
A map of the former surface of the Earth, now residing in the mantle - zeristor
https://arstechnica.co.uk/science/2017/10/scientists-release-an-atlas-of-the-underworld/
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zeristor
A link to the mentioned Atlas of the Underworld:

[http://www.atlas-of-the-underworld.org/depth-top/](http://www.atlas-of-the-
underworld.org/depth-top/)

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gus_massa
In particular, the image in the article is from [http://www.atlas-of-the-
underworld.org/Cocos/](http://www.atlas-of-the-underworld.org/Cocos/)

The site is designed for specialist, and current version is too technical. I
looked at it for a few minutes and read the explanations, but I'm not sure
about the interpretation.

It would be nice to have an informal blog post with a layman explanation of
each image and what is supposed to show each one. At least, a image edited
with paintbrush to add a few ellipses to mark the important parts and arrows
with a keyword. Do we have a geologist here?

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cardiffspaceman
As a nontechnical person I wonder what they need to say that they didn't say.

We have places around the world where one plate is diving under another plate.
Subduction zones. The extrapolation is that the diving plate persists in some
form as it dives into the mantle, and can be identified with the position it
once occupied on the surface. These pieces of crust are cooler than the mantle
they are immersed in. One reason we can track them is they propagate seismic
waves at a faster velocity than the surrounding mantle. I'm not a geologist
and I welcome feedback on my interpretation of the abstract and other
materials.

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mirimir
Do we know how much of slab movement in the mantle just reflects mantle
convection, and how much also reflects slabs' structural integrity? Near the
surface, I understand that slabs are load bearing. But below a few hundred km,
aren't they melting? Or does the fact that this technique detects them imply
that they haven't melted fully yet?

~~~
cardiffspaceman
As a layman I don't know what consensus geologists have about your questions.
But I think the answer to the last of your questions would be yes. The
colder/solid slab is showing up in data gathered about seismic wave
propagation speed.

I have the model in mind of an ice cube in water, except with higher viscosity
and of course more heat. There would be a penumbra of cold water surrounding
the melting cube until a point where there would just be cool water, vaguely
cube-shaped. Plus the motion would smear the penumbra but not the remaining
solid. Also my guess is there aren't a lot of precise measurements, so the
usual blur from that. Like an ice cube in water, if you test the solidity of
the cube you find that the cube hasn't softened, just shrunk.

I looked at a few of the plots but only the Cocos plot clearly suggested a
'slab' to my eyes, and probably for the wrong reason: The plot looks like it
is divided in half by the blue area. Looking more carefully and reading, I
think what I'm seeing is two depths of the same object. The plate subducting
under the West coast of the Americas is apparently appearing as an anomaly or
slab at two depths.

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mirimir
Thanks. I gather that at least some volatiles bake out of subducting crust
during the first 100-200 km. That's what drives subduction zone volcanism. But
maybe that increases the melting range for what's left. I have no clue.

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drtillberg
It's not clear from the article, but the site makes clear that the 'slabs' are
the parts of the map colored in blue.[1]

[1] [http://www.atlas-of-the-underworld.org/Atlantis/](http://www.atlas-of-
the-underworld.org/Atlantis/)

