<p>The technical difference is simply in the bulk percentage of <a href="https://en.wikipedia.org/wiki/Silicon_dioxide" rel="nofollow noreferrer">silica</a>, <a href="https://en.wikipedia.org/wiki/Basalt" rel="nofollow noreferrer">Basalts</a> are relatively low in silica, less than 52% in total, the rest of it being composed primarily of iron and magnesium oxides which makes it quite low <a href="https://en.wikipedia.org/wiki/Viscosity" rel="nofollow noreferrer">viscosity</a> compared to other lavas. Andesite lavas contain from 57 to 63% silica by weight and are thus more viscous as they contain a smaller percentage of other rock forming oxides. The changes in viscosity often reflect differences in dominant mineralogy but primarily are due to the higher <a href="https://en.wikipedia.org/wiki/Eutectic_system" rel="nofollow noreferrer">eutectic temperature</a> of the more pure silica. There is also the fact that andesites contain more <a href="https://en.wikipedia.org/wiki/Volatile_(astrogeology)" rel="nofollow noreferrer">volatiles</a> (particularly water and flourine) that evaporate near or at the surface taking heat out of the melt and freezing the flow, this often results in explosive eruptions. <a href="https://en.wikipedia.org/wiki/Rhyolite" rel="nofollow noreferrer">Rhyolites</a> tend to be even wetter and are even higher in silica and viscosity resulting in very few lava flows and many extremely explosive eruptions.</p>
<p>The colour difference is due to the higher percentage of dark iron/magnesium oxides and silicates in basalt versus andesites' more silica rich chemistry.</p>

CC BY-SA 4.0
2024-07-07T02:15:03.420Z
2024-07-07T02:15:03.420Z
Ash
<p>The first mineralogical difference is that hardly any quartz is found in basalt, but some  amount of quartz is found in andesite.</p>
<p>Another distinction is that basalt has higher concentrations of magnesium/iron, thus presents olivine, whiles andesite has more amphiboles.</p>
<p><a href="https://en.wikipedia.org/wiki/Amphibole#/media/File:Mineralogy_igneous_rocks_EN.svg" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/Amphibole#/media/File:Mineralogy_igneous_rocks_EN.svg</a> this picture might help.</p>

CC BY-SA 4.0
2024-07-06T23:36:07.893Z
Hmmm I have also read however that basalt doesn’t always have olivine
Harrychink
CC BY-SA 4.0
2024-07-06T15:30:57.713Z
2024-07-06T15:30:57.713Z
Mr.inquisitive
<p>I understand that basalt is darker in color and has less silica but do not understand how these reflect the underlying minerals/mineraloids (if mineraloids are even present in these rocks)???</p>

<geology><rocks><igneous>
What is the difference between basalts and andesites?
Harrychink
# What is the difference between basalts and andesites?

(Asked by: username_0 on 2024-07-06)

I understand that basalt is darker in color and has less silica but do not understand how these reflect the underlying minerals/mineraloids (if mineraloids are even present in these rocks)???

**Tags:** geology, rocks, igneous


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## 2 answers

### Answer by username_1 on 2024-07-07. Score: 3

The technical difference is simply in the bulk percentage of [silica](https://en.wikipedia.org/wiki/Silicon_dioxide), [Basalts](https://en.wikipedia.org/wiki/Basalt) are relatively low in silica, less than 52% in total, the rest of it being composed primarily of iron and magnesium oxides which makes it quite low [viscosity](https://en.wikipedia.org/wiki/Viscosity) compared to other lavas. Andesite lavas contain from 57 to 63% silica by weight and are thus more viscous as they contain a smaller percentage of other rock forming oxides. The changes in viscosity often reflect differences in dominant mineralogy but primarily are due to the higher [eutectic temperature](https://en.wikipedia.org/wiki/Eutectic_system) of the more pure silica. There is also the fact that andesites contain more [volatiles](https://en.wikipedia.org/wiki/Volatile_(astrogeology)) (particularly water and flourine) that evaporate near or at the surface taking heat out of the melt and freezing the flow, this often results in explosive eruptions. [Rhyolites](https://en.wikipedia.org/wiki/Rhyolite) tend to be even wetter and are even higher in silica and viscosity resulting in very few lava flows and many extremely explosive eruptions.

The colour difference is due to the higher percentage of dark iron/magnesium oxides and silicates in basalt versus andesites' more silica rich chemistry.


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### Answer by username_2 on 2024-07-06. Score: 0

The first mineralogical difference is that hardly any quartz is found in basalt, but some amount of quartz is found in andesite.

Another distinction is that basalt has higher concentrations of magnesium/iron, thus presents olivine, whiles andesite has more amphiboles.

<https://en.wikipedia.org/wiki/Amphibole#/media/File:Mineralogy_igneous_rocks_EN.svg> this picture might help.

> **Comments:**
> **username_0**: Hmmm I have also read however that basalt doesn’t always have olivine  

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