Under a Mantle of Secrecy

Studying the Earth’s mantle is no easy task. Although it makes up 85% of the volume of the planet, it is inaccessible to direct observation. The mantle is hidden between the crust and the core, starting at a depth of several tens of kilometers and going right down to 2900 kilometers. The mantle is divided in four layers: the upper mantle (33-410 km), the transition zone (410-670 km), the lower mantle (670-2798 km), and D” (2798-2998 km). To penetrate the secrets of this mysterious world, scientists have powerful allies in seismic waves, which provide abundant information about the distribution of densities throughout the planet and the nature of the materials that make it up. As a result, we know that the mantle consists of iron- and magnesium-rich silicates, which at upper mantle depth form a mineral known as olivine. At a depth of about 400 kilometers, where temperatures already reach 1500 °C and pressure is around 150,000 times atmospheric pressure, olivine transforms into wadsleyite and then into ringwoodite, two other silicates with a more compact structure. Below 660 kilometers, under the effect of increasing pressure, these minerals give way to an even denser material called perovskite, which alone makes up at least 75% of the constituents of the lower mantle.
Seismic tomography, which lets us visualize the Earth’s interior in much the same way as a CAT scanner, also turns out to be an essential tool to “understand what happens to the cold, hydrated material that descends into the mantle, and to locate the hot, water-deficient material that rises up from great depths,” explains Éléonore Stutzmann, from the seismology department at IPGP,¹ and who also directs the Geoscope observatory, a network of 29 seismic stations distributed all over the world.
Yet there are still unsolved mysteries. For instance, what role does the puzzling D” layer play in heat transfer between the core and the mantle? How is the huge quantity of water present in the mantle (roughly as much as in the oceans) stored? Is it trapped inside particular mineral structures, or does it circulate freely?

A Solid in Motion
Another important area of research focuses on the rheological behavior of the mantle–in other words, the way in which it continuously deforms. “On our time scale, the mantle appears to be totally solid,” IPGP’s Anne Davaille points out. “But over very long periods of time, we observe that it behaves like a viscous material, and ‘flows’ at a rate of a few centimeters per year. A good analogy would be the stained-glass windows found in cathedrals, whose glass inexorably flows down under the effect of gravity. Of course, the higher the temperature in the mantle, the less resistant it is to flow.” It is this convective motion that cools the planet’s interior. This is because the slow movement of the masses of rock carries the planet’s internal heat towards the surface. This is a much faster and more efficient way of evacuating heat than conduction, another process that plays a role in cooling the mantle. Yet for Xavier Le Pichon, who holds the chair in geodynamics at the Collège de France, things are far more complex. “We still do not know whether the mantle undergoes convection as a whole, or whether two separate levels convect independently on top of each other.”
Recently, Francis Albarède, professor of geochemistry at the École Normale Supérieure in Lyon², proposed a unifying theory of the fate of rare gases in the Earth that removes the strongest objection against material exchange across the entire mantle.³
Some seismological research points to the existence of another type of convection called “superplumes.” These are believed to be huge mushroom-shaped masses of hot, light matter that form at the core-mantle boundary under the Pacific and under Africa, causing upwelling of hot matter from the D’’ layer.
Be that as it may, convection is the driving force behind plate tectonics and its direct and often destructive effects, including mountain formation, earthquakes, and volcanic eruptions. The Earth’s surface is made up of a patchwork of around a dozen spherical plates, which may be 100% oceanic, 100% continental, or a mixture of the two. The mid-ocean ridges, the area where two plates are moving away from each other, have a total length of 67,000 kilometers, and it is here that approximately 90% of all the magma that flows out from the mantle is released. These spreading zones raise a number of questions. “For instance, we don’t know precisely how the magmatic liquid makes its way up to the surface,” says IPGP’s Mathilde Cannat, also a member of the MoMAR project, which focuses on the Mid-Atlantic Ridge south of the Azores. “We’d like to know if the magma travels along channels, and if so, what size they are. Apart from that, we still have to understand how magma gets onto the ocean floor, the frequency of eruptions, etc.”
Then there are subduction zones, which act as global “differentiation factories,” the places where the lithosphere (the rigid 100 kilometer-thick outer layer of the Earth made up of the crust and upper part of the mantle) continuously descends into the mantle to be recycled. Albarède compares the descent of the plate back into the mantle to a tablecloth that slides off a table because it’s too heavy. “The oldest, and therefore thickest, part of the crust–which was formed directly above the ocean ridges by rising basaltic lava–bends and sinks down into the deeper parts of the Earth together with the sediments on top of it.” If the underlying principles are well known, many questions still remain. Why do some plates sink right down to the base of the mantle while others remain stuck at a depth of between 400 and 670 kilometers? How long does it take for a plate to reach the depths of the mantle before blending with the ambient material and eventually rising back up towards the upper layers?

Intense Activity
To try to better understand the processes that occur at subduction zone, LGCA’s⁴ Catherine Chauvel focuses her research on the Lesser Antilles island arc, where the Atlantic plate dives under the Caribbean plate, causing intense volcanic (Mount Pelée, La Soufrière, etc.) and seismic activity. Because it is known that volcanic rocks “more or less directly record the processes active in the mantle ‘wedge’–the place where the plate descends–and the conditions prevailing in the subducted plate,” Chauvel explains, “we establish the chemical budget of the material entering the subduction zone, model the variations in the types and composition of volcanic rocks, and understand how transfers of fluids from the descending plate into the mantle wedge occur.”
While spreading zones and subduction zones produce volcanism directly related to plate tectonics, this is not the case for “hot spot volcanism.” It appears to be associated with the D” layer, and acts as a blowtorch beneath the moving plates. Huge plumes of hot material rise up from the core-mantle boundary like giant hot-air balloons, and spread out forming bulges beneath the plates. The pressure drop triggers partial melting and the production of spectacular amounts of basalt. “The gradual build-up of these reservoirs leads to the formation of millions of cubic kilometers of flood basalt, like the Deccan Traps in India,⁵ and this in a very short period of time, geologically speaking (a million years),”explains Pierre Schiano, director of the LMV,6 one of the world’s largest laboratories dedicated to volcanic activity. “On continents, these increasingly pronounced bulges can even lead to a plate rupture and the opening of a new ocean.” These shallower hot spots may form at the interface between the upper and lower mantle. For now, this extremely potent type of volcanism remains highly mysterious.

Philippe Testard-Vaillant

Left: A volcanic rock that contains peridotite (light green), a material that originates in the upper mantle. Above: A sample of perovskite, a material that is present everywhere in the lower mantle.

© H. Raguet/ CNRS Photothèque

© Antoine Dagan for le Journal du CNRS/CNRS Photothèque

Mass Extinctions The chief culprit behind the five largest mass extinctions of species on our planet over the past 500 million years has now been clearly identified: volcanic activity. But not just any volcanic activity. “The volcanism that formed the traps1 was exceptionally intense,” explains IPGP’s Vincent Courtillot. “Each time a series of eruptions occurred at close intervals, the atmosphere became poisonous, mainly due to sulfur. Each short, massive eruption probably gave rise to a cold period, caused by sulfur, followed by a warm period caused by carbon dioxide, which may in certain cases have led to the release of methane hydrates trapped in marine sediments–leading to an even more marked greenhouse effect.” Could a seventh mass extinction strike any time soon? “There is nothing to make us think that an event of this kind will threaten the Earth in the near future,” Courtillot reckons. “But it’s likely that it will happen again sometime in the next few millions, or tens of millions of years.” P.T.-V. 1. Gigantic continental flood basalts. Contact : Vincent Courtillot, courtil@ipgp.jussieu.fr

Pockets of magma Approximately 10 years ago, seismologists discovered partially melted zones at the base of the mantle. These small pockets of molten material could be the remnants of a massive magma ocean that might have formed during the highly energetic birth of the Earth.1 An important part of the radioactive elements still present in the Earth may be stored in these small reservoirs. P.T.-V. 1. S. Labrosse, et al. “A crystallizing dense magma ocean at the base of the Earth’s mantle,” Nature 2007. 450: 866-9. Contact: Stéphane Labrosse, stephane.labrosse@ens-lyon.fr