In 1912, meteorologist Alfred Wegener (1880-1930) proposed that the continents once formed a single supercontinent, Pangaea, and have been drifting slowly for hundreds of millions of years. The similarity of the African and South American coastlines, the continuity of geological formations from one continent to another, and the presence of identical fossils on lands now separated by oceans all support his theory. However, the scientific community rejected it, as there was no credible mechanism to explain the movement of entire continental masses.
It was not until the 1950s-1960s that mapping the ocean floors revealed the mid-ocean ridges. Plate tectonics then took its modern form, thanks to the work of Harry Hess (1906-1969), who proposed seafloor spreading, and J. Tuzo Wilson (1908-1993), who introduced the concepts of hotspots and transform faults.
The Earth is structured in concentric layers with very different properties. The temperature at the center reaches about 5,100 degrees Celsius, from two sources: the primordial heat inherited from accretion 4.5 billion years ago, and the heat continuously produced by the decay of uranium-238, thorium-232, and potassium-40.
| Layer | Thickness (km) | Temperature (°C) | State | Tectonic Role |
|---|---|---|---|---|
| Lithosphere | 5 to 70 | 0 to 300 | Rigid solid | Forms the moving tectonic plates |
| Astenosphere | 200 to 300 | 300 to 900 | Viscous, partially molten | Allows the sliding of lithospheric plates |
| Lower Mantle | 2,200 | 900 to 3,700 | Viscous solid | Site of deep convection currents |
| Outer Core | 2,260 | 3,700 to 5,000 | Liquid | Generates the Earth's magnetic field |
| Inner Core | 1,220 | 5,000 to 5,100 | Solid | Reservoir of primordial heat |
Over millions of years, the solid mantle behaves like a very viscous fluid and undergoes thermal convection: hot rocks rise, cool at the surface, and then sink back down. These convection cells drive the lithospheric plates above them, much like rafts on a river.
However, the mechanism is more complex: Don Anderson (1933-2014) and Claude Allègre (1937-) showed that the pull exerted by cold plates sinking in subduction contributes as much, if not more, than the push from the ridges. The weight of old, cooled plates is indeed a major driving force of the system.
The plates move between 1 and 15 centimeters per year: the Atlantic, for example, widens by about 2.5 centimeters each year, or the width of a fingernail, moving Europe and America a meter apart every forty years. The two continents, now separated by nearly 6,000 kilometers, began to drift apart about 180 million years ago during the breakup of Pangaea.
At divergent boundaries, two plates move away from each other and magma rises to form new oceanic crust. This is the mechanism of mid-ocean ridges: the Mid-Atlantic Ridge extends over more than 16,000 kilometers, and Iceland is one of the few places where it emerges above sea level.
At convergent boundaries, two plates collide. If one is oceanic, it sinks beneath the other in subduction, generating volcanoes, earthquakes, and tsunamis, as along the Peru-Chile Trench. If both are continental, the crust folds to form mountain ranges: the Himalayas were born this way, 50 million years ago, from the collision between the Indian and Eurasian plates.
At transform faults, two plates slide laterally past each other without creating or destroying crust. The friction accumulates stress that is released as earthquakes: the San Andreas Fault in California is the most famous example.
Rocks formed at the ridges record the orientation of the Earth's magnetic field at the time of their solidification. Since this field has reversed many times, symmetrical bands of alternating polarity are observed on either side of the ridges, direct proof of the expansion of the ocean floor. This work by Drummond Matthews (1931-1997) and Frederick Vine (1939-) was decisive in the 1960s.
| Plate | Area (106 km2) | Speed (cm/year) | Dominant Type | Notable Associated Phenomenon |
|---|---|---|---|---|
| Pacific Plate | 103 | 5 to 10 | Subduction and translation | Pacific Ring of Fire, Mariana Trench |
| North American Plate | 76 | 2 to 3 | Divergence (east) and translation (west) | Mid-Atlantic Ridge, San Andreas Fault |
| Eurasian Plate | 68 | 2 to 3 | Divergence (west) and collision (south) | Himalayas (collision with the Indian Plate) |
| African Plate | 61 | 2 to 3 | Multiple divergence | East African Rift, nascent ocean |
| Antarctic Plate | 60 | 1 to 2 | Divergence (edges) | Surrounded by ridges on almost its entire perimeter |
| Indo-Australian Plate | 58 | 6 to 7 | Rapid convergence (north) | Himalayas, Australian Alps, Sumatra earthquakes |
| South American Plate | 44 | 2 to 3 | Divergence (east) and subduction (west) | Andes, Peru-Chile Trench, active volcanoes |
| Nazca Plate | 16 | 7 to 8 | Rapid subduction | Subduction under South America, formation of the Andes |
| Philippine Plate | 5.5 | 6 to 8 | Subduction (east and west) | Philippine island arc, intense volcanism |
| Arabian Plate | 5 | 2 to 3 | Collision (north) and divergence (south) | Red Sea (nascent rift), Zagros, Caucasus |
N.B.: The speeds indicated are average values measured by space geodesy (GPS). They can vary significantly depending on the part of the plate considered and the axis of measurement. The area includes, in some cases, smaller adjacent plates when geologists group them under the same name.
Paleomagnetic and geochemical data allow us to trace the successive assemblies and dispersals of continental masses. This cycle, called the Wilson Cycle in honor of J. Tuzo Wilson (1908-1993), predicts that in about 250 million years, the continents will come together again to form a new supercontinent, sometimes nicknamed Pangea Proxima or Amasia, depending on the models.
| Supercontinent | Formation | Dispersion | Remark |
|---|---|---|---|
| Nuna / Columbia | ~ 1.8 billion years ago | ~ 1.5 billion years ago | First well-documented supercontinent; centered around 30-40° N, mainly in the northern tropical hemisphere |
| Rodinia | ~ 1.1 billion years ago | ~ 750 million years ago | Its breakup may have triggered a global glaciation; centered around 10-20° S, straddling the equator |
| Pangaea | ~ 335 million years ago | ~ 175 million years ago | Fragments into Laurasia (north) and Gondwana (south); centered around 10° N, extending from 85° S to 85° N |
| Pangaea Proxima / Amasia | ~ in 250 million years | - | Future supercontinent predicted by models; centered around 30-60° N depending on scenarios, around the Arctic pole for Amasia |
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