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 1963when 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 kilometers 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 kilometers. Then from multiple boreholes they detonated half a ton 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 kilometers thick.
Via Dr. Stefan Gruenwald