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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Sign up to join this communityA 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?
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.
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.