Documentary series Voyage of the Continents on
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Our planet is a sphere with a diameter of 12 756 kilometres, slightly flattened at the poles. However this sphere is not just a big homogeneous pebble: from its core to its surface, layers with different characteristics are piled on top of each other.
Right below our feet is the Earth's crust: a layer of solid rock, between 5 and 70 kilometres deep. This is the layer we walk upon, and the only one we really know. No drill has ever completely pierced through it to reach the levels beneath. The deeper layers are only known through seismology.
The Earth's crust rests upon the upper mantle, a layer approximately 700 kilometres thick. The part closer to the surface is solid and is attached to the crust, while the deeper part has a mushy consistency: with temperatures over 1000 °C, its rock is not completely molten, and moves a few centimetres a year. This motion is the main cause of the tectonic plates movement.
Underneath is the lower mantle, a very thick layer of rock between 700 and 2885 kilometres in depth. This layer is not quite solid: its matter can bend and stretch slowly, like very hard plasticine.
Still deeper is the outer core, a layer of molten metal 2270 kilometres thick with a temperature of over 3500 °C. The movements of this liquid layer are at the origin of the terrestrial magnetic field.
The deepest layer is the inner core, a sphere 2400 kilometres in diameter composed of a mixture of iron and nickel. Although its temperature is higher than 5000 °C, the inner core is solid: the pressure on it is so strong that its metals remain in a solid state despite the extreme heat.
For most of us, the word “ocean” simply evokes a vast stretch of salt water. But not geologists: to them, an ocean is a specific zone where the Earth's crust is very different from the crust of the continents. Therefore, they have defined two types of crust: continental and oceanic.
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The continental crust is made up of a complex mixture of different kinds of rocks. It is relatively thick - between 20 and 70 km - and on a terrestrial scale, very, even extremely old: some pieces of continental crust have existed for 4 billion years! They are almost as old as Earth itself, or 4.5 billion years.
The oceanic crust is younger, thinner (between 5 and 30 km), and heavier than the continental crust. It is composed of volcanic rock produced in oceanic crust “factories”: the ridges, long chains of underwater volcanoes. Unlike the continental crust, oceanic crust never lasts very long: it usually ends up in subduction zones, where it sinks under the continents and melts in the depths of the Earth.
For geologists, not all the areas covered by salt water are oceans: the sea has occasionally covered continental crust. This is the case of Hudson Bay in Canada and the North Sea in Europe: these bodies of water are actually flooded continents rather than true oceans.
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For us humans, the Earth looks stable. The mountains that existed when we were born will still be there when we die, and the continents' positions on a map will be the same. But in reality, the face of the Earth is perpetually changing. During a human lifetime, the continents move a few metres; these tiny advances in the human time scale are enormous in terms of geological time! At the whim of tectonic forces, the continents travel thousands of kilometres over the Earth's surface.
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The Earth did not always have five continents: at times, there were fewer and at other times, more. Scientists have given these past continents names like Gondwana, Laurussia, and Laurentia. The best known of the former continents is certainly Pangaea, a supercontinent that existed 250 million years ago and contained all the earth on the face of the planet. With only one continent, there was only one ocean at that time as well.
Surprisingly, the distribution of the continents on the Earth's surface alternates cyclically between periods when the continents come together to form a single super-continent, and others when they tend to separate from each other. Before Pangaea, there were other supercontinents, such as Rodinia, 1.1 billion years ago, and Vaalbara, 3 billion years ago.
Today, the continents are relatively evenly distributed over the Earth's surface and are in a phase in which the tendency is separation. However, scientists predict that about 250 millions years from now, the continents will again join together to constitute a single continent. A name-in fact several names-have already been suggested for this future super-continent: Pangaea Ultima, Novopangaea, or Amasia, depending on individual theories and the creative imagination of their authors.
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