the plates drift apart affects the composition of the melts forming below the Earth’s crust.

The Earth’s crust consists of tectonic plates that can collide and form mountains such as the Himalayas, or the volcanic ring of fire when one plate delves underneath the other.

The parts where plates drift apart, however, is less known, mainly because it is located 2 kms deep within the Earth’s ocean, according to study co-author Associate Professor Oliver Nebel, from the Monash University School of Earth, Atmosphere and Environment, and the Head of the Monash Isotopia facility, where the research was undertaken.

These suture zones form a global network of submarine mountain chains, where Earth’s crust is constantly rejuvenated.

Driven by the convection of the Earth’s mantle, plates drift at speeds of ca. 10 mm per year or 10 meters in a millennium.

Magma rises from deep within the Earth’s mantle to the ocean floor where lava instantly solidifies to a basaltic rock, the most common rock on the Earth’s surface, and a vital resource for nutrients leached into the oceans.

“The rate at which Earth’s plate drift apart is on average that of the speed at which a human’s fingernail grows,” said study author Dr Mariann Richter, a recent Monash University PhD graduate.

“However, there are subtle differences, where movement can be as slow as 6 mm per year or as fast as 150 mm per year,” she said.

“We know these differences translate into changes in lava chemistry.

“What we don’t know is which exact process deep underneath the ocean floor adds to this chemical diversity.”

The team from Monash and the US-based Woods Hole Oceanographic Institute analysed different weights of individual atoms of iron within basalts from the Arctic Ocean.

The iron isotopic differences in these basalts, and when compared to those from the East Pacific ridge, revealed that very slow plate movement  (< 10 mm per year) preserves the chemical signature of Earth’s mantle from which the ridge is formed.

At faster speeds (> 100 mm per year) these chemical memories are erased.

“We found another important piece of the puzzle as to how lava chemistry is related to plate tectonic forces,” said Associate Professor Nebel.

“Understanding these processes will provide us with a better understanding of our deep ocean basins today, and also how these formed millions or billions of years ago,” he said.

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