Download Earth is made of hard rock

Earth is made of hard rock
Composition of Earth
I’m sure you’re thinking (you! Yes, you! Don’t play the innocent!) that, like a goof number of your
fellows, that Earth is an ocean of magma (so liquid) where some little ships are floating (so solid)
which are tectonic plates.
Sorry to tell you you’re totally wrong!
As you already know (or don’t ;-) in this case, cf precedent articles), Earth is made up of different
layers of rocks. Some layers are “harder” than others. In particular the crust, because of its lower
temperature compared to lower layers.
All Earth’s layers, composed of siliceous and/or magnesic minerals (all in all, the “rocky” layers)
are SOLID (we aren’t speaking of the outer core, the composition of which is more metallic).
Some layers (asthenosphere) behave a little more like fluids : they are made of solid rocks with a
very high (result of very strong bonds between the elements which are composing them) viscosity
(flow resistance). To the extent that we could never make them flow from one tupperware to
another one.
To give you an exemple : plasticine is a solid. We can’t make it “flow” from a tupperware to
another one but nevertheless we can put it out of shape. So let’s imagine that asthenosphere is a
wonder-plasticine for a great-sturdy giant.
The asthenosphere’s viscosity is approximatively from 1018 to 1019 Pa.s. (Pa.s = Pascal x seconde)
and the lithosphere’s one is 1000 times higher. The mantle’s one is estimated to be 10 000 times
higher.
For information, water viscosity is about 10-2 Pa.s., the hawaian lava’s one (considered as very fluid)
is 400 Pa.s., the ice’s one at 0°C (alpine glacier) is 1011 Pa.s. and the ice’s one at -20°C (antarctic
ice) about 1013 Pa.s.
(data source : http://planet-terre.ens-lyon.fr/article/manteau-discontinu.xml)
So the asthenosphere, that we are considering more “soft” is even 100 000 times harder than the ice
in Antartica !
Plate’s sliding on the asthenosphere
You’re probably going to ask me : how could the plates (lithosphere with crust on it) move if they
are solid and the asthenosphere too?
Well, they do, because there is an area where the sliding is possible for this zone is more
deformable because it’s hotter. This zone is called LVZ (Low Velocity Zone) and is situated
approximately between 80 and 250 km deep. It was located thanks to a decrease of sismic waves’
speed at this depth. It’s linked to a temperature limit of approximately 1300°C (called isotherm,
which come from the greek isos = equal and thermos = hot, so an area with the same temperature).
The temperature and pressure zones in this area almost correspond to the peridotite’s fusion
conditions. Therefore the mantle nearly reaches it’s melting point and that can explain the decrease
of sismic waves’ speed because the material there, at this place, behaves a little less like a solid.
But beware, this sliding is very slow and is only about a few centimeters per year. Imagine that it’s
two hard toffees (lithosphere on top and asthenosphere below) with a soft toffee layer between them
(LVZ) which allows one to move horizontally relative to the other one.
You should know that when a solid material is subjected to high temperature, it could show some
deformation in the long term (not visible immediatly, with the naked eye) because of stress
(compression or distension) and temperature exerted on it. It’s what we called fluage. That’s what
happens in the asthenosphere, in the LVZ mainly.
Convection movements in asthenosphere
The fact that convection movements exist in the asthenosphere is now widely known. Thus, “hotter”
plumes go up from the depths and “colder” material goes down in the depths. The oft-shown
example is colored oil making convection movements inside a beaker under which we put a heat
source.
Video of oil convection : https://www.youtube.com/watch?v=HmBHKeemZVY
The problem with that model is it makes you believe that it happens quickly. But that’s not true! It
occurs on a geological scale. So this means you can quietly station sitting at a table, with your cup
of coffee, watching a plume moving on the TV screen. By the time it has moved one centimeter,
your bones will have crumbled into dust since a long time!
Conclusion : convection movements in the mantle are VERY VERY VERY VERY VERY VERY
VERY VERY (etc.) slow.
If you want to see something moving, do like the cows watching a train go by!
Magma’s formation at the oceanic ridges
Tectonic plates’ sinking in the subduction trenches create a distension at oceanic ridges, which thins
the oceanic crust and facilitates the ascent, near the surface, of an ascending plume coming from a
convection cell that exists in the mantle.
Because of this ascending movement, a pressure fall occurs but not a temperature fall (rocks are bad
conductors for temperature so they cool very slowly). The pressure fall at a constant temperature
drives the peridotite into melting. But this melting is only partial because it was proved that
peridotite doesn’t melt immediately when heated (we are talking about peridotite’s partial melting).
Only the most acid minerals (mica, plagioclase, quartz) enter the melting first. So they will form the
origin magma for basalt and gabbro. And the residual peridotite will be more basic than it was
previously (much more concentrated in olivine and pyroxene).
But this magma which forms from peridotite is first in a “solid” state, but more “soft” that the rest
of the rock, which allows its ascending between cristals that are not already melting (very slow
process). So it’s a process which takes place at a solid state (I know, that’s difficult to imagine but it
exists). It only becomes “liquid” in the upper part of the magmatic chamber.
Magma’s formation at subduction zones
At a subduction zone’s level, metamorphism (transformation of minerals which takes place while
they are solid) because of temperature and pressure rise, due to the diving of the plate, is leading to
water release (but be careful, not in a liquid form, rather an ionic form : HO- and H3O+) in the
asthenosphere (dehydration due to metamorphism which occurs in minerals). This hydration of the
mantle’s peridotite leads to a decrease of its melting temperature, which goes down below the
temperature of this zone. A partial melting of peridotite occurs then. As in the case with oceanic
ridges, this partial melting and the magma’s production that comes with it, take place at a solid state.
Animation (in french, sorry) :
http://www.larousse.fr/encyclopedie/animations/Subduction_et_formation_des_montagnes/110051
8