PLATE TECTONICS

The conveyor belt

First formulated as a theory in the early 20th century but not "proven" until the 1960s and 1970s, plate tectonics is the key to contemporary geology. The surface we inhabit - the Earth's crust - rests on a magma conveyor belt lying at a depth of several kilometres on which parts of the lithosphere have been riding for the past four billion years. These plates move and collide, at times merging to form a single supercontinent, the last of which is known as Pangea and disappeared 130 million years ago when the Atlantic opened up. The Earth today is the result of the displacement and collision of fragments of this megacontinent.

Aléa sismique en Iran. La faille active de Mosha qui coupe le massif de l’Alborz – au pied duquel se trouve Téhéran et ses 12 millions d’habitants – est provoquée par le décrochage, en extension, de la partie de la plaque eurasienne qui ceinture le bassin de la Mer Caspienne en plongeant sous le Caucase. L’image, prise en 2006 dans le cadre de la recherche franco-iranienne qui a analysé ce mouvement considéré comme récent (1 million d’années), montre bien les «fissures» dues à l’extension. Ce phénomène tectonique régional est associé à la compression globale entre la plaque arabique et la plaque eurasienne. © CNRS Phototheque/Jean-François Ritz
Seismic phenomena in Iran. The active Mosha Fault that cuts through the Alborz Mountains – at the foot of which lies Tehran with its 12 million inhabitants – was caused by the breaking off, through extension, of the part of the Eurasian plate that circles the Caspian Sea basin and passes under the Caucasus. The picture, taken in 2006 as part of a joint French and Iranian research project to study this movement regarded as recent (a million years ago), clearly shows the “cracks” due to this extension. This regional tectonic phenomenon is linked to the global compression between the Arabian plate and the Eurasian plate. © CNRS Phototheque/Jean-François Ritz
La dérive des continents est à l’œuvre au Proche-Orient où l’Arabie et l’Afrique s’éloignent imperceptiblement, séparées par un jeune bras d’océan, le golfe d’Aden. Plusieurs campagnes scientifiques étudient la structure de cet océan naissant. Cette carte montre le détail du relief de la Dorsale de Sheba située entre la côte d’Oman et la pointe de la Somalie. En vert, en haut, la côte d’Oman. En bas, l’Ile Socotra. La dorsale, de couleur rouge orangée, est orientée Est-Ouest. En bleu, à droite, les fonds de l’océan Indien. © Sylvie Leroy/CNRS-UPMC
Continental drift is at work in the Middle East, where Arabia and Africa are imperceptibly drifting apart, separated by a strip of ocean, the Gulf of Aden. A number of scientific campaigns are studying the structure of this expanding sea. This map shows a detail of the Sheba Ridge located between the coast of Oman and the point of Somalia. Shown in green at the top is the Oman coast. At the bottom, the island of Socrota. The ridge, shaded reddish orange, is facing east-west. The Indian Ocean is shown on the right, in blue. © Sylvie Leroy/CNRS-UPMC

Understanding what lies so very close to us, beneath the ground we walk on, has long defied the efforts of humankind. Beginning with Copernicus, Galileo, Kepler and Huygens, science first explored the distant - the Earth's place in the solar system and the latter's place in the galactic vastness of the Universe.

Meanwhile, the composition of the lithosphere and the nature of the telluric forces brought to bear on this layer several dozen kilometres thick that forms the foundation of continents and carpets the ocean floors - where it is just a few kilometres thick - remained a mystery.

The age of the adventurers

Yet since the 18th century geology has been the subject of sustained curiosity and intense observation. Many passionate and adventurous scientists of that time set out to explore some of the most extreme locations on the planet, travelling the peaks, valleys, plateaux and plains of the five continents in their quest for knowledge. They studied soil and sub-soil structures, collecting and classifying fossils, minerals, rocks and sediment of all kinds.

The existence of lava-spitting volcanoes and other "hotspots" on the Earth shows that its centre contains a molten liquid mass, known as magma. For a long time the prevailing belief was that, due to the effect of heat, the Earth's crust was subject to vertical pressure.This was seen to explain the existence of mountains, and their counterparts formed by oceanic or terrestrial depressions.

But this quite rudimentary and indeed rather ill-founded theory did not fully explain all that the geologists witnessed. In particular, they noted certain very distinctive - and at the same time very comparable - "geotypes" at different continental locations at great distances from each other and separated by oceans. These had in common rock formations, flora (ferns) or fauna (lemurs) of the kind found, for example, in Africa or Brazil, or in Madagascar and Indonesia.

The Afro-American anatomy

As geographical maps became increasingly precise and small scale, so the shape of continents was revealed. Their examination caused some to question the way Cape Recife in South America is the mirror image of the Gulf of Guinea in Africa, the shape of the two coastlines interlocking almost exactly.

Alfred Wegener was the first to formalise this "inter-continentality" of geological formations, without being able to provide a scientific explanation. In 1915, this meteorologist - who became a geologist out of curiosity and who was a great adventurer (he lost his life 16 years later while exploring Greenland) - published the visionary work entitled The origins of continents and oceans.

Wegener gave us the theory of "continental drift". He believed that the Earth's crust, resting several hundred kilometres below the surface on viscous magma - which is in perpetual motion due to the effects of heat - is subject to lateral and not vertical movement. He described the geographical congruence between the Americas and Europe and Africa, separated by the Atlantic Ocean, as the result of the break-up, which began 250 million years ago, of a former continent to which he gave the name "Pangea".

The explanation met with deafening silence on the part of a large majority of geologists.This notion of continental drift was revolutionary, going against all the accepted thinking in the Earth sciences. But once the idea had been launched the most curious scientists showed increasing interest in it. But it was not until the 1960s and 1970s that scientific evidence of plate tectonics reached the point where Wegener's visionary genius could be confirmed.

The tectonic revelation

Much of the confirmation did not come until almost a half century later. A decisive step was taken in the 1950s when bathymetric measurements by researchers on board the Verna, the oceanographic vessel of Columbia University's (USA) Lamont-Doherty laboratory, established the existence of the first underwater ridge. Known as the Mid- Atlantic Ridge, this runs from the Arctic Ocean north of Iceland to the South Atlantic. It was then understood that it was through this huge tear that molten lava welled up from the Earth's interior due to the effects of convection movements in the mantle, escaping and continuously pushing back the two sides of the ridge as it did so.

By the end of the 1960s the knowledge available, provided by bathymetrics, paleomagnetism and seismography in particular, was extensive. This formed the basis for a global formulation of plate tectonics by the Frenchman Xavier Le Pichon and the American Jason Morgan, who each published an article on the subject during the same year, 1968. During a pioneering abyssal exploration operation known as Famous, Le Pichon was also the first scientist to actually go down in a submarine to see for himself the North Atlantic Ridge off the Azores. This oceanographic campaign confirmed the formidable magmatic activity along the length of this huge vertical tear in the lithosphere.

The tectonic model marked a genuine revolution in the Earth sciences as it made it possible to understand the workings of the "puzzle" that is the Earth's crust, divided into a dozen large plates (consisting of elements of either continental or oceanic crust) moving around the globe on the viscous upper layer of the mantle, known as the astheno - sphere. As the Earth's diameter remains constant, the continuous creation of new crust at the site of the mid-oceanic ridges must be accompanied by a corresponding destruction of the crust. This destruction can be observed when the oceanic plate - finer but with a higher density than the continental crust - collides with the latter. These rifts are visible at some points of the Earth's surface: Iceland, the African rift, the French overseas territories of the Afars and Issas, etc.

This collision is generally expressed in the form of "subduction" - when the oceanic plate passes beneath the continental plate. At this location the tensions between the plates build up to cause earthquakes and most volcanoes are formed, materialising the fusion of the oceanic plate and rise of the lighter magmatic fraction.

The meaning of an earthquake

Moving from the realms of theory to practice, in the space of a few decades tectonics has become a useful science in understanding what constitutes one of the most dramatic natural disasters of them all: an earthquake. The build-up of stress by subduction generates large amounts of energy that, when breaking point is reached, is released causing a seismic shock. Most of the world's earthquake-prone regions are crossed by major faults, sometimes stretching for hundreds of thousands of kilometres with spectacular rifts and mountain chains as testimony to areas of tectonic activity. Zones of subduction and active faults, such as the famous San Andreas Fault (California) or the North Anatolian Fault (Turkey), are areas of fragility along which the most violent earthquakes are produced when the energy built up by one plate rubbing against another is suddenly released.

This release is accompanied by the simultaneous creation of two types of waves that travel across the adjacent area: those that travel most rapidly (reaching speeds of 6 km a second on the surface and the first to be detected by seismographs) are known as "compression" waves because they lead to a succession of dilatation and compression of land both parallel to and on the plane of their axis; secondly, there are the "shear" waves that trigger a phenomenon whereby the ground wrinkles like waves. It is these that cause the most devastating damage.

Volcanism revisited

Plate tectonics shed new light on our understanding of volcanism, whether on the seabed or continental. Most volcanoes are found along the major tectonic fault lines through which magma - the result of the partial melting of the lower layer of the lithosphere due to the action of the heat at the Earth's core - wells up to the surface at the site of subduction or the movements between two plates. Volcanic eruptions occur when this molten matter that accumulates in magma chambers is suppressed and expelled through the chimneys leading to the craters.

But apart from this phenomenon, certain volcanoes can be "hotspots" that are not linked to a boundary between tectonic plates.

A plume of hotter magma that rises up from the mantle can penetrate the lithosphere. If this hot place at depth remains fixed while the lithospheric plate as a whole moves, hotspots form in succession, as in the archipelago of the Hawaiian islands.

Every volcano is unique but has characteristics that allow experts to identify different categories. In particular, they distinguish between effusive eruptions in which the lava flows quite fluidly, and explosive eruptions during which large quantities of gas and burning ash can be projected to great distances, resulting in nuées ardentes and huge volcanic plumes.

The contribution of spatial geodesy

For several decades, detailed mapping of all the earthquakes occurring on the surface of the planet has been seeking to model the horizontal movements of the tectonic plates that form the Earth's crust. Apart from the ground geology observations, a major revolution has been brought by the very developed spatial geodesy instruments that equip a number of satellites, such as Envisat and Cryosat. These make it possible to measure with increasing accuracy the deformations caused by recent - or imminent - telluric shocks in areas of high seismic activity. As Xavier Le Pichon explains: "The tectonics that create mountain ranges do so through an accumulation of successive ruptures that seismology enables us to study. The study of earthquakes is thus in fact instant tectonics!"

This does not mean that current knowledge enables us to predict where and when all earthquakes are going to occur. However, when combined with an ever-improving knowledge of past events, it does permit an ever-more precise evaluation and mapping of the seismic risk that reflects the probability of an earthquake of a given intensity and the likely human, economic and environmental cost to the potentially affected region. The models developed make it possible to simulate the impact of an earthquake and to determine the zones that will be most affected. That was one of the goals of the Risk-EU (2001-04) project that studied the cities of Barcelona (ES), Bucharest (RO), Bitola (MK), Catane (FR), Nice (FR), Sofia (BG) and Thessaloniki (GR). The European Commission (Directorate-General for the Environment) is currently working on a strategy to reduce the risk of anthropic or natural disasters that could, ultimately, result in multi-risk mapping for those regions of Europe most vulnerable not only to earthquakes and volcanic eruptions but also to many other risks linked to geology.

Didier Buysse


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