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Cape Verde Volcanoes

  1. Aug 6, 2016 #1

    Astronuc

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    Wired published an article on volcanic eruptions this week, including one in Cape Verde Islands
    http://www.wired.com/2016/08/volcanic-eruptions-indonesia-interfering-flights/
    It seems Brava has been considered a dormant stratovolcano.
    http://volcano.si.edu/volcano.cfm?vn=384020

    The article mentions phonolite eruptions, which I take to mean phonolite deposits from previous eruptions. Phonolite has a high silica content that can produce explosive eruptions. As I recall, the Cascade volcanoes, like Mt. St. Helens have high silica content, which makes for explosive eruptions.

    Fogo - http://volcano.si.edu/volcano.cfm?vn=384010

    Fogo had activity in February 2015.


    Basanites are most commonly associated with continental rift and ocean island magmatism. They are sometimes parental magmas to a crystal fractionation series that leads to phonolites.

    https://wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/viewglossrecord.php?Term=basanite
    https://wwwf.imperial.ac.uk/earthsc...ocklibrary/viewglossrecord.php?Term=phonolite

    Apparently, the Canary Islands also have basinitic/phonolitic deposits.
     
    Last edited: Aug 6, 2016
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  3. Aug 10, 2016 #2

    davenn

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    That's interesting .... how is there phonolite ? I thought that was produced magma coming up through continental crust ?
    where is the continental crustal material coming from in the middle of basaltic ( silica poor) oceanic crust ?

    The Dunedin Volcano that I referred to in a relatively recent thread, is well known for it's phonolite volcanic pipes and general deposits



    Dave
     
  4. Aug 10, 2016 #3

    Astronuc

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    Excellent question - that requires some research.

    I'm curious myself, especially after finding the following from a Wikipedia article on Phonolite (https://en.wikipedia.org/wiki/Phonolite):

    "Phonolite is an uncommon extrusive igneous rock, volcanic rock, of intermediate chemical composition between felsic and mafic, with texture ranging from aphanitic (fine-grain) to porphyritic (mixed fine- and coarse-grain)." OK

    But, "Unusually, phonolite forms from magma with a relatively low silica content, generated by low degrees of partial melting (less than 10%) of highly aluminous rocks of the lower crust such as tonalite, monzonite and metamorphic rocks. Melting of such rocks to a very low degree promotes the liberation of aluminium, potassium, sodium and calcium via melting of feldspar, with some involvement of mafic minerals."

    So perhaps there is thermochemically-induced segregation or partitioning.

    However from http://www.geology.sdsu.edu/how_volcanoes_work/Unusual lava.html

    Phonolite apparently is a high silica, strongly alkaline lava.

    The Canary Islands are 7 large volcanic islands that have developed just off the African continental shelf. I'm guessing similar for Cape Verde Islands.
    http://www.geo.arizona.edu/~andyf/LaPalma/orient.html

    Basanite–Phonolite Lineages of the Teide–Pico Viejo Volcanic Complex, Tenerife, Canary Islands
    http://petrology.oxfordjournals.org/content/39/5/905.full

    There does not seem to be a lot of detail on the geology of Cape Verde Islands.

    https://en.wikipedia.org/wiki/Geology_of_Cape_Verde
    Not very definitive.

    However, perhaps the key is toward the bottom of this page.
    http://www.tulane.edu/~sanelson/eens212/cont_lithosphere.htm
    Evolution of Alkaline Rock Suites
     
    Last edited: Aug 10, 2016
  5. Aug 11, 2016 #4

    davenn

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    thanks for all the links, will have to do some reading and see if it sheds any light

    but as you say in the above, it may be a little difficult to come to some reasonable conclusions

    upload_2016-8-11_16-3-10.png

    The Canary Isl are much closer to the shelf edge than the Cape Verde Isl. but maybe there's a possibility that the base parts of both
    island chains are broken off and sunken remains of the continent/continental shelf from when South America and Africa rifted apart ?

    Both these sets of islands, and also Madeira to the north, are well away from the active volcanism of the Mid-Atlantic Ridge so I would suggest there is possibly hotspot volcanism at play here. With that in mind I googled ....
    "are the Canary Isl and Cape Verde Isl. volcanics caused by hot spots ?"

    and one link suggest that it is a possibility ....
    https://www.deepdyve.com/lp/elsevier/hot-spot-activity-and-the-break-up-of-pangea-0HN1epj4je

    so how about the possibility of hotspot volcanism intruding into continental remains of the breakup of SA. and AFR. during rifting ?
    just putting it out there as a plausible suggestion :smile:

    Dave
     
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