[JimBob (OP)]

OK Bass

Your mission impossible today, should you choose to accept it, is to explain how the volcano, whose name no one can pronounce except Icelanders and a few smart-arses, should begin its eruption with a basaltic lava flow which suddenly became andisitic? (It is described as "tephra" on the main Wikipedia page, linked below, and this was verified as andesiteic by the geologist talking about the mineral composition on the National Geographic Channel this evening during the 7 PM hour, central time.)

SOOOOOO ?


First eruption March 20th, 2010, - http://en.wikipedia.org/wiki/Fimmv%C3%B6r%C3%B0uh%C3%A1ls
Note the red lava fountain

Later ongoing eruption beginning on 14 April 2010 - Main article - http://en.wikipedia.org/wiki/2010_eruptions_of_Eyjafjallaj%C3%B6kull

[Geezer]

Whoopdy effin do!

Sounds like your typical after the fact geologic explanation. When you lot can explain things before they actually happen, we might start to take you seriously.

[JimBob (OP)]

Sounds like your typical grouchy old man who has nothing better to do except raise Holy Ned with others. Have a life buddy? Or is Mrs. Geezer that hard to deal with?

[Bass]

Whoopdy effin do!

Sounds like your typical after the fact geologic explanation. When you lot can explain things before they actually happen, we might start to take you seriously.

Which is not a whole lot different than your typical after-the-fact engineering explanations.  Just talk to a Toyota owner...

[Bass]

OK Bass

Your mission impossible today, should you choose to accept it, is to explain how the volcano, whose name no one can pronounce except Icelanders and a few smart-arses, should begin its eruption with a basaltic lava flow which suddenly became andisitic? (It is described as "tephra" on the main Wikipedia page, linked below, and this was verified as andesiteic by the geologist talking about the mineral composition on the National Geographic Channel this evening during the 7 PM hour, central time.)

SOOOOOO ?


First eruption March 20th, 2010, - http://en.wikipedia.org/wiki/Fimmv%C3%B6r%C3%B0uh%C3%A1ls
Note the red lava fountain

Later ongoing eruption beginning on 14 April 2010 - Main article - http://en.wikipedia.org/wiki/2010_eruptions_of_Eyjafjallaj%C3%B6kull

There are several possibilities, but two are the most likely:
Magma fractionation and differential melting.

Magma fractionation- ferromagnesium minerals crystalize leaving the melt richer in silicon (in the case of basalt fractionation, more andesitic).

More likely is differential melting.  As the basaltic magma slowly rises, it melts pre-existing rock in the crust.  The more silica rich minerals tend to melt first (especially with a bit of water) resulting in two magmas with different compositions.  These magmas may, or may not, mix.
Yellowstone is a good example of differential melting.  Basaltic magmas melt silica-rich crustal rocks resulting in the creation of rhyolite magmas (two ends of the volcanic spectrum).  Volcanic rocks in Yellowstone are bimodal- either basalt or rhyolite, indicating no mixing occurs. 

[frethack]

As an aside, Bass, would you happen to know James Gardner?  He is doing quite a bit of research at Yellowstone...they are about to head out for the summer to pick up samples of a rhyolite flow.  I didnt even know that rhyolite COULD flow, but apparently it has and they want to know why.

Have you seen anything like this out there before?

[Bass]

Like this rhyolite flow??

 [ Invalid Attachment ]
USGS photo, Newberry Caldera, OR showing Big Obsidian Flow (rhyolite).

There are numerous rhyolite flows and domes around Yellowstone.  Rhyolite doesn't flow far, but it does flow.

[JimBob (OP)]

OK Bass

Your mission impossible today, should you choose to accept it, is to explain how the volcano, whose name no one can pronounce except Icelanders and a few smart-arses, should begin its eruption with a basaltic lava flow which suddenly became andisitic? (It is described as "tephra" on the main Wikipedia page, linked below, and this was verified as andesiteic by the geologist talking about the mineral composition on the National Geographic Channel this evening during the 7 PM hour, central time.)

SOOOOOO ?


First eruption March 20th, 2010, - http://en.wikipedia.org/wiki/Fimmv%C3%B6r%C3%B0uh%C3%A1ls
Note the red lava fountain

Later ongoing eruption beginning on 14 April 2010 - Main article - http://en.wikipedia.org/wiki/2010_eruptions_of_Eyjafjallaj%C3%B6kull

There are several possibilities, but two are the most likely:
Magma fractionation and differential melting.

Magma fractionation- ferromagnesium minerals crystalize leaving the melt richer in silicon (in the case of basalt fractionation, more andesitic).

More likely is differential melting.  As the basaltic magma slowly rises, it melts pre-existing rock in the crust.  The more silica rich minerals tend to melt first (especially with a bit of water) resulting in two magmas with different compositions.  These magmas may, or may not, mix.
Yellowstone is a good example of differential melting.  Basaltic magmas melt silica-rich crustal rocks resulting in the creation of rhyolite magmas (two ends of the volcanic spectrum).  Volcanic rocks in Yellowstone are bimodal- either basalt or rhyolite, indicating no mixing occurs. 

I did consider differential melting but discarded it.

So correct me if I am wrong, but all of Iceland is made of mid-ocean ridge basalts isn't it? The small amount of sediments are derived from this as well.

[Geezer]

Yep! Works every time.

Y'all certainly know how send a crater to sleep.

Snork......snork......

[JimBob (OP)]

Go play in the traffic, will ya?

[Bass]

I did consider differential melting but discarded it.

So correct me if I am wrong, but all of Iceland is made of mid-ocean ridge basalts isn't it? The small amount of sediments are derived from this as well.

The whole point is that the melting is "differential".  It works like the Bowen reaction series in reverse- it melts the more silicic and volatile minerals first, especially in the presence of water.  If this differential melt is then separated, the resulting magma will have a different composition than the original rock.

[JimBob (OP)]

OK! Now I understand. I was thinking you were sating it was a melt derivative from existing crustal material. Do'h!

Jimmy Hutton didn't explain this to well when I apprenticed to him.