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Cape Verde Volcanoes

  1. Aug 6, 2016 #1


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    Wired published an article on volcanic eruptions this week, including one in Cape Verde Islands
    It seems Brava has been considered a dormant stratovolcano.

    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.


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


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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

  4. Aug 10, 2016 #3


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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.

    Basanite–Phonolite Lineages of the Teide–Pico Viejo Volcanic Complex, Tenerife, Canary Islands

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

    Not very definitive.

    However, perhaps the key is toward the bottom of this page.
    Evolution of Alkaline Rock Suites
    Last edited: Aug 10, 2016
  5. Aug 11, 2016 #4


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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


    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 ....

    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:

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