Listening to the Earth.
Research site: La Réunion, Indian Ocean
What?► Did a plume of hot rock shoot up from the earth’s core to form the volcanic island of La Réunion in the Indian Ocean? That’s the question geophysicists led by Karin Sigloch at the University of Oxford are trying to answer on their continuing mission to understand what is happening as the earth’s core cools since its formation 4.5 billion years ago.
The immediate mystery is this: La Réunion sits in the middle of a tectonic plate far away from the volcanic activity usually found at plate boundaries. Yet it still has an active volcano. How did it get there?
“It is one of the best suspects for a mantle plume, an upwelling of mantle rock heated by the fluid iron core 3,000 kilometres down,” says Sigloch. The hypothesis is that the fluid core warms the lower layers of mantle rock, which become buoyant and rise viscously in the shape of a plume that mushrooms up to the surface – though melting occurs only in the last 100 km. This plume, they suppose, is always anchored to the same spot on the core. But it burns through the crust at different points because, while the plume is stationary, the tectonic plates are moving. “So the plume leaves tracks and burn marks on the sea floor. We think the plume first rose in India, later forming the Maldives, then Mauritius and most lately Réunion,” she explains.
How?► At sea in the Indian Ocean for many weeks. To see if there really is a superhot magma conduit rising up beneath La Réunion, the European team decided to compute 3D images of the crust and mantle below the island in a novel way: using the seismic waves from earthquakes all over the planet to “illuminate” the deep rock structure. Heat distorts seismic waves and should give a telltale image of the plume.
This meant a sweltering, stormy five-week research voyage in October 2012 performing sonar surveys of the seabed, seeking clear, flat locations before dropping 57 seismic motion sensors in a 1500 x 1500-km area around the island. The sensors – 30 cm long cylinders of 30 cm diameter – were left logging earthquakes for a year and then retrieved in a six-week mission in November 2013. “I loved it, even though we had to endure a tropical storm. You’re working very hard and can be very focussed as there is no one to interrupt you like in the lab,” says Sigloch. One problem: they used sensors that were sensitive to seismically induced motions of just 1 nanometre. So they picked up everything from whale song to distant icebergs breaking up, storms and ship wakes. Cleaning the quake data of such signals means the construction of the 3D image has yet to start.
“But we’re hoping for a convincing detection of the plume,” Sigloch says.
Special report by Paul Marks