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 Post subject: Volcano questions
PostPosted: Sun Nov 27, 2016 3:25 pm 
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Can a volcano extrude more than one type of lava over the course of it's life?

In my area there are lava fields of different lava types overlapping one another that are supposed to be of roughly the same era. There are a number of places where rhyolite meets olivine basalt - isn't rhyolite a felsic type while basalt is a mafic type? Can a volcano spit up silica-rich magma, go quite for a few thousand years and then start spitting up silica-poor but magnesium-rich magma?

I would desperately love to get fossicking access to some of these spots - the mind boggles at what might be present in an area that has seen several totally different types of lava wash over it.


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 Post subject: Re: Volcano questions
PostPosted: Sun Nov 27, 2016 9:39 pm 
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You can certainly get rhyolite and basalt in close association with one another, it's pretty typical. I'm not a volcanologist so I won't try to get into the nitty gritty but it's not atypical to have multiple sources of magma in the same area which can also sometimes mix to give intermediate compositions. Eastern California is full of nice big felsic volcanoes and even calderas from supervolcanic eruptions but there are also little pods of basalt all over the place.

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 Post subject: Re: Volcano questions
PostPosted: Sun Nov 27, 2016 10:25 pm 
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Cheers Stephen.

So in areas where felsic and mafic have both flowed, it's not impossible that there might be say, agates and sapphires both present together?


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 Post subject: Re: Volcano questions
PostPosted: Sun Nov 27, 2016 10:34 pm 
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The river crossing where all that pet wood and chalcedony comes from is identified as a rhyolite flow, never seen a sapphire there. Conversely, from memory the Anakie fields are weathered basalts - plenty of sapphires and pleonaste but I've only ever seen a few small quartz crystals in all the years of digging and a few small bits of chalcedony-type materials.

Unusually, the volcanoes that brought up that labradorite at Springsure are basaltic and yet they brought up heaps of feldspar, the bare ground glitters with millions of tiny chips of the stuff in some spots. I guess they were already in existence deep down and simply got caught up in the eruption and brought to the surface that way. But then there is the now off-limits opalised wood deposit a short distance away, I assume opalised wood was not brought up from the mantle :) but silicified by something relating to the volcanics.

Our earth is damn complex (and my understanding is lacking :) )


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 Post subject: Re: Volcano questions
PostPosted: Sun Nov 27, 2016 11:16 pm 
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Gem labradorite is typically found in basalts rather than rhyolites. While felsic rocks are rich in both k-spar and plagioclase feldspars like labradorite, plagioclase feldspar is an integral ingredient of basaltic rocks as well. Chalcedonies and opals are also very typically found in basaltic lavas as a result of later mobilization of silica, though whether it's necessarily sourced from the basalt itself or from some other source I do not know.
Sapphires are most typical in basaltic magmas--according to (imo the best current) models the basalts are from below a subducting slab and they grab sapphires from the slab as they pass through it. Rhyolitic magmas are not sourced from that kind of depth.

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 Post subject: Re: Volcano questions
PostPosted: Sat Dec 10, 2016 1:51 pm 
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Lefty wrote:
Can a volcano extrude more than one type of lava over the course of it's life?

In my area there are lava fields of different lava types overlapping one another that are supposed to be of roughly the same era. There are a number of places where rhyolite meets olivine basalt - isn't rhyolite a felsic type while basalt is a mafic type? Can a volcano spit up silica-rich magma, go quite for a few thousand years and then start spitting up silica-poor but magnesium-rich magma?

I would desperately love to get fossicking access to some of these spots - the mind boggles at what might be present in an area that has seen several totally different types of lava wash over it.

Hi Lefty- I have done a little research about the volcanic activity that produced my favorite gemstone red beryl. There were two distinct miocene magmatism episodes. The early episode was a beryl bearing rhyolite flow while the second was more of a topaz bearing flow. The earlier lava flow along with the unique geochemical conditions of beryllium, manganese,aluminum and oxygen with cooling water allowed red beryl to crystallize in the fractures and cavities. So I do believe that a volcano can extrude more than one type of lava at different times.

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 Post subject: Re: Volcano questions
PostPosted: Sat Dec 10, 2016 6:04 pm 
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Nicky Newark wrote:
Lefty wrote:
Can a volcano extrude more than one type of lava over the course of it's life?

In my area there are lava fields of different lava types overlapping one another that are supposed to be of roughly the same era. There are a number of places where rhyolite meets olivine basalt - isn't rhyolite a felsic type while basalt is a mafic type? Can a volcano spit up silica-rich magma, go quite for a few thousand years and then start spitting up silica-poor but magnesium-rich magma?

I would desperately love to get fossicking access to some of these spots - the mind boggles at what might be present in an area that has seen several totally different types of lava wash over it.

Hi Lefty- I have done a little research about the volcanic activity that produced my favorite gemstone red beryl. There were two distinct miocene magmatism episodes. The early episode was a beryl bearing rhyolite flow while the second was more of a topaz bearing flow. The earlier lava flow along with the unique geochemical conditions of beryllium, manganese,aluminum and oxygen with cooling water allowed red beryl to crystallize in the fractures and cavities. So I do believe that a volcano can extrude more than one type of lava at different times.


Hi Nicky.

Yes, it's fascinating stuff isn't it? It seems that many gemstones can form under more than one set of conditions. I would have thought the main source of gem beryl crystals to be pegmatites in granite (granite is the intrusive version of extrusive rhyolite I think?) but it appears that even the actual extruded lava flow may have spaces in it where such things can crystalize. Then first and second stage flows you describe probably explain why it's apparently not all that rare to find beryl and topaz in the same spot (though I have only ever found topaz).

I'm just about to head off to a spot where I suspect the existence of grossular garnets - it appears based on my rather basic geological knowledge than some small mafic-type volcanoes punched up through a bed of limestone, which I guess would have resulted in contact metamorphism into some variety of marble. It rained here last night so we may not be able to get in but hopefully we'll get at least a quick look at something worth reporting back on.

Cheers


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 Post subject: Re: Volcano questions
PostPosted: Sat Dec 10, 2016 7:13 pm 
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Ooh, good luck! You never know what you could find in hat sort of setting.

The beryl/topaz rhyolites are realy quite unusual geologically, and wouldn't have too much to do with pegmatitic formations except on a very broad scale. Their geochemistry is a bit off-kilter, hence the uniquely Mn-rich red beryl and Mn pink topazes found in them, for instance.

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