Scientists shed new light on seafloor growth

A University of Plymouth-led team of international scientists has pioneered a novel geological technique and used it to shed new light on how the oceans form during ‘seafloor spreading’, the process that constantly ‘re-paves’ the crust of the Earth’s seas.

This new approach was developed following a multi-million dollar Integrated Ocean Drilling Program (IODP) expedition to the mid-Atlantic ridge in the heart of the Atlantic Ocean, on board the research ship the JOIDES Resolution. Over a period of three months, scientists drilled deep into the top of a huge 15km wide, 3km high mountain on the seafloor known as the Atlantis Massif.

It is only in the last decade that scientists have discovered these large mountains beneath the Atlantic Ocean and realized that they form when the Earth’s crust is pulled apart by faulting. Evidence had suggested that these ‘detachment faults’ took rocks that had been formed deep down in the lower crust and brought them up to the surface to create enormous dome-like massifs – but how the faults achieved this had remained a contentious issue until now.

The unique set of rock samples recovered by drilling at Atlantis Massif allowed researchers from the UK and US to use specialist magnetic equipment to discover how these faults evolve.

Antony Morris, lead author of the report, and Geophysics lecturer at the University of Plymouth, said: “We realised that we could use the record of the direction of the Earth’s magnetic field, that became locked into the lower crustal rocks at Atlantis Massif when they formed, to find out whether they have been rotated during their geological journey to the surface.

“This would allow us to distinguish between alternative models for the formation of these major oceanic fault structures, since different models make different predictions on the amount of rotation that should have occurred.”

To do this, however, the scientists first had to overcome a major problem related to the drilling itself. Morris said: “Core samples from the oceanic crust tend to break up and spin around during drilling and the passage back to the surface. Since we cannot tell which way they were originally oriented, it makes it impossible to measure their direction of magnetization relative to north.

“This problem has existed throughout the 40-year history of scientific ocean drilling. However, we succeeded in overcoming this obstacle by employing a novel method to independently restore our samples to their original orientation.”

The process involved the team using data from a geophysical device, lowered on a cable into the bore hole, that measured the electrical properties of the rocks – effectively creating an electrical image of the inside of the hole. They were then able to painstakingly match pieces of the drill cores to these images to restore them to their original positions, allowing the magnetic data to be re-orientated and used to measure the geological rotation experienced by the rocks.

“Our fully oriented magnetic data are the first of their kind from scientific ocean drilling," said Morris. "They provide conclusive proof that oceanic detachment faults, like that at Atlantis Massif, that are currently exposed on the seafloor at very low angles, were actually steeply dipping structures when they first initiated.

“As the plates pull apart during seafloor spreading, these faults start to progressively rotate to lower angles as they take up the motion of the tectonic plates. In doing so, they drag lower crustal and upper mantle rocks up onto the seafloor. Our data have allowed us to quantify this fundamental process in a way which would have been impossible to achieve before.”

Morris said that this success was not only a milestone for future drilling expeditions, but could shed new light on material from past expeditions. He said: “This is a methodology which now unlocks the potential for renewed study of materials from numerous previous ocean drilling expeditions, all of which are carefully archived in the IODP core repositories.”

The research team consisted of: Morris, University of Plymouth; Nicola Pressling, University of Plymouth (now at the National Oceanography Centre, Southampton); Jeff Gee, University of California (San Diego); Bobbie John, University of Wyoming; and Chris MacLeod, Cardiff University. The project was funded by the National Environment Research Council (NERC).


Notes to Editors

For more information, please telephone Andrew Merrington in the University of Plymouth Press Office on 01752 588003.

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