<p>You are correct that arc magmas are produced by melting of the mantle wedge above the subducting slab. In the case of felsic rocks in island arcs, a model I have not seen debunked is that these volcanoes typically produce early melts that are no more felsic than andesite. These andesites are then re-melted as the source for more differentiated dacites and rhyolites. This is supported by their geochemistry, which is more enriched relative to continental arc and rift rhyolites in 'typical' mafic elements like Ca and Al.</p>
<p><a href="https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1440-1738.2010.00746.x" rel="nofollow noreferrer">https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1440-1738.2010.00746.x</a></p>

CC BY-SA 4.0
2022-12-29T04:03:07.483Z
2022-12-29T04:03:07.483Z
desander
<p>I previously believed that continental crust owes its lower density to the partial melting of oceanic crust; the mantle would partially melt at mid-ocean ridges to produce basaltic crust, and when this oceanic crust eventually subducted, the basalt would then partially melt to produce an even lighter magma. This lighter magma would rise up and eventually erupt, building up a less dense continental crust above.</p>
<p>However, I have recently learned that the magma produced at subduction zones comes from the partial melting of the mantle above the subducting plate and that very little comes from the plate itself. This means that mid-ocean ridges, hot spots, and subduction zones all derive their magma directly from the mantle, yet they somehow produce magmas of differing compositions.</p>
<p>I have seen <a href="https://upload.wikimedia.org/wikipedia/commons/4/48/Gesteinskreislauf_plus_2019.png" rel="noreferrer">explanations</a> which state that felsic magma forms due to fractional crystallization of magma while working its way through the thick continental crust, but this still wouldn't explain how it forms under thin oceanic crusts at island arcs.</p>
<p>I am trying to understand how a hot spot under oceanic crust produces shield volcanoes with basaltic rock while oceanic-oceanic subduction zones produce stratovolcanoes with felsic rock.</p>

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2022-12-28T02:29:56.353Z
Comments are not for extended discussion; this conversation has been [moved to chat](https://chat.stackexchange.com/rooms/141587/discussion-on-question-by-richard-how-is-felsic-magma-produced-at-subduction-zon).
f.thorpe
<plate-tectonics><petrology><subduction><volcanic-eruption><oceanic-crust>
How is felsic magma produced at island arcs?
richard
# How is felsic magma produced at island arcs?

(Asked by: username_0 on 2022-07-15)

I previously believed that continental crust owes its lower density to the partial melting of oceanic crust; the mantle would partially melt at mid-ocean ridges to produce basaltic crust, and when this oceanic crust eventually subducted, the basalt would then partially melt to produce an even lighter magma. This lighter magma would rise up and eventually erupt, building up a less dense continental crust above.

However, I have recently learned that the magma produced at subduction zones comes from the partial melting of the mantle above the subducting plate and that very little comes from the plate itself. This means that mid-ocean ridges, hot spots, and subduction zones all derive their magma directly from the mantle, yet they somehow produce magmas of differing compositions.

I have seen [explanations](https://upload.wikimedia.org/wikipedia/commons/4/48/Gesteinskreislauf_plus_2019.png) which state that felsic magma forms due to fractional crystallization of magma while working its way through the thick continental crust, but this still wouldn't explain how it forms under thin oceanic crusts at island arcs.

I am trying to understand how a hot spot under oceanic crust produces shield volcanoes with basaltic rock while oceanic-oceanic subduction zones produce stratovolcanoes with felsic rock.

**Tags:** plate-tectonics, petrology, subduction, volcanic-eruption, oceanic-crust

> **Comments:**
> **username_1**: Comments are not for extended discussion; this conversation has been [moved to chat](https://chat.stackexchange.com/rooms/141587/discussion-on-question-by-username_0-how-is-felsic-magma-produced-at-subduction-zon).  

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## 1 answers

### Answer by username_2 on 2022-12-29. Score: 1

You are correct that arc magmas are produced by melting of the mantle wedge above the subducting slab. In the case of felsic rocks in island arcs, a model I have not seen debunked is that these volcanoes typically produce early melts that are no more felsic than andesite. These andesites are then re-melted as the source for more differentiated dacites and rhyolites. This is supported by their geochemistry, which is more enriched relative to continental arc and rift rhyolites in 'typical' mafic elements like Ca and Al.

<https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1440-1738.2010.00746.x>


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