A lower average temperature is, apparently, only part of the answer....
Several sites said composition, not temperature, explains most of the difference...
And the difference in viscosity can be up to seven orders of magnitude!!!
Why?
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2 Answers
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Several sites said composition, not temperature, explains most of the difference...
They are correct. All other factors remaining equal, felsic magmas are more viscous. This is the result of higher silica (SiO2) contents in the melt.
In melts and rocks, silicon atoms are (almost) always surrounded by four oxygen atoms in tetrahedral shape. These tetrahedrons have an extremely strong affinity to bond to each other, forming a polymerised network of silica tetrahedrons.
More silica = better capacity to form the polymerised network. Increasing silica contents also make each instance of silica polymer bigger and longer.
When you have a low silica melt (basalt) those polymerised silica networks are tiny and not well connected, so moving them around is easy. Once they grow bigger, it becomes much harder for them to move. So the viscosity goes up.
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Partial crystallisation products are the primary factor, iron and magnesium are many times more common in mafic melts than felsic ones. The minerals that are highest in iron and magnesium are the olivines they are also quite soft and slippery due to their microstructure (individual silica tetrahedra that stack together to form grains that don't share any internal atomic bonds) which means they flow more easily past each other in a melt at a given temperature. Felsic melts being much lower in magnesium and iron and relatively higher in sodium, potassium, lithium, and calcium, as well as silicon, have very little olivine and are high in feldspar minerals which have a very different micro structure (they are composed of long chains of interlinked silica tetrahedra that share atoms similar to polymers but with much higher melting points) and are far less able to flow past and around each other.
Due to these differences in chemistry a felsic melt at 1000ᵒC will be many times more viscous than a mafic melt at the same temperature. Add the fact that felsic melts tend to be water saturated and lose a lot of heat when they dehydrate near the surface and so tend to be much colder when observed and they are very viscous indeed. Felsic mineral crystallisations occur at higher temperatures and produce lower heat of crystallisation so felsic melts cool as they set where basalts, and even moreso ultramafic lavas, can actually, initially, increase in temperature as they crystallise.
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1$\begingroup$ I disagree, even above liquidus (i.e., without crystals) felsic melts have higher viscosity than mafic melts. It is essentially a matter of strong Si$-$O bonds (and other network formers like Al), while mafic magmas have less Si and more network modifier cations. Crystals then have a role in increasing viscosity of lava, depending on their packing and shape, but to me the primary factor of viscosity is Si$-$O atomic bonds. $\endgroup$ Dec 12, 2021 at 7:22
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$\begingroup$ @Jean-MariePrival I've never heard of, or seen data for, get a totally liquid melt, all magmas are, by definition, partially crystalline, even in the mantle. However the reason that a completely liquid felsic melt would be more viscous than a mafic melt at the same temperature is simple; it is still closer to its temperature of crystallisation. $\endgroup$– AshDec 13, 2021 at 4:44
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$\begingroup$ @Jean-MariePrival is correct. All else remaining equal, it is the silicon tetrahedral network that determines viscosity, often much more than any other factors if those are unequal. $\endgroup$– GimelistDec 13, 2021 at 4:57
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$\begingroup$ @Gimelist "all else being equal" the non-silicon chemistry of these melts is not equal so what are you even trying to say exactly? $\endgroup$– AshDec 13, 2021 at 5:01
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1$\begingroup$ @Ash this is absolutely incorrect. Sodium is one of the elements best at breaking silica chains, in silicate melts! If you're interested in sources for education to address some of your misconceptions, I'm happy to provide them. Otherwise, I'm done here. $\endgroup$– GimelistDec 13, 2021 at 5:22