Scientists study Earth's missing crust

[Evo]

Interesting article.

"SANTA CRUZ DE TENERIFE, Canary Islands - British scientists have embarked on a mission to study a huge area on the Atlantic seabed where the Earth's crust is mysteriously missing and instead is covered with dark green rock from deep inside the planet.

The 12-member expedition to take an unprecedented peek at Earth's mantle left the Canary Islands on Monday with a new high-tech vessel and a robotic device named Toby that will dig up rock samples at the site and film what it sees.

The main site — there is at least one other in roughly the same area and a third is suspected — is about three miles below the surface of the Atlantic and located about 2,000 nautical miles southwest of the Canaries.

It is part of a globe-spanning ridge of undersea volcanos, the kind of structure that forms when Atlantic tectonic plates separate and lava surges upward to fill the gap in the Earth's crust.

But that apparently did not happen this time. Where there should be a four-mile-thick layer of crust, there is instead that much mantle — the very dense, dark green rock that makes up the deep inner layer of the Earth.

Scientists have seen chunks of mantle that have been spewed up with lava, but never such a large, exposed stretch."

continued...

http://news.yahoo.com/s/ap/20070306/ap_on_sc/seabed_expedition

 

[Astronuc]

More information here - http://www.noc.soton.ac.uk/gg/classroom@sea/JC007/about.html - Drilling the Mid-Atlantic Ridge, RRS James Cook cruise JC007, 5 March 2007 – 17 April 2007

Scientists have discovered a large area thousands of square kilometres in extent in the middle of the Atlantic Ocean where the Earth's crust seems to be missing entirely. Instead, the mantle - the deep interior of the Earth, normally covered by crust many kilometres thick - is exposed on the seafloor, 3000m below the surface. It has been described as being like an open wound on the surface of the Earth. What scientists don't know is whether the ocean crust was first developed, and then ripped away by huge geological faults, or whether it never even developed in the first place.

In March-April 2007, a team of scientists from Durham University, Cardiff University and NOCS will board the RRS James Cook to visit this special area of the Mid-Atlantic Ridge, which is called the Fifteen-Twenty Fracture Zone (FTFZ for short - a map shows where this is located - see link below).

http://www.noc.soton.ac.uk/gg/classroom@sea/JC007/background.html

 

[billiards]

Yes this is an interesting story, the question about whether the ocean crust formed first and then ripped away, or whether it was never developed there in the first place is interesting.
I would think intuitively that here is an area that has stopped producing any crust but is still being torn apart by the tectonic forces. The moho has been pushed upwards by the (ambiently?) rising asthenosphere beneath it, however there must be some kind of chemical anomaly with the upper mantle here because it has decided not to melt under the reduced pressure. Clearly the whole region is faulted, it's a plate boundary for crying out loud! and clearly no new melt has been emplaced here. Perhaps I haven't grasped the point in this question but if this mantle material wasn't fault emplaced then wouldn't it have to have formed at the surface? To my knowledge that is simply not possible, they're not sedimentary rocks are they? And for them to be exposed to the surface, they cannot be covered by new ocean crust, so it seems clear to me that the two mechanism are not mutually exclusive and this feature exists by a combination of them.

 

[matthyaouw]

I've been doing a bit of detective work here... The rock exposed here is serpentinite, which is a metamorphic rock. The parent material of serpentinite is peridotite, which is the residue left over when partial melting has occurred, so from this I'm assuming there was once basalt etc on top of the section which has since been removed. If the rock is old enough, it could have cooled sufficiently so that decompression melting did not occur. Peridotite is denser than serpentinite: 3.3g/cm3 compared to 2.7 g/cm3 so metamorphosis would cause some adiabatic uplift, going some way to explain why the rock is exposed. It also expands by 40% upon metamorphosis- I wonder if this would lead to faulting & fracturing of any crust that was above it. It seems the timing of the metamorphosis is going to be relevant here.

 

[billiards]

I've been doing a bit of detective work here... The rock exposed here is serpentinite, which is a metamorphic rock. The parent material of serpentinite is peridotite, which is the residue left over when partial melting has occurred, so from this I'm assuming there was once basalt etc on top of the section which has since been removed. If the rock is old enough, it could have cooled sufficiently so that decompression melting did not occur. Peridotite is denser than serpentinite: 3.3g/cm3 compared to 2.7 g/cm3 so metamorphosis would cause some adiabatic uplift, going some way to explain why the rock is exposed. It also expands by 40% upon metamorphosis- I wonder if this would lead to faulting & fracturing of any crust that was above it. It seems the timing of the metamorphosis is going to be relevant here.

Yes but you need to explain how the peridotite got serpentinized in the first place, I guess it's a kind of chicken and egg situation.

 

[matthyaouw]

True... I'm not too sure about that. Perhaps the study will shed some light on it.

 

[billiards]

What kind of tectonic regime is there around there? Is there a possibility this might be some kind of extensionally exposed metamorphic core complex? (c.f. Wernicke model)

EDIT: Although that would have to be on continental crust wouldn't it?