On 22. September, a French-German team sets out from La Reunion to map the upwelling of hot magma that powers one of the oldest and most active regions of volcanic activity in the world. An expedition blog will keep interested landlubbers up to date.

Volcanic activity on and around La Reunion is driven by a localized upwelling of hot buoyant magma. Unlike most magma sources, this is not located on the boundary between two tectonic plates, and rises from much greater depths.

It is a so-called hotspot, and has left behind on the overlying mobile crust a track of volcanic activity that stretches 5500 km northwards to the Deccan Plateau in India. Some 65 million years ago, in a process that had a massive impact on world climate, the Deccan area was covered with enormous amounts of lava as the Indian Plate passed over the hotspot.

Such a long-lived upwelling of hot molten rock, which penetrates the overlying material like a blowtorch, is referred to as a mantle plume. Where exactly mantle plumes originate is the subject of a controversial debate among geoscientists.

During the course of a French-German expedition, LMU geophysicist Dr. Karin Sigloch, leader of the German contingent, wants to find out more about the putative plume under La Reunion. The goal is to determine the depth of the plume and to map the conduits by which the magma reaches the Earth's surface.

The largest plume survey campaign ever
"We want to look deeper into the Earth's interior than any previous expedition, down to the bottom of the mantle at a depth of about 2900 km; earlier efforts reached half that depth, at most," says Sigloch. To achieve this goal, the researchers must deploy a dense array of seismometers over a wide area.

On 22 September, the team will board the French research vessel Marion Dufresne on a cruise that will place nearly 60 seismometers on the seabed, dispersed over an area of some 4 million km2. As 30 additional instruments will be installed on land, this will be the largest such campaign ever undertaken. Data from a further 70 or so observatories located along the coasts of the Indian Ocean will complement the results obtained with the new network.

The data collected will be used to create three-dimensional tomographic images that will give us a picture of the Earth from the bottom of the crust to the core, and provide new insights into the structure, dynamics and history of the Earth.

As they effectively short-circuit the transport of heat from the core to the surface, plumes may play an important role in the Earth's heat budget, and are a major force in shaping the Earth's surface. Analysis of the new data will begin in a year's time, after the German RV Meteor retrieves the newly deployed seismometers from the seabed.

In addition to the team from LMU, researchers from the Alfred Wegener Institute in Bremerhaven, the universities in Frankfurt and Munster, the University of La Reunion and the Institut de Physique du Globe in Paris (IPG Paris) are involved in the project.

Read the expedition blog here.