(Image: New Scientist)
Dynamite explosions have revealed the layer that lubricates the movement of Earth's tectonic plates in unprecedented detail.
These plates interlock like a massive jigsaw to make up the planet's outer layer, or lithosphere, which is around 70–100 kilometres thick. They sit on top of a lubricating channel that separates them from the asthenosphere underneath. "Tectonic lubrication is a valid description of what we think happens in this 10-kilometre-thick channel," says Tim Stern of Victoria University in Wellington, New Zealand. The lubricating layer contains viscous melted rock that allows the plates to move independently of the layers below (see enlarged diagram here).
Stern and his colleagues directed explosions from Earth's surface down into a zone beneath New Zealand, then captured reflections of the resulting seismic waves to build up images of the base of the tectonic plate. Previous studies relied on distant, low-frequency seismic waves from real earthquakes, which dramatically limited the resolution. By setting off 12 of their own mini earthquakes with dynamite buried in 50-metre-deep shafts, Stern's team obtained much sharper images with a resolution of less than a kilometre.
This study is the first to image the lubricating layer, and has identified the base of the lithosphere in much greater detail than before, showing that it is shallower and sharper than previously believed.