<p>When an oceanic plate is subducted below a continental plate, it dives into the mantle, taking water with it and producing frictional heat which melts some of the rock (rock with a high water content melts more easily). This produces plumes of magma beneath the continental plate margins. As the magma is lighter than the mantle material, it rises until it meets the continental plate above. The plumes try to break through, but are not always successful. Those that break through form volcanoes, those that don't quite manage to break through form plutons.</p>

<p>Looking at your map, we see gaps where the magma plumes have so far failed to break through, perhaps in places where the crust was thicker. In places where it seems there should be more volcanoes, there could be plutons marking a failed attempt, or there could still be a rising plume which might one day succeed in reaching the surface. Close to a line of volcanoes, it could be that these volcanoes are bleeding off so much of the rising magma that the remainder finds it hard to break through to the surface.</p>

CC BY-SA 4.0
2020-01-05T10:24:34.067Z
Your answer implies that there is magma under the gaps, but it failed to reach the surface. It's wrong: there is no magma under the gaps because the slabs don't go deep enough to trigger melting.
Jean-Marie Prival
CC BY-SA 4.0
2020-01-05T08:36:51.257Z
2020-01-05T08:36:51.257Z
Michael Walsby
<p>As you said, the Andean belt is divided into four segments, usually called the northern, central, southern, and austral volcanic zones (NVZ, CVZ, SVZ and AVZ, respectively; your map lacks the AVZ). This has been interpreted as a difference in the angle of subduction. Under the active zones, the subducting plate (called "slab") sinks into the mantle, releasing water, and the hydrated mantle partially melts, a process called "metasomatism". But <strong>under the gaps, the slab has a low angle ("flat slab subduction"), so it doesn't sink deep enough to trigger the melting process</strong>. There is no partial melting of the mantle under these zones.</p>

<p>Now, why has the slab failed to sink under these zones? This is due to the presence of submarine ridges. Between the NVZ and CVZ, there is the <a href="https://en.wikipedia.org/wiki/Nazca_Ridge" rel="nofollow noreferrer">Nazca Ridge</a>. Similarly, there is the Juan Fernandez ridge between the CVZ and SVZ. Those ridges are young, making the oceanic crust less dense, thus more buoyant (usually it is old, dense oceanic crust that subducts). The ridges also make the oceanic crust thicker, thus harder to subduct.</p>

<p><a href="https://i.sstatic.net/zs7t1.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/zs7t1.jpg" alt="enter image description here"></a><br>
<sub><a href="https://en.wikipedia.org/wiki/File:MAGMAARC1.jpg" rel="nofollow noreferrer">Image source</a>; by <a href="https://en.wikipedia.org/wiki/User:Chiton_magnificus" rel="nofollow noreferrer">Chiton magnificus</a> in 2011 and placed in the public domain.</sub></p>

<p>Two papers if you want to go further:</p>

<ul>
<li><a href="https://doi.org/10.1016/0040-1951(83)90112-9" rel="nofollow noreferrer">Volcanic gaps due to oblique consumption of aseismic ridges</a>  </li>
<li><a href="http://doi.org/10.1029/1999TC001152" rel="nofollow noreferrer">Geodynamics of flat subduction: Seismicity and tomographic constraints from the Andean margin</a></li>
</ul>

CC BY-SA 4.0
2020-01-05T14:14:06.110Z
+1 for linking to sources and further reading material
samcarter_is_at_topanswers.xyz
CC BY-SA 4.0
2020-01-05T16:51:59.780Z
Amazing answer! Thanks
caverac
CC BY-SA 4.0
2020-01-05T10:22:39.367Z
2020-01-05T21:35:18.183Z
2020-01-05T21:35:18.183Z
Jean-Marie Prival
<p>The figure below is taken from <a href="https://ui.adsabs.harvard.edu/abs/2019JGRB..124..195R/abstract" rel="noreferrer">Reath, K. et al. (2019)</a></p>

<p>A couple of things can be seen from this representation</p>

<ol>
<li><p>Volcanism seems to be strongly correlated with subduction zones, in this case the Nazca plate and South America plates.</p></li>
<li><p>There is a significant gap of volcanoes in both Peru and Chile. Which suggests that subduction is not the whole story. There must be something else happening</p></li>
</ol>

<p>Question is: do we know what? What is the reason for these gaps?</p>

<p><a href="https://i.sstatic.net/wJhvW.png" rel="noreferrer"><img src="https://i.sstatic.net/wJhvW.png" alt="enter image description here"></a></p>

<volcanology><plate-tectonics><geodynamics>
Gaps in locations of volcanos (Peru & Chile)
caverac
# Gaps in locations of volcanos (Peru & Chile)

(Asked by: username_0 on 2020-01-04)

The figure below is taken from [Reath, K. et al. (2019)](https://ui.adsabs.harvard.edu/abs/2019JGRB..124..195R/abstract)

A couple of things can be seen from this representation

1. Volcanism seems to be strongly correlated with subduction zones, in this case the Nazca plate and South America plates.
2. There is a significant gap of volcanoes in both Peru and Chile. Which suggests that subduction is not the whole story. There must be something else happening

Question is: do we know what? What is the reason for these gaps?

[![enter image description here](https://i.sstatic.net/wJhvW.png)](https://i.sstatic.net/wJhvW.png)

**Tags:** volcanology, plate-tectonics, geodynamics


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## 2 answers

### Answer by username_2 on 2020-01-05. Score: 12 | Accepted answer

As you said, the Andean belt is divided into four segments, usually called the northern, central, southern, and austral volcanic zones (NVZ, CVZ, SVZ and AVZ, respectively; your map lacks the AVZ). This has been interpreted as a difference in the angle of subduction. Under the active zones, the subducting plate (called "slab") sinks into the mantle, releasing water, and the hydrated mantle partially melts, a process called "metasomatism". But **under the gaps, the slab has a low angle ("flat slab subduction"), so it doesn't sink deep enough to trigger the melting process**. There is no partial melting of the mantle under these zones.

Now, why has the slab failed to sink under these zones? This is due to the presence of submarine ridges. Between the NVZ and CVZ, there is the [Nazca Ridge](https://en.wikipedia.org/wiki/Nazca_Ridge). Similarly, there is the Juan Fernandez ridge between the CVZ and SVZ. Those ridges are young, making the oceanic crust less dense, thus more buoyant (usually it is old, dense oceanic crust that subducts). The ridges also make the oceanic crust thicker, thus harder to subduct.

[![enter image description here](https://i.sstatic.net/zs7t1.jpg)](https://i.sstatic.net/zs7t1.jpg)  
[Image source](https://en.wikipedia.org/wiki/File:MAGMAARC1.jpg); by [Chiton magnificus](https://en.wikipedia.org/wiki/User:Chiton_magnificus) in 2011 and placed in the public domain.

Two papers if you want to go further:

* [Volcanic gaps due to oblique consumption of aseismic ridges](https://doi.org/10.1016/0040-1951(83)90112-9)
* [Geodynamics of flat subduction: Seismicity and tomographic constraints from the Andean margin](http://doi.org/10.1029/1999TC001152)

> **Comments:**
> **username_3**: +1 for linking to sources and further reading material  
> **username_0**: Amazing answer! Thanks  

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### Answer by username_1 on 2020-01-05. Score: 0

When an oceanic plate is subducted below a continental plate, it dives into the mantle, taking water with it and producing frictional heat which melts some of the rock (rock with a high water content melts more easily). This produces plumes of magma beneath the continental plate margins. As the magma is lighter than the mantle material, it rises until it meets the continental plate above. The plumes try to break through, but are not always successful. Those that break through form volcanoes, those that don't quite manage to break through form plutons.

Looking at your map, we see gaps where the magma plumes have so far failed to break through, perhaps in places where the crust was thicker. In places where it seems there should be more volcanoes, there could be plutons marking a failed attempt, or there could still be a rising plume which might one day succeed in reaching the surface. Close to a line of volcanoes, it could be that these volcanoes are bleeding off so much of the rising magma that the remainder finds it hard to break through to the surface.

> **Comments:**
> **username_2**: Your answer implies that there is magma under the gaps, but it failed to reach the surface. It's wrong: there is no magma under the gaps because the slabs don't go deep enough to trigger melting.  

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