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Wednesday, December 14, 2016

Thinner Crust: Not Just for Pizza Anymore -- Oceanic Crust has Thinned Since Pangaea Jurassic Time

      Oceanic crust created by the earth today is significantly thinner than crust made 170 million years ago during the time of supercontinent Pangaea, according to U. of Texas, Austin, scientists. The Earth's crust under the oceans is up to 1 mile thinner today than it was during that Pangaean time.



      "The thinning is related to the cooling of earth's interior prompted by the splitting of Pangaea, which broke up into the continents that we have today," said Dr. Harm Van Avendonk, the lead author of the study. The research published in Nature Geosciences on December 12, 2016, illuminates how plate tectonics has influenced the cooling of the Earth's mantle throughout geologic history.




      "What we think is happening is that the supercontinent was like an insulating blanket," Van Avendonk said. "So when these continents started opening up and the deeper mantle was exposed, more or less, to the atmosphere and the ocean it started cooling much faster."






      The mantle is the very hot, but mostly solid, layer of rock between the Earth's crust and core. Magma from the mantle forms oceanic crust when it rises from the mantle to the surface at spreading centers and cools into the rock that forms the very bottom of the seafloor. 



     Since about 2.5 billion years ago, the mantle has been cooling, a phenomenon that does not influence the climate on the surface of the Earth and has nothing to do with the issue of short-term human-made climate change (which is a real phenomenon, DT!). This study suggests that since the breakup of Pangaea, the cooling rate of the mantle has increased from 6 - 11 degrees Celsius per 100 million years to 15 - 20 degrees per 100 million years. Since cooler mantle temperatures generally produce less magma, it is a trend that's making modern day ocean crust thinner. The illustration below of a slice of earth ties in well with the pizza analogy ;-).



      "It's important to note the Earth seems to be cooling a lot faster now than it has been over its lifetime," Dr. Van Avendonk said. "The current state of the Earth, where we have a lot of plate tectonic events, this allows the Earth to cool much more efficiently than it did in the past."






      The research that led to the connection between the splitting of the supercontinent and crust thickness started when Dr. Van Avendock and Ph.D. student Jennifer Harding, a co-author, noticed an unexpected trend when studying existing data from young and old seafloor. They analyzed 234 measurements of crustal thickness from around the world and found that, on a global scale, the oldest ocean crust examined, Jurassic in age, is 1 mile thicker, as noted above. The oldest oceanic crust (or sima, short for silica and magnesium, mainly basalt) is Jurassic in age due to the "recycling" nature of this denser crust versus less dense continental crust (or sial, short for silica and aluminum).




     The link between crust thickness and age prompted two possible explanations, both related to the fact that hotter mantle tends to make more magma. (1) Mantle hot spots, highly volcanic regions, such as the Hawaiian Islands and Iceland, could have thickened the old crust by covering it in layers of lava at a later time. Or, (2) the mantle was hotter in the Jurassic than it is now.


      The analysis ruled out the hot spot theory; thick layers of old crust formed just as easily at distances greater than 600 miles from hotspots, a distance that the researchers judged was outside the influence of the hotspots. In contrast, the analysis supported the hypothesis of mantle cooling after the breakup of the supercontinent.

      The discovery that breaking up Pangaea cooled the mantle is important because it gives a more nuanced view of the mantle temperature that influences tectonics on earth. The researchers also note that the study illustrates the success that can come from spontaneous collaboration and leveraging basic research on a global scale.

Coolly and Warmly,
Steph








25 comments:

  1. Replies
    1. So, now we know where to find that fantastic beast.

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    2. Exactly. . .and then there's this passage in the link:

      'These spiders are large, but not all over. “Males have unusually small genitals,” reports National Geographic.'

      Showing great restraint. . .

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    3. Note that your smirking is about the Flying Ghost Spiders,and as the article notes they are in a Chile place....

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    4. eco, glad to see you here.

      Smirking, moi?

      Btw, the long and the short of it is that I had a certain New Yorker in mind, the one with the "beautiful flowing sentences."

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    5. I used to work with Mark Liberman, the linguist cited in that article, when we were at Bell Labs. And, just to show you how interconnected things are, that elderly friend of my inlaws I mentioned last year, who used to play duplicate bridge with my son on Martha's Vineyard? He gave Mark bar mitzvah lessons.

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  2. I ordered a slice of ocean-floor-thin-crust pizza the other day. I explicitly said "No anchovies!" But when I bit into it, the sauce above the thin crust was lousy with anchovies.

    LegoAddThatTheSliceWasAlsoMoreSaltyThanHeLikesIt(OneMightEvenSayMoreBriny)

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    Replies
    1. Lousy with anchovies, Lego? You do have such a way with words.

      Briny? Brainy? Salt of the earth, for sure.

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  3. So, does Chicago's position in the middle of the continent, where the Earth's crust is so much thicker, explain their weird, thick-crust, deep-dish pizza? Or their crusty, conservative freshwater economists, so often at odds with more liberal coastal saltwater economists?

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    1. jan, there must be a reason for that deep dish Chicago crust pizza. This is as good a reason as any!

      Had not heard the fresh water vs. salty water economics dichotomy before. It all comes down to fish, as it so often does; and now we're back to anchovies. . .

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  4. Speaking of continental crusts, when I hear "sial", the rocks I think of are salivary gland stones, not silica and aluminum. They can be pretty unpleasant, like mumps without the infection.

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    1. Never thought of stones in salivary glands. Have you seen them up close, jan? Interesting to note they are often idiopathic.

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    2. No. I've treated patients with sialadenitis, but if I suspect stones, I'll send them to an ENT.

      It's interesting how many different kinds of stones plague us besides these salivary gland stones: gallstones, kidney stones, fecaliths causing appendicitis, otoliths causing vertigo, not to mention calcification of arteries and tendons. Medical geology isn't a happy place.

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  5. 25 F, snow and freezing rain. No biking today. So, why did early humans ever leave nice, warm Africa 100,000 or so years ago, and set off for such inhospitable climes? This fascinating short video from the AMNH explains when and where, but not why. The population back then certainly wasn't high enough that they were crowded out. I'm not sure that human curiosity alone is enough to get me to leave comfortable surroundings and familiar food sources. Maybe the lack of zoonotic diseases outside their original range explains their success there, but it doesn't provide a motivation for making the trek in the first place.

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    1. Fascinating video, jan. I watched it twice. Seeing the population numbers go down during the Bubonic Plaque was chilling.

      Indeed, why did they explore elsewhere? "Because it was there" may have to do for an explanation.



      -3 degrees here and about 10 inches of new snow. We surprised a friend last night with a tree lighting ceremony on their 45' spruce. We all chipped in to have a company out to hang the lights while she was at a meeting for the day. Her brother read a beautiful poem about the hands and love that planted the tree and how it grew so much since 1992. A very strong love. Her husband is in a care facility with early onset Alzheimer's disease. We were weeping and laughing, tears freezing, drinking champagne to toast the lights and love. In the Peruvian tradition, we gave the spruce a drink of champagne, too.

      The snow just started to fall as we left our friends and the magical tree. . .

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    2. Nice pics, WW, and interesting video Jan. It's supposed to go down to 35 degrees here tonight, absolutely barbaric!

      Why did our ancestors explore? Personally I suspect some form of drought in Africa that decimated the food supply. A geologically short term drought, say 10 years, is pretty rough on the population.

      Some things in the video I question: some believe the population in the "Americas" was much higher than what they show, estimates range from 18 to 125 million, or as much as Europe at the time.

      It certainly takes a large, sophisticated and organized population to take on massive public works projects, like the temple compounds in Teotihuacan or Mexico City, the walls of Cusco, Chaco Canyon, etc. Or the earthen structures along the Mississippi and Ohio Rivers. Not to mention vast irrigation fields in the southwest, and trade that stretched from Peru to Arizona. And Democrats in the government to tax and spend all that money!

      I believe it was Cortez who described massive populations as he sailed up the Mississippi, a river lined with people in 1498 or thereabouts. Yet when he came back less than a decade later they were all gone. It's hard to know the structures of the eastern portions of this continent as they were likely using the ubiquitous wood, which doesn't leave much behind.

      The institutional museums have long underplayed the extent of habitation in this continent, and the role that the European migration, with communicable diseases, played in their demise.

      One other tidbit: some say there is evidence of anthropomorphic climate change going back 8-10,000 years, coinciding with the advent of agriculture, which releases carbon into the air. Maybe this has been debunked, but I also read that there was a significant drop in CO2 levels coinciding with the Bubonic Plague, ascribed to the farmers' fields laying fallow and reabsorbing the carbon. Maybe there is a solution.

      And why is it called the Black Plague? It mostly affected white Europeans! Please don't reply with comments about dark patches on the skin.

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    3. eco, lots to ponder here. An African drought makes sense. Ethiopia and other African countries have been going through a similar drought now.

      The drop in CO2 levels during the Bubonic Plague could also be the result of reforestation as those fields, not only lay fallow but were repopulated with trees.

      The possible large population you describe in the Americas is interesting. "Massive populations" along the Mississippi River in 1498? Mostly native peoples, presumably. What was "massive" to Cortez?

      The disappearance of the Ancient Puebloans (formerly called Anasazi, except by the U.S Forest Service, which still uses that term) from Mesa Verde, Chaco Canyon, and environs was likely also drought/environment related. Or, maybe it wasn't. . .

      I passed on the rhetorical questions.

      Much more to cogitate on. . .

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    4. Thanks WW for completing the thought about fields laying fallow; the reforestation that would happen fairly quickly (as we see after wildfires or Mount St Helens) would have absorbed carbon.

      Yes, Cortez did mean indigenous/ native peoples: there wasn't much European settlement in 1498, but I don't know that he took a head count. He and the explorers of Amazonia described the lands as filled with people. Trade and exploration brought many communicable diseases - influenzas, smallpox, yellow fever, etc. - which were no fun for Europeans but absolutely devastating to the native populations with no previous exposure and hence no resistance. Early East Coast explorers described the forests as a park land through which one could march armies; much different than the brambles that one sees even in mature woods. Best explanation is controlled burning to make it easier to hunt in the days before Roundup.

      Can't say I like the term Puebloans, uses the Spanish term to describe people who surely had their own name. But the evidence I've read does point to SW drought as the demise of those civilizations.

      Climate aside: there is also a theory that the Amazon rainforest was a human-made structure, +/- 13,000 years ago.

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    5. I rather liked Anazasi. It had pizzazz.

      So much interesting research out there about the Amazon Rainforest .

      Also, if you type relentless.com into your browser you are redirected to Amazon.com. It was Bezos's original name for the corporation. Weren't we just talking about being relentless?

      Go ahead, try it. You know you want to. . .

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  6. Replies
    1. Thanks, Paul. And this is silly Putty,too. . (Need a better adjective than silly, though.)

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  7. New post on "Western Pacific Biotwang: Whale Noises in Deepest Mariana Trench" is now up. Enjoy!

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