What tectonic plates float on

Any geologist will tell you the Earth’s crust is broken into tectonic plates that “float” around like gigantic rafts. But just what these rafts have been floating upon, has been a mystery – until now.

A team of New Zealand scientists detonated tons of dynamite and listened for echoes to reveal the underbelly of the Pacific plate. They found a 10 kilometre thick channel of lubricating jelly-like rock, which they say allows the plate to slide above it, according to a report in Nature.

German meteorologist Alfred Wegener proposed the idea of rafting continents back in 1912 after perusing maps and noticing that the east coast of South America and the west coast of Africa would fit together like jigsaw pieces. But scientists only started taking the idea seriously in 1963 when geophysicists Fred Vine and Drummond Matthews showed that the crust on the ocean floor, on either side of the mid-oceanic ridges, was indeed moving.

These days plate tectonics is “obvious”, says Louis Moresi, a geologist at the University of Melbourne. “You can log on to Google Earth and actually plot the movement.”

The plates themselves are composed of a thick layer of hard rock known as the lithosphere that lies above a softer layer known as the asthenosphere. But no one knew what lay at the lithosphere asthenosphere boundary (LAB).

In the past geologists relied on earthquakes originating on the other side of the planet of the planet to try and find out. Like doctors placing a stethoscope to the Earth’s surface, they detected seismic waves.

The fact these waves move at different speeds through different layers allowed geologists to sketch a coarse picture of the medium through which they travelled. But natural seismic waves are 10-40 kilometres in length – too long to resolve the fine-grained structure below the plates. So the New Zealanders took matters into their own hands.

“Rather than relying on earthquake waves that come from below we create our own ‘earthquakes’ with dynamite shots,” says Tim Stern at Victoria University, Wellington, who led the project. The resulting waves are about 500 metres long and  able to resolve finer structures. The blast zone was sited on the southern tip of New Zealand’s North Island where the 73-kilometre thick Pacific plate dips beneath the Australian plate at the rate of about 40 millimetres a year.

The team set up 877 Coke can-sized seismometers strung like beads along 85 kilometres. Then from multiple boreholes they detonated half a tonne of TNT in each.

The seismic echoes revealed something unusual stuck to the Pacific plate’s underbelly – a channel of jelly-like rock about 10 kilometres thick.

Researchers used blast waves to get a view of what lies beneath the Pacific plate as it dives below New Zealand’s North Island. At the base of the plate they found a 10 km thick jelly-like channel, the lithosphere asthenosphere boundary (LAB), which decouples it from the underlying asthenosphere. Credit: Cosmos Magazine

“We always thought the boundary would be gradual and defined by temperature. This study shows it’s an abrupt transition and requires something more than temperature alone to explain it,” says geologist Andrew Gleadow, also at the University of Melbourne.

The New Zealand team suggests the jelly rock gains its consistency from a higher concentration of water or magma than is present in the lithosphere above it. But it would not have to be too high. While the lithosphere contains 0.1% magma, even a 2% concentration of magma might be enough to explain the consistency of the rock in the channel. “On a million-year time scale this would appear weak and jelly-like,” explains Stern.

The finding of the jelly channel might also help resolve a 50-year debate about whether the plates move as a result of being pushed or pulled. An early idea was that magma being extruded from the mid-oceanic ridges was pushing the plates apart. Another pushing force might come from slowly creeping convection currents beneath the plates that act like rollers beneath a conveyer belt.

On the other hand the major force might be a pulling one. As one edge of an oceanic plate dives back into the mantle beneath – as the Pacific one is doing – it pulls the rest of the slab after it. The finding of the jelly layer makes the pushing and rolling mechanisms less likely, says Gleadow. “If the plates are mechanically disconnected from the mantle below, there can’t be good coupling to underlying convection movements.”

On the other hand, the jelly layer adds weight to the idea that gravity is the driving force pulling the plates along. As one edge of the plate is being dragged under, the low friction jelly layer means the rest of the plate just slithers after it like a ski on snow.

The next question is how this channel was formed and if it is present all over the world, says Moresi. Evidence from previous studies hints at a similar structure beneath the coast of Norway and another off Costa Rica. If it is found everywhere, “it would change our understanding of the internal dynamics quite a lot”.

Additional reporting by Elizabeth Finkel

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