Giant Slab of Earth's Crust Found Near Core Options

[ljk4-1]

Giant Slab of Earth's Crust Found Near Core

http://www.livescience.com/forcesofnature/...side_earth.html

A huge slab of folded Earth that scientists think used to be part of the ocean
floor has been detected near the planet's core.

[Guest_Richard Trigaux_*]

WOW!


Conceptually we were accustomed to the idea of Earth crust diving into the lava hot mantle. But to actually see it working is another thing... Pity their image is so small.

[The Messenger]

Atlantis?

[Guest_BruceMoomaw_*]

I always thought the place was falling apart.

[chris]

So that's where I left it...

[tty]

Atlantis?

Uh.... I think that was on the other side of North America.

tty

[Jyril]

WOW!
Conceptually we were accustomed to the idea of Earth crust diving into the lava hot mantle. But to actually see it working is another thing... Pity their image is so small.

there is a bigger picture available.

[Guest_DonPMitchell_*]

Wow. That's remarkable. But I always thought the crust was less dense (less mafic) than the mantle material. Is it actually sinking, or is it being carried down by the convection of the mantle?

[tty]

Wow. That's remarkable. But I always thought the crust was less dense (less mafic) than the mantle material. Is it actually sinking, or is it being carried down by the convection of the mantle?

It is only the basaltic oceanic crust that can do this. The continents are too light and "float" on top of the mantle.
That this process actually happens has been known from seismic records since quakes outline the sinking crustal slabs. However a few hundred kilometers down the pressure and temperatures are such that materials deform plastically without quakes, so this is the first time anyone has been able to follow the sinking slab all the way down through the mantle.

tty

[edstrick]

One reason that basaltic crust can be subducted is that it's chemically different from the upper mantle. Magma that forms the oceanic crust is formed by partial melting of upper mantle and the unmelted residue is depleted in the elements that dominate basaltic minerology. The magma, as a liquid, is lighter than the depleted mantle and rises towards the surface.

However, when oceanic crust is subducted into the mantle, something quite odd and distinctive happens: The olvivine/pyroxene/feldspar of basalt/gabbro undergoes a solid state reaction to form a completely different high-pressure minerology of pyroxene and garnet called eclogite. What's odd and important is that eclogite is DENSER than upper mantle rocks and sinks, rather than floats!

This requires a rather specific combination of temperatures and pressures. As I recall, theoretical studies indicate that in the probably dry and hot upper mantle of Venus, the basalt --> eclogite phase change cannot occur, thus basaltic crust cannot be subducted in large quantities and terrestrial style plate tectonics cannot occur on Venus.

[Guest_DonPMitchell_*]

One reason that basaltic crust can be subducted is that it's chemically different from the upper mantle. Magma that forms the oceanic crust is formed by partial melting of upper mantle and the unmelted residue is depleted in the elements that dominate basaltic minerology. The magma, as a liquid, is lighter than the depleted mantle and rises towards the surface.

However, when oceanic crust is subducted into the mantle, something quite odd and distinctive happens: The olvivine/pyroxene/feldspar of basalt/gabbro undergoes a solid state reaction to form a completely different high-pressure minerology of pyroxene and garnet called eclogite. What's odd and important is that eclogite is DENSER than upper mantle rocks and sinks, rather than floats!

This requires a rather specific combination of temperatures and pressures. As I recall, theoretical studies indicate that in the probably dry and hot upper mantle of Venus, the basalt --> eclogite phase change cannot occur, thus basaltic crust cannot be subducted in large quantities and terrestrial style plate tectonics cannot occur on Venus.

Thanks, very interesting. I own a "Xenolith", a chunk of olivine embedded in basalt. Is that a piece of the mantle coughed up by a volcano, or just a partial crystalization of minerals? Is there any kind of rock that is essentially what the mantle is made of?

[Guest_Richard Trigaux_*]

there is a bigger picture available.

Thanks Jyril.

We understand better on this larger image.


What I wonder now is what happens ith that crust in the bottom of the mantle. At a moment, it must melt and mix with the remainder of the mantle. perhaps the mantle if highly heterogenous, formed by slabs of crusty material piled since billions of years, ad slowly rising to the top while new slabs are pushing from the bottom.
Or there is a faster melting at the very bottom of the mantle, forming the the D'' layer, which is some kilometres to tens of kilometres thick. This layer, in direct contact with the core, is much hotter and more plastic, so that it quickly forms bulging and mushroom shaped diapirs, which climb to the surface independently of the overal convection, forming the hot spots. Also the D'' layer could chemically interact with the core, although we don't know in which way.

[edstrick]

That xenolith is more likely a piece ripped loose from the bottom of a magma chamber. When the basaltic magma was being "stored" before eruption onto or near the surface, it was cooling and crystalizing and some of the heavier crystals settled out as a crystal mush on the magma chamber bottom. A new injection of magma broke up some of the deposit and carried some up with the basalt. It's likely that the basaltic magma that was erupted was already partly molton and partly crystallized with large "phenocrysts" of pyroxene embedded in fine grained crystalized "matrix"

Google "magma chamber", phenocryst, and especially "Stillwater intrusion" and I bet you'll find more then you ever want to know on igneous petrology and geochemistry.

As far as intact chunks of mantle rock, to some extent, yes. Mangled bits of mantle get (probably) included in "ophiolite sequences" of oceanic rock "obducted" onto continental terrain instead of subducted beneath them. Other chunks get carried up in various eruptive events, including diamond pipe "kimberlite" eruptions. I've forgotten most of what I picked up on those subjects. A lot of them tend to be badly altered.

There are some absolutely gorgeous eclogite <I think> or other upper mantle rock samples that are made of green pyroxene or olivine and almost ruby red garnet, I think from Scandanavia.

[Guest_BruceMoomaw_*]

One reason that basaltic crust can be subducted is that it's chemically different from the upper mantle. Magma that forms the oceanic crust is formed by partial melting of upper mantle and the unmelted residue is depleted in the elements that dominate basaltic minerology. The magma, as a liquid, is lighter than the depleted mantle and rises towards the surface.

However, when oceanic crust is subducted into the mantle, something quite odd and distinctive happens: The olvivine/pyroxene/feldspar of basalt/gabbro undergoes a solid state reaction to form a completely different high-pressure minerology of pyroxene and garnet called eclogite. What's odd and important is that eclogite is DENSER than upper mantle rocks and sinks, rather than floats!

This requires a rather specific combination of temperatures and pressures. As I recall, theoretical studies indicate that in the probably dry and hot upper mantle of Venus, the basalt --> eclogite phase change cannot occur, thus basaltic crust cannot be subducted in large quantities and terrestrial style plate tectonics cannot occur on Venus.

The downward pull from that newly created dense eclogite pulling at the edge of the basalt crustal plate to which it's still attached is, it turns out, much more important as a driving force for the crustal conveyor-belt tectonics of Earth than the upward push on the other end of the crustal plate is (where newly melted basalt is surging upwards). There has, in fact, been a recent abstract -- if I can (*sigh*) find the thing -- attempting to compare the push/pull ratio that is possible on the various inner planets to determine just how much possible force they may have to drive crustal tectonics.

[Guest_Richard Trigaux_*]

The two last posts strongly suggest that there are conditions which authorize or forbid plate tectonics, depending on planets. These conditions seem strongly linked to the mantle chemistry, and not to the presence of liquid water on the surface, as I wrote earlier.

A planet similar to Earth, but where plate tectonics would not work, would have its heat escaping under the form of hot spots, like on Venus. Even on earth, the plate tectonics, despites its incredible efficiency, is not enough to evacuate all the heat, and hot spots still do a significant part of the job.

On the contrary plate tectonics is very efficient to expose a large fraction of the mantle material to the surface. This is perhaps why there are large oceans on earth, with all the water which escaped from the mantle. (The mantle seems to contain a small amount of water). On venus, the mantle contains less water, and the hot spots volcanism is less efficient to bring it to the surface.