Geologists solve tectonic plate mystery with half a tonne of explosives
A New Zealand experiment using explosive slurry poured into 50m-deep holes helps measure rock layers on the border of Pacific and Australian plates
Michael SafiG
Thursday 5 February 2015
Geologists may have finally explained how tectonic plates shift by blowing up hundreds of kilograms of dynamite in New Zealand.
Half a tonne of explosive slurry was pumped into a dozen, steel-cased holes spread nearly 100km across New Zealand's north island.
The seismic waves produced by the subsequent explosion reached the base of the tectonic plate and rebounded to the surface, where they were recorded by more than 1,000 seismographs.
Because seismic waves encode information about each of the layers they pass through, an international team of researchers was able to use them to produce detailed images of what lies beneath the earth's surface.
They revealed the existence of a narrow, lubricating layer of rock about 73km deep, over which the plate "skied" several centimetres a year.
The results are published in the latest issue of the scientific journal Nature.
One of the researchers, professor Tim Stern of Victoria University, Wellington, said earlier studies had relied on recording the seismic waves produced by earthquakes.
"But those earthquake waves are very low-frequency, wobbly looking waves, and they haven't really given us the details we need," he said.
Detonating 500kg of dynamite – the equivalent of more 2,600 sticks – provided far sharper echoes.
The 50m-deep explosive holes were dug in a line parallel to the border of the Pacific and Australian plates, Stern said. The two plates meet at a relatively shallow 12- to 15-degree angle, making the area ideal for rebounding seismic waves, which he likened to "bouncing light off a mirror".
Once triggered, the explosion caused the earth to shake and produced a "big whoomp" that could be heard from 10km away. "But that's about all," Stern said.
While the discovery of the slippery 10km layer explains how the plates move, what causes them to do so remains unclear.
One leading theory suggests the plates are pulled or pushed along their edges. Another, posits that the plates are connected to a deeper layer of hot, convecting mantle and getting dragged.
Stern said the research showed "there's still fundamental discoveries to be made" in the theory of plate tectonics, which has dominated earth science since the 1960s.
A forerunner of the idea was suggested in 1912 by a German meteorologist, Alfred Wegener, who observed that the east coast of South America and the west coast of Africa fit together like a puzzle.
He suggested the world's continents had once been fused together in a supercontinent he named Pangaea ("all the world"), but had since broken up and drifted to their present locations.
Earlier theories had suggested the world's continents had been connected by massive land bridges, which have since broken off and sunk.
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