Iceland earthquakes, 18000 in a week! Fagradalsfjall Volcano

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  • #76
Astronuc
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The microscopic analysis shows mainly plagioclase, clino- (~ MgCaSi2O6) and orthopyroxenes (~Mg2Si2O6) where Mg may be substituted by iron, and magnetite (Fe3O4), but no quartz or sanidine (K-feldspar).
So, bascially, a dark, fine-grained basalt composed largely of plagioclase with pyroxene and olivine?
 
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Yes!
 
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In the debris of the basalt, I found one small splinter of olivine, whose optical properties I measured yesterday on a spindle stage attached to the microscope. Check
https://www.researchgate.net/publication/339054634_steven_guidebook
for a description of the technique.
The olivine has an angle 2V_x between the two optical axes of 89.15 degrees and is optically negative. The axis angle is a monotonous function of the forsterite content of the olivine (W. E. Tröger, Optische Bestimmung der Gesteinsbildenden Minerale, Part 1 Bestimmungstabellen) from which an forsterite content of about 85 % is inferred.
This is quite in line with the values reported for various icelandic basalts in https://academic.oup.com/petrology/article/54/4/745/1548250
which range from 78.1 to 90.2 % Forsterite.
Of course my measurement was made on one crystal, only, so that the statistics is rather poor :-)
 
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Here a picture of the olivine between crossed polarizers. Note the interference colours due to the strong birefringence.
 

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  • #80
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Fagradalsfjall in Geldingadalur is still producing.

Live camera has been discontinued
 
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Wow! How far is the camera from the volcano and hot might it be at that spot?

The camera conveys the feeling of being close but being safe which means its likely not safe at all if the volcano erupted.
 
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Wow! How far is the camera from the volcano and hot might it be at that spot?

The camera conveys the feeling of being close but being safe which means its likely not safe at all if the volcano erupted.
I believe the cameras are some km from the fissures and up-hill. The close up views are done with zoom lens. The cameras have been moved back from the original site as new fissures opened closer to the cameras.

https://www.camscape.com/webcam/fagradalsfjall-erupting-volcano-webcam/
 
  • #83
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During vacations I found some time to look at the plagioclase in the basalt under the microscope using a so-called "universal stage" which allows to rotate the thin section around 4 axes, see the picture. This makes it possible to determine the orientations of the principal optical axes relative to some characteristic crystal faces.

As these orientations are known as a function of the composition of the plagioclase, namely the albite (NaAlSi3O8)-anorthite (CaAl2Si2O8) ratio, it is possible to estimate this ratio quite accurately without the necessity to do a chemical analysis or a X-ray microprobe analysis. It turned out that the plagioclase is about 95% anorthite, which is what one would expect for a MORB.

In comparison to albite, anorthite contains less SiO2 and is therefore said to be more basic. The low SiO2 content means that the temperature, where crystals begin to form in the lava is quite high, and the viscosity of the lava is low. This is in line with pictures where the lava can be seen to flow like water. The high anorthite content also points to the lava being quite primitve, meaning that it is still quite similar to the primary magma that formed when the mantle material melted.

So quite a lot can already be learned about this kind of rocks without high-tech equipment.
 

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