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Andrew Alden

Granitic Seafloor Reported Off Brazil

By May 8, 2013

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It's exciting in a geeky way to learn that the deep-sea plateau called the Rio Grande Rise appears to contain granitic rocks. Dredging two years ago brought up samples, and this week the "video evidence" from a submersible dive to the Rise was cited as further proof. That doesn't seem like much evidence, and nothing has been published yet, and the video I've seen doesn't look at all granite-y, but I'll go with it. Granite is a big deal, because the Rio Grande Rise is supposed to be a hotspot track created by a "mantle plume," and mantle plumes are not supposed to create granites.

The Rio Grande Rise is a long welt of mostly volcanic rocks that extends from southern Brazil out to sea nearly all the way to the Mid-Atlantic Ridge at the island (and hotpot) of Tristan da Cunha. On the other side, a similar feature named Walvis Ridge extends to northern Namibia, in Africa. They aren't sexy places to study because they seem to be well explained for now. But lo and behold, a closer look has brought forth evidence of a noteworthy amount of granite out there in the South Atlantic.

What prompted this closer look? I surmise that it was money and shipboard time. Japan and Brazil arranged a joint mission, using Japan's Yokosuka research ship and its manned submersible Shinkai 6500, to survey the waters off Rio de Janeiro. On the way across the Atlantic the vessels took the opportunity to look at the Rio Grande Rise.

There are models for the Rio Grande Rise and Walvis Ridge that don't involve hotspots, explaining them instead as stretch marks in the South Atlantic crust made as the South America and Africa plates separated. Perhaps a chip of the South American continent got lost and sank with the ocean crust around it, like the Seychelles Islands in the Indian Ocean or Jan Mayen island in the North Atlantic. Maybe an amalgamated model that mixes mantle plumes and plate cracking will arise to explain the granite.

I would never have heard about this, except that the Brazilian-Japanese project decided to splash its news as the discovery of a "Brazilian Atlantis." If my understanding is correct, the media, as usual, have totally missed the point.

Background:
Hotspots
The non-hotspot Earth
Plate-tectonic divergence
Brazilian video from the Shinkai 6500 dive
About granite
The Portuguese Atlantis of 2005

Comments

May 13, 2013 at 5:38 am
(1) R.K. Stevens says:

Don’t forget Trondhjemite, a granitic rock found as a minor but common
part of the ophiolite suite and probably of oceanic crust.

May 13, 2013 at 1:16 pm
(2) jim meyer says:

It seems best to wait and see here-it might be a missing link-or not. The Atlantis link can be developed if somehow landmass can sink which is a better option than anything else if the story holds up.

August 7, 2013 at 2:00 pm
(3) nonbeliever says:

Maybe someone needs more claim for exploration licenses. If this is a piece of the south american continent, Brazil can extent their marine economy zone and drill for oil.

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