Our planet welcomes us, carries us, feeds us, supports us... but what do we really know about "her"?
Third planet of the solar system, the Earth is the only known planet on which are living beings!
The revolution of the Earth around the Sun, which defines the sideral year, lasts 365days and 6 hours.
The planet Earth rotates on itself in 23h and 56min and 4sec (not 24h), which defines the duration of a sidereal day.
The Earth and the Sun are about the same age, 4.5 billion years.
From the surface to the center, there are about 6400km, the solid part is 66km thick.
So, if the Earth was an apple, this solid part would be the skin of the apple and if it was an egg, this part would represent the shell!
So walk carefully, we walk on eggs!
The planet Earth is made up of different layers of different thicknesses and constitution!
From the nucleus to the outer crust, distinct worlds are declined...
Let's Visit them!
The nucleus is located in the center of the Earth.
It is divided into two parts:
A liquid part, the outer core, closer to the surface and a solid part, the inter core located deeper.
The two parts of the nucleus form 17 % of the terrestrial volume.
They are separated from one another by the Lehmann discontinuity.
The outer core consists essentially of iron and reaches a temperature of 4000 ° C!
It is known to be liquid, and because of its fluidity, the molten rocks it contains are constantly moving.
It is this movement which is at the origin of the magnetic field of the Earth.
Over the millennia, the outer core loses volume, as it solidifies; It then becomes a part of the inner core!
The inner core is sometimes called "seed".
It's probably solid, but it's not certain.
The temperature there is from 3800 to 5500degree C according to a study of 2013.
If the internal core is really solid in spite of the temperature, it is because the pressure is very strong; The matter is so compressed that it can not spread, and therefore be liquid.
It is thought that the internal core is also in motion: it would turn, under the effect of the magnetic field, this being facilitated by the liquid with which it is surrounded.
This is called the "differential rotation", because the rotation of the nucleus and that of the entire planet are not related.
The nucleus weighs about... 1000 billion billion tons... it's the heaviest seed on the planet!
Beyond the nucleus is the mantle!
The mantle represents 80 % of the volume of the Earth and 65 % of its mass.
It is separated from the crust by the discontinuity of Mohorovicic, also called "Moho" by abbreviation; And of the nucleus by the discontinuity of Gutenberg.
There are two levels of mantle: the upper mantle (which extends from 35 to 700km deep), plastic;
And the lower mantle (700 to 2900 km deep).
The two coats are therefore a little less than 2900km thick.
The greater part of the earth's mantle is composed of a rock called peridotite.
The differences between the mantlecomponents are mostly related to the depth: the deeper it is, the hotter it is and the higher is the pressure.
The most common rock in the mantle, the perovskite, appears from a depth of 670 km.
The upper mantle is mostly plastic, but under the effect of the heat emitted by the core, some areas are liquid, closer to the surface.
It is animated by movements called "convection movements".
Which means that the rocks move according to the temperature, the hot rock is less dense and climbs, the cold rock is denser and goes down.
It is these mantle movements that are responsible for the displacements of tectonic plates, and therefore for the activity of volcanoes and earthquakes.
Nothing is homogeneous in the earth's mantle:
The heat from the radioactivity and the viscosity of the layer are examples of parameters very difficult to determine correctly...
The mesosphere refers to the lower mantle in the region between the asthenosphere and the outer core.
It is the thickest layer of Earth.
As the depth increases, the pressure and strength of atoms in a structure is denser and more rigid.
Thus, the difference between the mésosphère and the asthenosphere is probably due to differences in density and stiffness, ie physical factors, and not any difference in chemical composition!
The mesosphere is synonymous with the lower mantle!
The asthenosphere (from the Greek asthenos, meaning 'without resistance') is the ductile part of the earth's upper mantle.
It extends from the lithosphere to the lower mantle over 700 km.
This concept appeared in the late 1960s with the revolution of plate tectonics, along with the concept of mesosphere.
The asthenosphere is between the lithosphere and the mesosphere.
The depth of the asthenosphere therefore depends directly on the thickness of the lithosphere.
The latter varies between about 100 km under the oceans (a few kilometers in the oceanic rifts) and about 170 km under the continents.
The distinction between these two regions is based on criteria of mechanical behavior of rocks, the passage from one domain to another is often defined using an isotherm.
The value of this isotherm arouses debate in the scientific community.
We can find 1300 Kelvin, a little more than 1000 degree celcius, or 1350 degree C.
At this temperature most rocks become quite ductile, allowing the lithospheres to move.
The upper part of the asthenosphere is called the low-speed area, often abbreviated LVZ according to the English Low VelocityZone.
Its lower limit is between 640 and 700 km, at the intermediate zone which separates it from the lower mantle.
It is the seat of plastic deformationsassociated with simultaneous mechanisms of sliding,
dislocations (discontinuities in the crystal structure) and the migration of ions and gaps.
This process is called diffusion creep or self-diffusion.
It is also the seat of convection, but many arguments indicate that this convection has at least partial continuity with the convection of the lower mantle.
It has been imagined that the convection movements of the asthenosphere were the cause of the superficial displacement of the lithospheric plates.
The ductile properties of the asthenosphere, causing mechanical decoupling with the lithosphere, nevertheless make it a bad candidate for this setting in motion.
It is currently estimated that the displacement of the lithosphere and the displacement of the asthenosphere are an integral part of the mantle convection system, and neither of them can be considered as the cause of the other...
The asthenosphere is composed of ductile peridotite(malleable) to more than 1300degree C at its limit with the lithosphere.
More precisely, it is composed of three mineral phases silicates:
55% olivine, 28% pyroxene, 17% garnet, and optionally a molten fraction.
The asthenosphere thus allows the phenomenon of convection (displacement of material due to heat).
The average density of rocks increases with depth from 3.4 to the upper limit of the asthenosphere(with the lithosphere) to 4 at its lower limit, with the lower mantle...
The velocity of the seismic waves P also increases with the depth.
It is 7.8 km/s at the level of the lithosphere.
The speed of the seismic waves S increases from 4km/s to 5.5km/s.
The speed of seismic waves depends on the stiffness: the greater the stiffness, the faster the waves.
The lithosphere(literally, the "stone ball") is the rigid terrestrial envelope of the Earth 's surface.
It includes the earth's crust and part of the upper mantle.
It is divided into a number of tectonic plates, also called lithospheric plates.
The lithosphere, which is relatively rigid over time scales of the order of 1 to 10 Million years, is based on the solid but ductile asthenosphere,
which is more easily deformable because it consists of rocks under physico-mechanical conditions (pressure, temperature, Increased strain rate), leading to a relatively low viscosity.
Carrier of two types of crusts, continental and oceanic, the lithosphere exists under two corresponding types:
The continental lithosphere, 60 km thick sometimes up to 200 km, aged in billions of years, and segmented according to the history of the continental provinces.
The oceanic lithosphere, organized continuously (if not laterally at the level of the transforming faults) according to the age of the oceanic crust that it bears.
Within the lithosphere, the earth's crust is separated from the upper mantle by the Mohorovicic discontinuity (more commonly called " Moho"),
a discontinuity marked by a change in the propagation speed of seismic waves, corresponds to a drastic change in the Material comprising these units, namely:
Ultramafic rocks for the lithospheric mantle, basaltic rocks for the oceanic crust, rocks of granulitic types for the lower continental crust, the upper continental crust being of granitoid nature.
The lithospheric-asthenosphere limit corresponds to a rheological limit situated inside the upper mantle:
on both sides, they are the same rocks, of the same chemical composition, but due to variations in physico-mechanical condition,
Mechanical and thermal behaviors processes are different, passing upward from fluide rheology and advective heat transport to the top of the asthenosphere, rigide rheology and conductive thermal transport from the base of the lithosphere.
For the purposes of modeling, the limit between lithosphere and asthenosphere is generally defined by the isotherm at 1300 degree C.
The crust-mantle passage is more superficial (it lies in the heart of the lithosphere), and corresponds to a change in the chemical composition and mineralogy of the rocks.
The transfer of heat into the lithosphere is by thermal conduction as opposed to convective heat transfer in the asthenosphere.
The temperature gradient is higher in the lithosphere than in the asthenosphere, of the order of about ten degrees per km.
The transformation of a rigid mantle into the lithosphere into a more deformable(ductile) mantle in the asthenosphere is responsible for a decrease in the velocity and a marked attenuation of the seismic wavesP and S at the level of the " Low velocity zone".
The plates which compose the lithosphere are animated by relative movements of divergence, convergence, or displacement, called sliding.
The movements of divergence reflect a distance of two plates with respect to each other at the level of a ridge.
The convergence is an approximation of the two plates.
Convergence can be a subduction, a plate passes over another.
There are two cases of subduction, a convergence of a continental and oceanic plate, with the formation of a cordillera or an insular arc.
In the second case, the convergence takes place between two oceanic plates with the creation of an insular arch or a double insular arc and a marginal sea formed by the filling of a rear-arc basin.
In the case of two continental plates, the two plates collide with a local blocking of the convergence, which gives rise to a collision chain.
The amount of lithosphere on the surface of the Earth is always the same, the movements of convergence and divergence compensate!
The lithosphere forms the bulk of the planet in contact with the biosphere, considering mass and volume, far in front of water and air.
It supports life, biodiversity and contains most of the residual fossil carbon in the form of coal, gas, oil and carbonaterocks.
The oceanic crusts, which have an average thickness of 6 km, cover about 70 % of the earth's surface!
The rest consists of continental crusts.
The oceanic crust corresponds to that part of the earth's crust that forms the oceans.
It is located above Moho(Mohorovicic discontinuity) and is mainly composed of basic rocks (basalts then gabbros) and ultrabasic ones such as peridotites.
Its thickness varies from 5 to 8 km, while it has a density of 2.7.
Continental crusts, which have an average thickness of 30 km.
They cover about 30 % of the earth's surface.
The rest consists of oceanic crusts.
The continental crust corresponds to the parts of the earth's crust that form the continents.
It is located above Moho (the Mohorovicic discontinuity) and is mainly composed of granitic rocks with a density of 2.7 to 2.8.
Its thickness varies between 15 km under the plains to more than 70 km under the mountains!
From the seed, in the center of the Earth, we crossed various layers to finish on a carpet unrolled and extended for our pleasure: the sedimentary cover!
There are five large sedimentary complexes: rocks, vases, sands(fine sands, muddy sands), gravels(containing gravel-sand), and pebbles(with pebbles and gravel, gravel-vases).
The sedimentary cover varies according to its position on the coast!
From the surface of the Earth to the center, there are about 6400 kilometers, the solide part is 66 km thick.
The hottest spot ever recorded is in Libya with a record temperature of 57.8 dégrées Celsius.
The coldest point was measured in Antarctica at -89 degrees C.
The highest mountain in the world is a working volcano : the Mauna Loa. It is located in Hawaii in the Pacific and measures 10230 meters high, higher than Everest, which measures 8850 meters.
But much of it is underwater. In fact, its altitude is 4205 meters.
The largest mountain range is underwater: the Mid-Atlantic Ridge.
The ridges of the highest mountains form islands.
This chain was discovered in the 1950s.
The deepest underwater pit is the Mariana pit.
Located in the Pacific, the deepest point is at 10916 meters.
Jacques Piccard, a Swiss oceanographer, son of Auguste Piccard, inventor of the bathyscaphe, a small submarine very resistant to pressure, goes down on board.
He is extremely surprised to find shrimp and abyssal fish!
The pressure is equal to 1000 times the atmospheric pressure.
97 % of the total water on Earth is in the oceans!
70 % of fresh drinking water is found in the Antarctic ice and Greenland.
If the Antarctic ice melted, the sea level would rise by 61 meters.
The largest lake in the world is the Caspian Sea.
The longest river in the world is the Amazon, 6992 km long.
The second is the nile with 6952 km.
The place where it rains the most is in Colombia, with 13 meters of water on average per year in Lloro!
A third of the Earth on the planet is desert, and the largest desert in the world is the Sahara!