Cape Verde Volcanoes: Recent Eruptions & Alerts

[Astronuc]

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/

Earthquakes on Brava in the Cape Verde Islands noticed on August 1 have prompted a heightened alert and evacuations of over 300 people from the slopes of the volcano. Brava has no known eruptions in the past 10,000 years, although many of the cones and flows on the volcano look relatively young.

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.

Only two years ago, Fogo in the Cape Verde Islands had one of its most specular eruptions in decades, with lava flows that overran multiple villages.

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/Earth's...rocklibrary/viewglossrecord.php?Term=basanite
https://wwwf.imperial.ac.uk/Earth's...ocklibrary/viewglossrecord.php?Term=phonolite

Apparently, the Canary Islands also have basinitic/phonolitic deposits.

 

[davenn]

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.

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

 

[Astronuc]

That's interesting ... how is there phonolite ?

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%20lava.html

One way to classify lavas is by their alkali content, reflected in their weight percent of Na2O + K2O. In contrast to the common lava types (basalt, andesite, dacite, and rhyolite), there exists less common lavas that define mildly alkaline trends (e.g., with increasing silica content: alkali basalt, trachybasalt, trachyandesite, trachyte, and comendite), and strongly alkaline trends (e.g., with increasing silica content: tephrite, phonotephrite, tephriphonolite, and phonolite).

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.

Cape Verde is a volcanic archipelago situated above an oceanic rise that puts the base of the islands 2 kilometers (1.2 mi) above the rest of the seafloor. Cape Verde has been identified as a hotspot and it has been argued that a mantle plume might be underneath it causing the volcanic activity and associated geothermal anomalies.

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

Note how a slightly silica-saturated basalt will evolve through hawaiites, mugearites, benmoreites and trachytes that will eventually continue to produce rhyolites. While a slightly silica-undersaturated composition will follow a similar path, but eventually produce phonolites with decreasing SiO₂.

Note how small amounts of crustal contamination of silica-undersaturated basalts could also cause these silica-undersaturated magmas to become silica-saturated, and result in the bifurcation of the trends.

Thus, upwelling of the mantle beneath the continental rift zones likely results in various degrees of melting of the mantle by decompression melting. The presence of continental crust, favors small amounts of contamination of these already alkali rich magmas resulting in the production of basalt-trachyte-rhyolite suites. Basaltic magmas that reach low pressure and are still silica-undersaturated results in basalt-trachyte-phonolite suites. Only small changes in composition of the original mantle-derived magmas are necessary to produce these diverging magma types.

 

[davenn]

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

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

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

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

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.

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

Dave