Under the oceans, the Earth

What if the secrets of the Earth's making were to be found under the sea? Last May, geologist Benoît Ildefonse participated in the Arc-en-Sub campaign organized by the French oceanographic fleet. Destination Rainbow, an underwater dome formed on the Atlantic ridge whose dynamics still elude researchers.    

 " In marine geology, we work on the oceans but without the water because what interests us is what's underneath. And to dig once again "what's underneath ", Benoît Ildefonse took part last May in the Arc-en-Sub campaign. 26 days at sea on board the Pourquoi pas? one of the four ships of the French oceanic fleet. " There were about twenty scientists on board: geophysicists, petrologists, people interested in rocks , others We were about twenty scientists on board: geophysicists, petrologists, people interested in rocks, others in earthquakes, but only geologists," says the director of Géoscience Montpellier, who no longer counts the campaigns carried out at sea since the very first one in 1997.

And for this mission, the Rainbow Massif, a submarine dome culminating at 1,500 meters below sea level, located on the Atlantic ridge two days west of the Azores. "What we call the oceanic ridges is a chain of essentially volcanic mountains 60,000 kilometers long, located between about 1,000 and 4,000 meters below sea level, and which correspond to the limits of tectonic plates," explains the geologist. Plates that can clash or move apart at speeds that can vary from one centimeter per year for the slowest, such as the Atlantic ridge, to fifteen centimeters per year for the fastest, in the eastern Pacific.

A crust factory

Veritable factories for making the Earth's crust thanks to the magma that escapes from their faults, the ridges are also zones of intense hydrothermal activity. The cold ocean water will slip into the faults and on contact with the magma, will heat up to several hundred degrees," continues Benoît Ildefonse. When this water, which has been loaded with a certain number of chemical elements, reaches supercritical conditions, it rises and comes out in the form of plumes. These plumes, in contact with colder seawater of different chemical composition, precipitate minerals such as manganese or iron sulfides.

In addition to the unique biodiversity they harbor and the valuable mineral deposits they contain, these ridges play a crucial role in cooling the planet. " It is estimated that the entire volume of the ocean is recycled in this circulation more than 100 times in 80 million years. In one day this represents more than 1 million Olympic swimming pools or 45 Salagou lakes."

If the Rainbow site has been known for 25 years, its functioning has not yet been fully deciphered by scientists, hence this latest mission. " The topography of the site corresponds rather to that of an area that is not very active volcanically, yet we note hydrothermal activity that implies the presence of pockets of magma underneath, but we do not know exactly where they are located ," summarizes the researcher. Another element: scientists have discovered near this site at high temperature, an older site at low temperature with different rocks. " So that indicates geographic and temporal variability in this system and this campaign was about exploring that complexity."

Change of scale

To achieve this, the geologists brought up more than 320 rock samples from the depths of the sea and took nearly 200 hours of video. Victor, a remotely operated submarine, and IdefX, an autonomous underwater vehicle, were also used to make a micro-bathymetric map of the area at a scale of a few centimeters, as well as systematic photographic coverage of the bottom. " This detailed mapping revealed extraordinary things to us. The exploitation of the data and samples will now allow us to resituate all this in a complete integrated scheme associating magmatism, hydrothermalism and tectonics ", Benoît Ildefonse is pleased to say.

Since their return from the mission, the scientists have been making the samples speak in the language of their own specialties: chemical composition and variability, mineralogical changes related to reactions with hydrothermal fluids... The geologist from Montpellier is interested in their deformation by measuring the orientation of the crystals with an electron microscope on 30-micron thick slides. " These microstructures give indications of the temperature at which these rocks were deformed," explains Benoît Ildefonse, before concluding: " In geology, we often work on a very small scale to understand the dynamics that occur on a very large scale.