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Tectonics : the Earth quakes [3]
Tectonique : la Terre tremble [3]
The Earth : It folds or it breaks ! An active planet [Genista]
By Nicolle Mathé, 'Genista Informations' n° 295, July-August, 2003 (Plate tectonics)
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![la Terre [the Earth]](../../i/globe.gif)
« My wife and I have decided to live on the San Andreas Fault to experience, at least once in our life,
the sensations caused by an earthquake ».
A deliberate flirtation with danger, like Katia and Maurice Krafft on the slopes of the Unzen, a Japanese volcano which
has been dormant for centuries, and which woke up to take their lives away...
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A wound 37,000 miles (60,000 km) long
Under the pressure and the traction of the underlying magma, the rifts a a security valve, necessary to let out the
ever-renewed energy, under the cover made by the Earth's crust, because of the decomposition of radioactive magmatic elements
such as uranium.
The oceanic range, lifted and broken on its whole length, is therefore the permanent centre for earthquakes
and an outpouring of lava. We cannot view this turbulence because it takes place under water !
![Iceland produces 1/5 of the total lava in the world [l'Islande produit 1/5 de la lave du monde]](it/islande.gif)
But, in Iceland, a 16 miles long piece (25 km) of the medio-atlantic Range, the rift,
a succession of 135 cones, is out in the air, with a production of lava estimated at 1/5th of the wold production !
The last volcano, formed on 14th November, 1963, liberated materials which gave birth to the island of Surtsey, with an area
of 700 acres (280 ha) !
The Icelanders live on this piece of range, with a ringside seat on the coasts, of course !
In the Aden Gulf, the rift of the recent range, which is at the origins of the Read Sea,
is also visible, and is the favourite of all sorts of specialists.
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Some countries hold concurrently...
Situated vertically over a zone of subduction, suffer from the whims of the oceanic plate which slowly slides
under the continental plate, exerting pressure on it and creating heat, because of friction.
...earthquakes
Seismographs in those places record micro-earthquakes daily ! The energy which is accumulated during the sliding along
can be liberated at once and therefore cause the breaking of rock layers at a more or less great depth. From this particular place,
the seismic centre, rings of waves go away in all directions.
They first reach the surface of the ground which is located above vertically, called the epicentre, a place where
the horizontal and vertical vibrations of the ground are strongest, with a possibility of important damages.
...volcanoes
As the temperature increases with the depth, the basalt oceanic plate releases a large quantity of water which diffuses
in the rocks located under the continental plate and causes them to melt.
This magma can reach a high pressure very quickly, through cracks in the continental layers, and build
majestic volcanoes like Mount Saint Helens in the Rocky Mountains, Paracutin in Mexico,
Nevado del Ruiz in Colombia... which periodically draw attention to them when the intramagmatic gas pressure is at its height.
![how the mountains are formed [comment les montagnes se forment]](it/subductn.gif)
...orogenesis
Pockets of hot magma slowly rise through the continental rocks that are on their way, and absorb them.
By getting cooler, this magma becomes granite which constitutes granitic massifs, surrounded with rocks which have
become modified after getting close to the hot magma ; they are the metamorphic rocks.
The oceanic and continental plates being moving in opposite directions, the continental rocks, soft and therefore plastic,
get folded and rise, forming impressive massifs, or subduction ranges, like the Rocky Mountains, the Andean Cordillera
(Andes)... the altitude of which increases ceaselessly in spite of erosion.
All these phenomena, which are linked to subduction, are inevitable.
The people who settle to live in these areas, or who merely visit them, must be aware and ready to live a natural catastrophe.
And what about "volcanic islands"... ?
![the meeting of two plates, and islands [la rencontre de deux plaques, et des îles]](it/arcinsul.gif)
There are numerous islands made from the meeting of two oceanic plates, one plunging under the other.
The fusion of the rocks located above the plunging plate create magma which escapes through the fracture in the oceanic crust,
to make important volcanic complexes all along the fracture zone, and build volcanic islands disposed along a curve.
On these islands, earthquakes are frequent and violent, and volcanism is active.
This is the case of island arcs in the West Indies, Japan, Indonesia... where volcanes which have been
dormant for centuries may wake up, like the Unzen in Japan or the Pinatubo in Indonesia.
Other volcanic islands are strangely located in the middle of an oceanic plate, in a line, at various distances,
because they have been formed from a fixed magmatic zone, with a high-temperature flow which breaks the ocean floor
in one point only.
The ocean floor, by moving over this hot spot during its expansion, breaks each time in another place
and creates new volcanic islands in a row like those in the Pacific.
![the meeting of two continents : a collision [la rencontre de deux continents : une collision]](it/collison.gif)
The activity of rifts cause the expansion of oceanic floors, and some continents drift away from one another.
In their drift, they necessarily come across other continents, and then there is a collision.
The layers of continental rocks get folded, se cassent, get to a higher altitude and form massive mountain ranges
with multiple large faults.
This confrontation is permanent and the layers, which accumulate a lot of energy, break at last,
causing violent earthquakess as in Caucasus, Himalaya, the Pyrenees, the Alps, Turkey, Greece, Indonesia...
![Indian plate and the formation of the Himalaya [plaque indienne et formation de l'Himalaya] - Wiki](it/inde.gif)
Formation of the Himalaya :
The Indian plate has been travelling 3,750 miles (over 6,000 km) before colliding with the Eurasian plate
some 40 to 50 million years ago.
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![the San Andreas fault and San Francisco [la faille de San Andreas et San Francisco]](it/sanandrs.gif)
A fault which is talked about a lot !
Two plates may slide horizontally and generate important earthquakes.
Such are the Pacific plate and the North-American plate, which slide along the San Andreas Fault,
620 miles (1000 km) long, on which the city of San Francisco, in particular, has been built, and the devastating
earthquakes of which we duly know.
The opposite forces exerted on the two continental blocks cause a deformation of the elastic rocks on both sides of the
fault, along a distance of 25 miles (40 km).
The constraints which have thus been accumulated during several months finally get liberated in the breaking of the rocks
in a few seconds.
In 1857, the two blocks were displaced along a distance of 20 ft (6 m) in 60 seconds.
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