Xenoliths From the Mantle- Little Green Rocks From Deep Inside the Earth

Posted by Kenneth W W Sims of Dept of Geology and Geophysics, University of Wyoming in Explorers Journal on November 12, 2012
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Written by Kenneth W W Sims

We are familiar with the geology we see on the surface of the Earth—the granite cliffs of Yosemite, the volcanoes of Hawaii, Yellowstone and Mount Saint Helens, or the red sandstone spires in the deserts of the American southwest. But what does deep inside of the Earth look like?  As geologists we have many different methods to discern the composition and structure of the Earth’s mantle indirectly and sometimes we are lucky enough to find samples of the mantle transported up to the surface by volcanic lava flows.

Ken Sims looking at mantle xenoliths near the summit of Mount Terror with the East Ridge in the background. This East Ridge is where we have been sampling for the past two weeks. Photograph by Erin Phillips Writer

Plate 2: Picture of mantle xenoliths in outcrop at a lava flow on the East Ridge of Mount Terror. Photograph by Paul Wallace

One of the cool things about working in remote settings like Antarctica, where there are locations no geologist has ever before explored, is that you get to make exciting discoveries. One of those discoveries for us has been finding lots of “mantle xenoliths” along the East Ridge of Mt Terror (Plate 1 and 2).  These mantle xenoliths (which means foreign rock in Greek) are made up of beautiful green olivine and black pyroxene crystals (Plate 3). Mantle xenoliths represent the deep mantle below the crust (~20- 100 kms deep) that is being melted to form the magmas of the Ross Island volcanoes.  We find them in cinder cones and sometimes in the lava flows; the green xenoliths are round nodules that stand out in stark contrast to the black and dark gray lava.

Picture of mantle xenolith in the laboratory. Green crystals are olivine and black crystals are pyroxene. Photograph by Paul Wallace

Next we are off to camp at Cape Bird to start sampling along the coast and on the summit of Mt Bird.  This should be another exciting part of the trip, as once again there are very few samples that have been collected from Mt Bird and we will also be camping next to a penguin rookery.

Woods Hole Oceanographic Institution scientist Glenn Gaetani in the ice cave in the Erebus Glacier Ice Tongue. Photograph by Ken Sims

Some other good news is that Glenn Gaetani, who was delayed in Woods Hole on account of Hurricane Sandy, has made it to Antarctica. Our team is now complete. So yesterday to get Glenn oriented and ready to sample we did a snowmobile tour on the sea ice along Hut Point Peninsula , which included sampling at Turtle Rock and an essential side trip to the nearby ice cave on the Erebus Glacier tongue (Plate 5). Glenn will be writing the next blog from Cape Bird – so stay tuned.

Hexagonal ice crystal in the Erebus Glacier Ice Tongue. Note hand for scale. Photograph by Alasdair Turner

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  2. MARIA HELENA
    Brasília.
    December 1, 2012, 12:27 pm

    Maravilhosa foto dos Cristais!
    fantástica!
    Parabéns!.

  3. juan antonio rodriguez martin
    gijon
    November 24, 2012, 5:38 am

    en que zona son las fotos..muy bonitas…

  4. Kenneth W W Sims
    November 21, 2012, 5:29 am

    All,

    Thanks for your questions and comments. They came in while we were away and since we have been back I have been flying and working long hard days including a few night flights.

    Today was a “bad weather day” and so we didn’t sample and I was able to catch up with preparing samples to ship back to the US, responding to work emails (it follows you everywhere these days), writing these responses and even getting some desperately overdue laundry washed.

    Anyway, below are my responses to everyone’s questions and comments.

    All the best,
    Ken
    —————————————–
    Vimala Jothi. F

    Thank you for your positive feedback. These caves and their crystals are truly awesome. I have been going into ice caves down here in Antarctica since 1989 and I am still amazed and always will be!
    ——————————————————-
    Ashley and Peggy’s Class –

    Regarding the Penguin picture, I hope you saw our blog written by Glenn Gaetani after we came back from our trip to Cape Bird. Lots of penguin pictures there.

    Great question regarding ice crystals! I have been sitting here in our office in McMurdo station with two other scientists (Don Voigt and Glenn Gaetani) debating/dicussing this for a long time tonight. The crystals are indeed large. Our answer is that they are like humongous snowflakes. They grow from vapor and they are flat and they like to grow on the edge. But because they have a longer time to grow they get very large.

    ———————————————————-
    Ansel and the Visser Family.

    Good to hear from you.

    Our inability to sample the lower cliffs of Mt Bird along Lewis Bay was a big disappointment and will leave a hole in our dataset as these are likely the oldest rocks of Mt Bird.

    They ship the fuel down in ships at the end of the season as the sea ice moves out. So we are using fuel that they brought down last year. Good questions.
    —————————————————————–

    Kai- Laramie Montessori School
    Great question. There are two types down here- Adele Penguins and Emperor Penguins.
    ———————————————-

    Christian, Laramie Montessori School-

    I answered this very astute question before in my response to Ashley and Peggy’s Class. If this was not what you are looking for please tell me and I will try to be more explicit.

    ———————————————————–
    Rielee-Laramie Montessori School

    Good question. Xenoliths are generally round and are covered by lava. This one was broken naturally.

  5. Kenneth W W Sims
    November 21, 2012, 5:20 am

    Dear LHS geology students,
    Thank you for the great questions.

    1. If Mount Erebus erupted, how powerful would it be (Megan, Nikka), what kind of damage would occur (Kaylee), and how would it affect the animals (and people) living in Antarctica (Brandee)?

    Megan and Nikka,
    There is evidence of previous calderas on Mt. Erebus, which means that there were large eruptions in the past. However, the current eruptive phase of Mt. Erebus involves much smaller eruptions. In 1984 there was a period of increased Strombolian eruptive activity on Mt. Erebus that ejected car-sized bombs of molten lava out of the crater, and in 1993 there was a phreatic eruption that Ken witnessed.

    Kaylee and Brandee,
    If a person happened to be on the crater rim when a period of increased Strombolian activity started, he or she would definitely be in danger. There is also a hut where scientists stay near the top of Mt. Erebus that could potentially be damaged by an eruption. An eruption large enough to damage McMurdo station, which is approximately 40 km to the south, is unlikely and has not been documented in historical times. Keep in mind that volcanoes are volatile and that there is always a chance that the eruptive regime at Mt. Erebus could change. An eruption of Mt. Erebus could also have an effect on air travel in this part of the globe. Seals, penguins, and whales live at the water’s edge or in the ocean, so it is unlikely that an eruption would have a significant direct effect on these animals. But this is part of the reason we study volcanoes.

    2. How many rocks will you collect from each site (Aundrea), and are you looking for any specific rocks (Nathan)?

    Aundrea,
    It depends on what kinds of rocks we find at a particular sampling site, but we typically take two to four samples from a site. We usually split up into groups of two, so we are collecting lots of samples. We have over 90 samples at the moment and still have more to collect!

    Nathan,
    For the work that Ken and I will be doing, which is to examine mantle sources for Ross Island volcanic rocks using isotopes, the most desirable rocks are primitive rocks like basanites. These provide the best information about mantle sources because they have not sat in a magma chamber and differentiated into more evolved rocks.

    3. If you throw water into the air, will it freeze instantly (Andrew, Kolton, Justin, and Nathan)?

    Andrew, Kolton, Justin, and Nathan,
    Today, with temperatures around 25°F, water thrown into the air would not have frozen instantly. The phenomenon of water freezing when it is thrown into the air is temperature dependent. You live in the perfect place to test this! Once winter arrives in Wyoming, I suggest trying this experiment at different outdoor temperatures and different water temperatures and figuring out the conditions at which water freezes instantly.

    Sincerely,
    Erin Phillips Writer

  6. Vimala Jothi. F
    India
    November 18, 2012, 9:46 am

    I think I ve seen hexagonal ice crystals only in some drawings or some cartoons…. This is the first time i m seeing it for real….. Thanx Nat Geo team… Cheers for your further discoveries…..

  7. Ashley and Peggy's Class Laramie Montessori
    Laramie, Wy
    November 14, 2012, 1:21 pm

    Could you take pictures of the penguins while you are camping? We would love to see this. We have one more question: Why are the ice crystals so large?

    Thanks,

    Ashley & Peggy’s Class

  8. ansel visser
    laramie, wy
    November 14, 2012, 1:16 pm

    Hello all,

    In a previous blog post you mentioned that the sea ice in Lewis Bay was too thin and unsafe for landing the helicopter. It appears that there are five proposed sampling sites along the bay, do the ice conditions make it impossible to sample at all five sites? If so, how does the inability to sample there affect your research?

    We are curious about where you get the helicopter fuel?

  9. Kai-Laramie Montessori School
    November 13, 2012, 4:57 pm

    How were the ice crystals formed?

  10. Christian-Laramie Montessori School
    November 13, 2012, 4:54 pm

    What kind of penguins have you/do you think you will see?

  11. Rielee-Laramie Montessori School
    Laramie Montessori
    November 13, 2012, 4:52 pm

    Did you have to break the mantle xenolith to make it look like that, because it looks like the crystals are only in the middle, or was it just like that when you found it?

  12. LHS Geology Students
    Laramie High School
    November 13, 2012, 3:23 pm

    Here are some more questions!
    1. If Mount Erebus erupted, how powerful would it be (Megan, Nikka), what kind of damage would occur (Kaylee), and how would it affect the animals (and people) living in Antarctica (Brandee)?

    2. How many rocks will you collect from each site (Aundrea), and are you looking for any specific rocks (Nathan)?

    3. If you throw water into the air, will it freeze instantly (Andrew, Kolton, Justin, and Nathan)?

    Thanks :)