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SOUND ON🔊 Louie Brings Plenty sends a powerful prayer out over the badlands towards the Stronghold on the #PineRidgeIndianReservation , South Dakota. Stronghold Table was the site of the last Ghost Dances before the Wounded Knee Massacre in 1890, and a protected sanctuary for the Lakota in the battles that followed.
Volcano sheds a tear.
Many of the world's volcanoes are too remote for geologists to install monitoring networks, particularly when no humans are at risk to justify the expense. Vulcanologists wishing to track activity at such peaks therefore rely on satellite monitoring to keep an eye on events, with occasional visits when possible. As part of this process, NASA's Earth Observing satellite caught Mawson peak on faraway Heard Island shedding a lava tear onto its glacial blanket. The eruption deposited the small black streak on the south western side.
These images were taken as follow up, after infrared satellite monitors showed signs of activity. The 2750 metre peak is named after the famous Australian geologist and Antarctic explorer Douglas Mawson. It is one of three volcanoes on Heard island in the southern Indian Ocean, about 2000 Km north of Antarctica and 2200 Km southeast of Africa. It is one of Australia's most remote territories, highest non-Antarctic mountain, and one of its only two currently active volcanoes, having erupted regularly since 1985.
Mawson Peak is a 20 Km wide resurgent stratovolcanic cone sitting on the edge of the larger Big Ben caldera (a collapse feature resulting from magma chamber roofs falling in as they empty in an eruption). Heard Island is the exposed part of the caldera, jutting out of the Kerguelen Plateau. It lies deep in the prevailing westerlies of the furious fifties that circle Antarctica, and sees cold temperatures and sea level snow throughout the year. Fourteen glaciers radiate out from its vent area. An air force base was maintained there in the 1950's, but is now long abandoned, since maintaining it was very expensive. It is so remote and harsh that only a few expeditions have explored it since its discovery in 1853, the last scientific one being in the summer of 2003-4.
The plateau is a micro-continental strip left behind when Gondwana rifted apart. Rising 3700 metres above the abyssal plains, it was overlain between 115 and 110 million years ago by the eruption of a large igneous province during the rifting of India and Australia. Intense volcanism was interspersed with longer periods of quiet. Soil layers with charcoal, fossils and fragments of gneiss indicate the plateau was above the waves for three extended periods since the eruptions started, finally subsiding under the waves about twenty million years ago. Its sedimentary rocks are similar to parts of India and Australia, implying it sat somewhere between them until they split, when it became a tectonic ribbon covered in kilometres of lavas from a hotspot under the Indian Ocean. It is the second largest oceanic plateau on Earth.
Monitoring global volcanism in this manner allows us to build up a more representative sample by including remote areas, where humans won't notice (or report on) any effects of an eruption. This helps us grow our understanding of plate tectonics and volcanism as a process affecting the entire planet.
Loz
Image credit: NASA.
Lots of subducted nitrogen
There is an active, ongoing debate over how thick Earth’s early atmosphere must have been. When the planet formed, the sun only put out about 80% of its current energy, and to keep the planet Earth from freezing it had to have an even stronger greenhouse effect than our current atmosphere. One way to make this happen is to have a higher atmospheric pressure – gases like CO2 and methane are even more effective greenhouse gases when the total pressure of the atmosphere (driven largely by the gas nitrogen) is higher.
However, only a couple weeks ago we described new results pointing exactly the opposite way. Arguments based on the size of vesicles in lava flows (https://tmblr.co/Zyv2Js26Ipk4j) built on previous arguments based on the size of raindrops to argue that Earth’s early atmosphere was actually thinner than today’s. Those results would require that the planet Earth has outgassed a huge amount of nitrogen since the Achaean, enough to double the mass of our atmosphere.
The last thing you’d expect to see two weeks after writing about that study is…of course…a paper arguing for subduction of a lot of nitrogen throughout Earth’s history. Subducting nitrogen would take it out of Earth’s atmosphere and thin the atmosphere over time, the exact opposite of what the previous study would require. That is, of course, what the study I’m writing about found – rather than outgassing nitrogen, new results from volcanic rocks in the Indian Ocean require subduction of nitrogen over geologic time.
Scientists Peter Barry from Oxford University and David Hinton from Scripps Oceanographic Institute collected samples from the Indian Ocean Ridge, a standard oceanic spreading center in the Indian Ocean, and from nearby Reunion Island, a volcanic island thought to represent a lower mantle plume (its major volcano, Piton de La Fournaise, is shown in this photo).
Mid-Ocean ridges form igneous rocks by allowing whatever rocks are directly below them to melt. Typically they sample the average upper mantle just because that’s what is below them. On the other hand, a plume like Reunion must sample something else – something that is hotter, distinct in chemistry, and stored somewhere in the lower mantle.
They started measuring isotopes of the element nitrogen in those igneous rocks. If nitrogen is coming out of the mantle, erupted lavas will store a bit of that nitrogen, and isotopes of nitrogen could be used to tell where it came from.
Nitrogen has 2 stable isotopes; a form with a mass 14 and a heavier isotope with mass 15. Nitrogen that comes out of the mantle has less nitrogen 15 than the atmosphere, so it is “lighter” than the atmosphere. On the other hand, heavy nitrogen preferentially gets stuck in sediments that go down subduction zones, so a subducted component will be “heavier” than the atmosphere (it will have a higher ratio of 15N to 14N).
When they analyzed nitrogen isotopes in those rocks, they found that the ocean ridges carried the normal upper mantle nitrogen isotope signature, while the island lavas were much heavier. The Reunion island basalts represent melts of material in the mantle that was subducted long ago and is now coming back to shallow levels and melting today.
But not only is this a subducted component, it represents a very large component. Based on the amount of nitrogen trapped in those rocks, and the ratios of nitrogen to other gases (like neon) they also could say that there was a very large amount of nitrogen going down through subduction zones. For these rocks to have this quantity of nitrogen with that isotopic ratio, the subducted nitrogen component must Dominate every other component in the mantle. That means a lot of nitrogen must be going down, not coming out!
The scientists calculated how much nitrogen subduction their results would require that Earth’s atmosphere was about 33% thinner than it was when the planet formed. The rest of the nitrogen would have to have gone down subduction zones, where it gradually builds up in the mantle and occasionally comes back out.
This result directly contradicts the result I wrote about two weeks ago, which suggested that Earth’s atmosphere was thickening over time as the mantle degassed. It seems like we can’t explain the chemistry of erupting rocks today without a lot of nitrogen going down into the mantle, but we can’t explain physical properties of ancient rocks without a lot of nitrogen coming out and very little going down.
Perhaps there’s one way around this conundrum. The authors proposing the thin, ancient atmosphere needed a mechanism that could keep the ancient atmosphere thin and they proposed subduction before the presence of oxygen might do that. If the nitrogen coming out of Reunion Island was subducted 3 billion years or so, then that could explain how the mantle got a lot of nitrogen put in it while still having a thin early atmosphere. However, at the same time, estimates today suggest that more nitrogen is being subducted into the mantle than we see erupted at the surface, so there’s still a major mass balance issue.
The data in these studies simply disagree. One suggests a lot of nitrogen going down, one suggests a lot of nitrogen going up. Figuring out how to make these results fit together in a way that makes sense will provide a fundamental new insight into how the planet we see today developed.
-JBB
Image credit: https://flic.kr/p/rwVbN1 Reference/original paper: http://bit.ly/1WnlhCO http://bit.ly/1WnllT4 http://bit.ly/1Ruo4lI
HIDDEN MICRO-CONTINENT FOUND IN THE INDIAN OCEAN
Beneath the islands of Reunion and Mauritius lies a hitherto undiscovered microcontinent. The continental fragment Mauritia is believed to have detached about 60 million years ago, while Madagascar and India were drifting apart. The fragment was hidden under huge masses of lava.
Continental break-up is usually associated with mantle plumes, which is where giant blobs of hot rock rise from the mantle, intruding tectonic plates until the plates break apart at the hot spots. The ancient supercontinent Gondwana began to break apart in this fashion about 184 Mya (million years ago). This break up was accompanied by massive eruptions of basalt lava, when East Gondwana (Antarctica, Madagascar, India and Australia), started to separate from Africa. About 130 Mya South America started drifting westward from Africa and the South Atlantic Ocean; this resulted in open marine conditions by 110 Mya. About 120 Mya East Gondwana began to separate, as India started moving northward.
Mantle plumes currently underneath the islands of Marion and Reunion may well have played a role in the formation of the Indian Ocean. If the zone of the rupture is situated at the edge of a landmass, then fragments of the land may separate off; the Seychelles are a prime example of this.
A team of geoscientists from Norway, South Africa, Britain and Germany studied lava sand grains from the Mauritius beach. Their study suggested there were more continental fragments. The sand grains contained zircons aged between 660 and 1970 million years; the lava carried these zircons when it pushed through subjacent continental crust of the same age.
This dating method was supplemented by recalculating the hotspot trail. This showed the position of the plates relative to the two hotspots at the time of rupture, and also showed that the continent fragments continued to wander over the Reunion plume; this explained how they were covered with volcanic rock. What had previously been thought of as the trail of the Reunion hotspot turned out to be continent fragments. This research suggests micro-continents occur ore frequently than previously realised.
The coloured track (left colour scale) west of Reunion in the image is the calculated movement of the Reunion hotspot. The black lines with yellow circles and the red circle indicate the corresponding calculated track on the African plate and the Indian plate, respectively. The numbers in the circles are ages in millions of years. The areas with topography just below the sea surface are now regarded as continental fragments.
-TEL
http://www.gfz-potsdam.de/portal/gfz/Public+Relations/Pressemitteilungen/aktuell/130224_Mauritia;jsessionid=5025B9C7341A54D4B5DB8C3C2714C008?template=gfz Trond H. Torsvik, Hans Amundsen, Ebbe H. Hartz, Fernando Corfu, Nick Kusznir, Carmen Gaina, Pavel V. Doubrovine, Bernhard Steinberger, Lewis D. Ashwal, Bjørn Jamtveit. A Precambrian microcontinent in the Indian Ocean. Nature Geoscience, 2013; DOI:10.1038/NGEO1736
Image: © GFZ/Steinberger
