<blockquote>
<p>Several sites said composition, not temperature, explains most of the
difference...</p>
</blockquote>
<p>They are correct. All other factors remaining equal, felsic magmas are more viscous. This is the result of higher silica (SiO<sub>2</sub>) contents in the melt.</p>
<p>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.</p>
<p>More silica = better capacity to form the polymerised network. Increasing silica contents also make each instance of silica polymer bigger and longer.</p>
<p>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.</p>

CC BY-SA 4.0
2021-12-13T04:56:28.810Z
2021-12-13T04:56:28.810Z
Gimelist
<p>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 <a href="https://en.wikipedia.org/wiki/Olivine" rel="nofollow noreferrer">olivines</a> 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 <a href="https://en.wikipedia.org/wiki/Feldspar" rel="nofollow noreferrer">feldspar minerals</a> 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.</p>
<p>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.</p>

CC BY-SA 4.0
2021-12-12T07:22:01.987Z
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.
Jean-Marie Prival
CC BY-SA 4.0
2021-12-13T04:44:52.380Z
@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.
Ash
CC BY-SA 4.0
2021-12-13T04:57:41.467Z
@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.
Gimelist
CC BY-SA 4.0
2021-12-13T05:01:14.620Z
@Gimelist "all else being equal" the non-silicon chemistry of these melts is not equal so what are you even trying to say exactly?
Ash
CC BY-SA 4.0
2021-12-13T05:04:16.127Z
@Ash Take a basalt. Melt it completely at a certain temperature. Then, take the exact same basalt, add silica to it, and melt it again at the same temperature. It will be more viscous.
Gimelist
CC BY-SA 4.0
2021-12-13T05:08:32.873Z
Also, in your answer you are making the link between a mineral's internal structure (eg the nesosilicate olivine vs the tectosilicate feldspar) and its ability to "slide" in a melt. There is no such link. The mechanical properties of how solid minerals affect the viscosity of their host magma has very little to do with a mineral's internal crystallographical structure. If you think otherwise, please provide a reference for this.
Gimelist
CC BY-SA 4.0
2021-12-13T05:09:12.730Z
@Gimelist Yes because you have changed the relative volume of non-silicon elements thus reducing the relative availability of iron and magnesium for making non-chain minerals in the melt. Iron and magnesium break up the natural long chain forming process that silica goes through left to itself. Also changes the eutectic but that's a slightly different issue.
Ash
CC BY-SA 4.0
2021-12-13T05:14:11.990Z
Well these things always sum to 100%, right? Increasing silica necessarily decreases everything else. But there's nothing special about Mg and Fe - it could be any other element (Ca, Na, K, Ti, etc). What is important is that the melt itself can now form polymerised silica chains. The effect of solid minerals in the magma (regardless of their internal structure, doesn't matter if it's olivine or quartz) also affects viscosity, but it has nothing to do with the mineral composition.
Gimelist
CC BY-SA 4.0
2021-12-13T05:22:12.737Z
@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.
Gimelist
CC BY-SA 4.0
2021-12-12T05:14:09.197Z
2022-02-13T17:23:48.713Z
2022-02-13T17:23:48.713Z
Ash
<p>A lower average temperature is, apparently, only part of the answer....</p>
<p>Several sites said composition, not temperature, explains most of the difference...</p>
<p>And the difference in viscosity can be up to seven orders of magnitude!!!</p>
<p>Why?</p>

<geology><volcanology><rocks><volcanic-eruption>
Why is felsic (granitic) lava so much more viscous when flowing (or trying to) than mafic (basaltic) lava?
Kurt Hikes
# Why is felsic (granitic) lava so much more viscous when flowing (or trying to) than mafic (basaltic) lava?

(Asked by: username_0 on 2021-12-12)

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?

**Tags:** geology, volcanology, rocks, volcanic-eruption


---

## 2 answers

### Answer by username_1 on 2021-12-13. Score: 7

> 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.


---

### Answer by username_2 on 2021-12-12. Score: 2

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](https://en.wikipedia.org/wiki/Olivine) 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](https://en.wikipedia.org/wiki/Feldspar) 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.

> **Comments:**
> **username_3**: 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.  
> **username_2**: @username_3 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.  
> **username_1**: @username_3 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.  
> **username_2**: @username_1 "all else being equal" the non-silicon chemistry of these melts is not equal so what are you even trying to say exactly?  
> **username_1**: @username_2 Take a basalt. Melt it completely at a certain temperature. Then, take the exact same basalt, add silica to it, and melt it again at the same temperature. It will be more viscous.  
> **username_1**: Also, in your answer you are making the link between a mineral's internal structure (eg the nesosilicate olivine vs the tectosilicate feldspar) and its ability to "slide" in a melt. There is no such link. The mechanical properties of how solid minerals affect the viscosity of their host magma has very little to do with a mineral's internal crystallographical structure. If you think otherwise, please provide a reference for this.  
> **username_2**: @username_1 Yes because you have changed the relative volume of non-silicon elements thus reducing the relative availability of iron and magnesium for making non-chain minerals in the melt. Iron and magnesium break up the natural long chain forming process that silica goes through left to itself. Also changes the eutectic but that's a slightly different issue.  
> **username_1**: Well these things always sum to 100%, right? Increasing silica necessarily decreases everything else. But there's nothing special about Mg and Fe - it could be any other element (Ca, Na, K, Ti, etc). What is important is that the melt itself can now form polymerised silica chains. The effect of solid minerals in the magma (regardless of their internal structure, doesn't matter if it's olivine or quartz) also affects viscosity, but it has nothing to do with the mineral composition.  
> **username_1**: @username_2 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.  

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