Professor of Physics, and specialist in mineral physics, at Lille University, in this article, explains how tectonic shape various phenomena of earth surface, gravity, among others.
Jules Verne made us dream with his extraordinary journeys and most of those dreams have come true. Cousteau took us to the bottom of the oceans, man walked on the Moon more than 50 years ago and today our robots roam the surface of Mars while we have probes exploring the far reaches of the solar system.
Yet one of Jules Verne’s promises is still refused us: the journey to the center of the Earth. The man does not descend more than 4 kilometers below the surface (in gold mines in South Africa) and the deepest drillings hardly exceed 12 kilometers, a scratch compared to the distance which separates us from the center of the Earth which borders 6,400 kilometers. What is under our feet remains beyond the reach of the explorer, but not of the scientist!
Mountain and tectonic plates
The scientist begins by observing. Our planet is surrounded by a layer of gas which allows life: the atmosphere, it is covered with a film of water or this life has undoubtedly appeared: the hydrosphere, but the geologist is interested in the Earth solid, made of rocks: the lithosphere. This one, rigid, seems immutable, yet mountains have formed in the past under the influence of tectonic forces.
The mountains, let’s talk about them. Why does the roof of the Earth, Everest, only culminate at 8,848 meters when Mars can be proud of Olympus Mons which rises to nearly 25,000 meters? Could we imagine such summits on Earth, or even higher ones? It is the physicist who is able to provide some answers to this question from the point of view of mechanics. A mountain weighs heavily on the surface of a planet which must support its load.
The Earth is less dense on the surface than at depth
These forces are all the more important as the planet is big and the gravity is high. On Mars, gravity is one-third that of Earth: the enigma of the Everest – Olympus Mons difference begins to light up. But the forces exerted do not explain everything. What matters is the mechanical strength of the earth’s crust in response to this load. In 1914-1915, Joseph Barrell, professor of structural geology at Yale, published a landmark series of articles on the rigidity of the earth’s crust.
For a very long time , the hypothesis had been advanced that the excess mass at the surface due to the mountains had to be compensated by a deficit of mass at depth. In 1735, the Academy of Sciences sent an expedition to Peru to determine the exact shape of the Earth, namely whether it was spherical, flattened at the poles, or elongated along the axis of rotation. Pierre Bouguer, who was part of the expedition, had made extraordinarily precise measurements of Earth’s gravity (using a simple pendulum!). Gravity depending on the distribution of masses, he wanted to study the way in which it is affected in the vicinity of Chimborazo (Ecuadorian volcano culminating at 6,263 meters).
Mechanical properties of Earth’s interior varies
Its results were clear: the Earth is much less dense on the surface than it is at depth: what is accessible to us on the surface does not therefore reflect the entire globe. Barrell goes further, the mechanical properties of the interior of the Earth must also be very different. The mechanical balance of the reliefs requires deformations in depth. The Earth, which seen from the surface appears as a sphere of rigid rocks (the lithosphere) must therefore give way in depth to a softer, more deformable Earth that he calls “asthenosphere” (from the Greek ἀσθένης, asthenes, without resistance).
Since Barrell, our knowledge of the globe has improved considerably. For about fifty years, plate tectonics has established itself as the conceptual framework which describes the dynamics of the lithosphere from the relative displacements of a small number (about fifteen for the main ones) of rigid plates. Even more, we know today that these surface translations are animated by vast convection movements which stir up the Earth’s mantle, a rocky envelope which extends to the core (that is to say up to nearly 2,900 kilometers deep).
Transition between lithosphere and asthenosphere
It is these slow, very slow, convective movements that by transporting hot rocks from the depths to the surface allow our planet to evacuate its internal heat. The transition between the lithosphere and the asthenosphere proposed by Barrell therefore plays a very important role since it is this interface which ensures the mechanical coupling between the deep convection movements and the displacements of the plates on the surface. But what is its origin, its nature? This seemingly simple question has still not received an answer that wins the support of the scientific community.
We know that the border between the lithosphere and the asthenosphere passes into the upper mantle, so it does not correspond to a change in the nature of the rocks. We also agree to think that it is associated with a characteristic temperature close to 1000° C. We must therefore understand how the rocks of the upper mantle see their mechanical properties suddenly collapse at this temperature. Rocks are made up of minerals which are crystals. These crystals are welded together to form rock.
What if the answer was in olivine?
In the upper mantle, one mineral is particularly important: olivine, a silicate of magnesium and iron, green (olive) in color. On the one hand, olivine is the most abundant constituent in the mantle, it is also observed that its deformation controls that of the upper mantle.
It is in this mineral that our team is particularly interested in a research program funded by the European Research Council (ERC) called TimeMan.
TimeMan tackles another major challenge: how ephemeral beings that we are can they understand on the scale of a human (professional) life of mechanisms that act infinitely slowly over hundreds of millions of years. The bet of this project is that the solution to the great questions of geology which concern objects of several hundred kilometers and time scales of hundreds of millions of years can be found in mechanisms that operate on a microscopic scale. in minerals.
Deformation mechanisms of olivine
We observe, we analyze, we therefore model the deformation mechanisms of olivine with the greatest attention. The study described in this article published in the journal Nature is based on the careful observation of deformed olivine aggregates at high temperature and high pressure, in the laboratory, by our colleagues at the universities of Montpellier and Bayreuth in Germany who showed a marked change in mechanical properties… around 1000° C!
Our observations are based on transmission electron microscopy, a sophisticated technology which makes it possible to study the structure and chemistry of matter almost down to the atomic scale, and including the Lille laboratory and that of our colleagues in Antwerp and Louvain-la-Neuve are specialists.
What was our surprise to note that the deformation of these samples was strongly localized at the border which welds the crystals together. Fascinating analogy with plate tectonics which locates deformations at the boundaries of plates when more than nine orders of magnitude separate the dimensions of these two phenomena. Looking more closely, at the atomic scale, we see that these walls between grains are made up of a thin (10,000 to 100,000 times thinner than a human hair) which has lost the regularity of its structure. crystalline, it is a glass whose atomic arrangement is disordered.
However, this glass layer was not present in the samples before deformation. Moreover, olivine is known to only vitrify with great difficulty. Here, it is under the influence of the stresses that are concentrated on these walls that the crystalline structure has collapsed. This observation sheds a completely new light on the behavior of the rock. Indeed the mechanical properties of a glass are completely different from those of a crystal. Glassblowers have long known that glass, that quintessential hard and fragile solid, suddenly becomes soft, pasty, and even sinks when a characteristic temperature is reached. However, for olivine glass, this characteristic temperature, called the glass transition temperature, is close to… 1000° C.
Do we have the key to the boundary between the lithosphere and the asthenosphere in this thin layer of glass between the olivine grains? The future will tell and science must encourage people to remain cautious and modest. It is clear, however, that this discovery justifies pushing further our investigations into the mechanical properties of this particular glass of olivine.
Beyond this particular discovery, this work sketches the outlines of a new science that we could call nanogeodynamics. Based on the conviction that the intimate properties of matter, due to its atomic structure, are expressed on infinitely larger scales, this approach mobilizes the resources of mineralogy, and more generally of materials science, physics, of chemistry to provide answers to the major questions of the Earth sciences. Who would have imagined that an electron microscope would be our vessel to explore the interior of the Earth?