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When the zaniness
of a planet has stirred up myriacubes of earth, we can
find this star muddled, but it is the genius of disorder.
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Un peu de tectonique
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The Mid-Atlantic Ridge emerging here in Iceland
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  Our planet was formed 4
billion years ago by the accretion of elements from
the pre-solar system. They then organized themselves to form
three large structures. Seismic studies make it possible to
highlight them and evaluate their compositions and dimensions.
From the center of the earth to the surface, there follows the
core, the mantle
and the earth's crust. We
could add the atmosphere,
resulting from the degassing of the planet. The continents
and ocean floors make up
the earth's crust.
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Structures :
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Quelques dimensions :
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Core
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Diametre: 2900 km
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Lower mantle
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Depp : 2800 km
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Upper mantle
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Thickness : 25 à 70 km
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Earth crust
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Thickness : 12 à 70 km
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 The energy from the accretion of primitive materials,
but also for a large part, from nuclear reactions, are the origin
of the internal heat of our Planet. The interior of the Earth
thus presents extreme temperatures
and, moreover, under the effect of gravity, considerable
pressures. Contrary to what one might think, the
coat is not liquid. It is a solid medium (due to pressures),
but it remains ductile.
The mantle is responsible for the effects on the Earth's crust.
The latter, less dense and cold, “floats” on the coat. The large
temperature differences, the differences in density of materials,
create convection movements
within the mantle, a bit like the agitation of water when it
is heated in a pan. The earth's crust, fragmented into large
sectors, called plates,
then moves according to these movements. This phenomenon, discovered
during the 20th century, is called “continental
drift” or “plate tectonics”.
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The continents moving, tectonic plates
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tectoniq.gif
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A changing surface : the mobility of the tectonic
plates
Since its birth, the Earth has been constantly
transformed. The numerous plates which form the earth's crust
cause the continents and oceans
to expand and then disappear. They are born in a movement of
accretion,
collide, dislocate,
are lost under other plates.
This dynamic, invisible on a human scale, combined with erosion
makes our planet a star in perpetual transformation and with
a changing surface. The mountains,
born from the collision of plates, are the direct consequence.
Other effects, perceptible to humans, are earthquakes
and volcanism. The slow
drift of soils is nevertheless highlighted by high-precision
topographical measurements in areas with high displacement (several
cm/year on the East African rift).
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Starting of the plates: the accretion
(divergent boundary)
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 Oceanic plates are formed by the rise
of magmas from the upper mantle. They are built symmetrically
by the influx of products from the depths of the mantle. Then
in a slow horizontal movement,
the resulting crust drifts on either side of the emission zone,
floating on the upper layers of the mantle. This phenomenon
is called accretion and
the middle axis, rifting.
This is how the ocean floor is created, because in most cases
the accretion is underwater. However, there are terrestrial
rifts, such as in Iceland
or in Afar. One of the characteristics
of the rift is intense volcanic activity. Thus, the Atlantic
ridge formed by the rift separating the American plates from
the European and African plates, is a succession of submarine
volcanoes extending as far as Iceland.
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Titanic shocks: collisions (Transform
boundary)
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 Driven by the movements of the mantle, the plates
“float”. When two plates
come together, they rub against each other. This confrontation
does not take place without consequences. We are thus witnessing
the most powerful earthquakes
on the planet, generating deadly disasters in populated regions.
This is the case on the Anatolian fault in Türkiye, where the
Eurasian plate collides with the African plate. The forces between
plates are also at the origin of large
mountain formations, such as the Himalaya
and Tibet, born from the meeting of India and the Asian plate.
Closer to us, the Alps are
the result of Italy's push on the Eurasian plate.
A plate is not made up of a single block. The slow drift, the
collisions, make it a strongly dislocated
whole, composed of faults, overlapping folds, raised zones (mountains),
collapse basins. These accidents are also the site of earthquakes
and volcanism.
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The disappearance of the plates: subduction
(convergent boundary)
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 The disappearance
of a plaque is a surprising phenomenon. Inexorably pushed by
the movements of the mantle, it passes beneath its neighbor,
entering the depths of the Earth. The materials,
swallowed by the mantle, will join the internal material. The
confrontation generates folds on the absorbing plate, causing
the formation of mountain chains
or archipelagos. Thus the
Andes or the Rockies were formed (at the end of the Secondary
for the Rockies), by the
subduction of the Pacific
plate under the American plate. The Indonesian, Philippine,
and Japanese arcs bear witness to the disappearance of the Pacific
plate beneath neighboring plates. It is in these formations
that we also find the deepest marine
trenches on the globe. The visible manifestations
of this dynamic are intense seismic activity, due to the friction
of the two plates, and especially the partial
fusion of the lithospheric mantle under the upper
plate at the origin of the volcanism. It is in these regions
that the latter is characterized by an explosive
dynamism that is sometimes cataclysmic, like Pinatubo
in 1991, or Mount Saint Helens in 1980.
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