<p>You and @jeffronicus are right in saying that it is the exsolving gas that drives the eruption of magma at the surface and gives it momentum. However, I'd like to point out that in order for the gas to exsolve from the magma, it first needs to reach a certain depth in the conduit. See for instance the solubility curve for water, the most common volcanic gas:</p>
<p><a href="https://i.sstatic.net/h55pE.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/h55pE.png" alt="enter image description here" /></a></p>
<p><em>H<sub>2</sub>O solubility in basaltic and rhyolitic melts at typical magmatic temperatures based on experimental data. From <a href="https://doi.org/10.1016/C2015-0-00175-7" rel="nofollow noreferrer">The Encyclopedia of Volcanoes (2nd edition, 2015)</a></em></p>
<p>It shows that if you take a rhyolitic magma with a typical, initial content of 4 wt% H<sub>2</sub>O at depth (i.e., in the chamber), you need to rise it to ~4 km (1 kbar) to reach the saturation curve, thus to begin water exsolution. Which means that <strong>gas exsolution usually happens in the conduit, while the magma has already started ascending</strong>.</p>
<p>So, your question actually leads to another: Why does magma start ascending from the chamber in the first place? Magma can stay for decades in its chamber, at equilibrium with the surrounding country rock (we speak of &quot;lithostatic&quot; pressure rather than hydrostatic, but it's the same thing with a different density). So why does it decide to start ascending one day? It is indeed a matter of <strong>overpressure, although not from above but from below</strong>. Crustal magma reservoirs are sometimes recharged by inputs of &quot;fresh&quot; magma from deeper, generating an overpressure in the chamber that triggers the eruption. Some examples of this:</p>
<ul>
<li><a href="https://doi.org/10.1007/s004450050274" rel="nofollow noreferrer">Effusive eruption of viscous silicic magma triggered and driven by recharge: a case study of the Cerro Chascon-Runtu Jarita Dome Complex in Southwest Bolivia</a></li>
<li><a href="https://doi.org/10.1130/0-8137-2375-2.175" rel="nofollow noreferrer">Mafic magma injection triggers eruption at Ilopango Caldera, El Salvador, Central America</a></li>
<li><a href="https://doi.org/10.1130/G37333.1" rel="nofollow noreferrer">Magma reservoir response to transient recharge events: The case of Santorini volcano (Greece)</a></li>
</ul>

CC BY-SA 4.0
2021-02-17T00:07:33.513Z
Oh this provides *really helpful* insight, thank you! I'll give it some time to sink in that exsolution can begin many km below the surface.
uhoh
CC BY-SA 4.0
2022-12-20T14:17:40.277Z
If you'd like to know more on the topic of volatile solubility in magmas, here is a nice video at e-Volcano: https://evolcano.iavceivolcano.org/volatile-solubilities-in-magmas/
Jean-Marie Prival
CC BY-SA 4.0
2022-12-20T23:49:46.043Z
The constantly evolving hats in the video remind me of the winter hats in Stack Exchange :-) and are somewhat distracting me from what seems to be an interesting talk. Thanks for the link!
uhoh
CC BY-SA 4.0
2021-02-16T15:29:13.337Z
2022-07-08T16:06:42.010Z
2022-07-08T16:06:42.010Z
Jean-Marie Prival
<p>Convection also plays an important part in making the eruption column rise further than it would just from its initial upward velocity alone.</p>

CC BY-SA 4.0
2021-02-13T01:09:32.633Z
Welcome to Stack Exchange! Once you reach 50 reputation points you can leave comments under other people's posts. Right now your answer post seems to disagree with the other answer, but provides no support for this. Can you cadd some supporting links or citations to make this a proper Stack Exchange answer? *Thanks!*
uhoh
CC BY-SA 4.0
2022-07-08T19:26:10.653Z
I'm not sure if this disagrees or not (it's just not mentioned in other answers, but could still play an additional important part?).  But if you happen to come back, it'd be best to explain more, give reference/reasoning/experience to support the answer so those of us who now little on the subject can better process the information and validity! :-)
JeopardyTempest
CC BY-SA 4.0
2021-02-12T17:35:38.657Z
2021-02-12T17:35:38.657Z
James W. McHardy
<p>The ejecta and their momentum appear to be entirely the product of the dissolved gasses in magma, according to this page on <a href="http://www.geology.sdsu.edu/how_volcanoes_work/Controls.html">Physicochemical Controls on Eruption Style</a> on the How Volcanoes Work pages of the Geology Department of San Diego State University:</p>

<blockquote>
  <p><strong>The amount of dissolved gas in the magma provides the driving force
  for explosive eruptions.</strong> The viscosity of the magma, however, is also
  an important factor in determining whether an eruption will be
  explosive or nonexplosive. A low-viscosity magma, like basalt, will
  allow the escaping gases to migrate rapidly through the magma and
  escape to the surface. However, if the magma is viscous, like
  rhyolite, its high polymerization will impede the upward mobility of
  the gas bubbles. As gas continues to exsolve from the viscous melt,
  the bubbles will be prevented from rapid escape, thus increasing the
  overall pressure on the magma column until the gas ejects explosively
  from the volcano. As a general rule, therefore, nonexplosive eruptions
  are typical of basaltic-to-andesitic magmas which have low viscosities
  and low gas contents, whereas explosive eruptions are typical of
  andesitic-to-rhyolitic magmas which have high viscosities and high gas
  contents.</p>
</blockquote>

<p><a href="http://www.tulane.edu/~sanelson/Natural_Disasters/volcan&amp;magma.htm">A similar description is offered</a> by Stephen A. Nelson, associate professor in the Department of Earth &amp; Environmental Sciences at Tulane University:</p>

<blockquote>
  <p>At depth in the Earth nearly all magmas contain gas dissolved in the
  liquid, but the gas forms a separate vapor phase when pressure is
  decreased as magma rises toward the surface of the Earth.   This is
  similar to carbonated beverages which are bottled at high pressure.
  The high pressure keeps the gas in solution in the liquid, but when
  pressure is decreased, like when you open the can or bottle, the gas
  comes out of solution and forms a separate gas phase that you see as
  bubbles.   Gas gives magmas their explosive character, because volume
  of gas expands as pressure is reduced.</p>
</blockquote>

<p>So the gross extrusion of magma from the earth is not innately gas-dependent (though gas influences the composition of the magma), but the violent processes are caused more by gas coming out of solution and forming pressurized bubbles than gas or magma being under pressure, if that makes sense.</p>

CC BY-SA 3.0
2017-02-06T02:23:56.023Z
It makes perfect sense, thank you! This is really an excellent example of a stackexchange answer. It is both thoughtful and well supported. I learned a new word also - * exsolve*.
uhoh
CC BY-SA 3.0
2017-02-06T02:42:24.820Z
This seems to be quite an extreme, but very visual example of low-viscosity lava; https://www.wired.com/2014/03/strangest-magma-earth-carbonatites-oldoinyo-lengai/ direct link to the video *Oldoinyo Lengai Volcano - Amazing Eruptions of Silvery Carbonatite Lava* https://youtu.be/qputaVyn7TE
uhoh
CC BY-SA 3.0
2017-02-06T02:50:41.057Z
Thank you! My job involves science communication, but mostly in a support role, so Stackexchange gives me a productive outlet.
jeffronicus
CC BY-SA 3.0
2017-02-05T20:18:15.953Z
2017-02-05T20:18:15.953Z
jeffronicus
<p>When I was very young I was always amazed that water didn't come shooting up out of the first ice-fishing hole of the season, due to the weight of all the ice. It doesn't, because the weight of the same volume of liquid water would be even heavier. Ice floats, and the thicker the ice, the lower the level of water sits down inside the hole. </p>

<p>There is no additional hydrostatic pressure in the water beyond that due to gravity, because if there were, the ice would have been slowly pushed up to a new equilibrium position fairly quickly.</p>

<p>If I extended this thinking to volcanism, I'd wonder why volcanos can sometimes have high velocity ejecta, and also build cones from liquid that rise high above the local surface. I am guessing that the height of the cone represents effects due to hydrostatic pressure <em>beyond that due to the weight of material above</em> (rock being less flexible than ice) but the high speed ejecta that falls far from the cone is accelerated by gas coming out of solution.</p>

<p>Is the momentum of a volcano's high-velocity ejecta mostly due to expanding gas?</p>

<p><a href="https://i.sstatic.net/XjaJb.jpg"><img src="https://i.sstatic.net/XjaJb.jpg" alt="enter image description here"></a></p>

<p><strong>above:</strong> Example of an ice-fishing hole, from <a href="https://commons.wikimedia.org/wiki/File:Ministry_of_Defence_of_Russia_-_035.jpg">here</a>.</p>

<volcanology>
Is the momentum of a volcano's high-velocity ejecta mostly due to expanding gas?
uhoh
# Is the momentum of a volcano's high-velocity ejecta mostly due to expanding gas?

(Asked by: username_0 on 2017-02-05)

When I was very young I was always amazed that water didn't come shooting up out of the first ice-fishing hole of the season, due to the weight of all the ice. It doesn't, because the weight of the same volume of liquid water would be even heavier. Ice floats, and the thicker the ice, the lower the level of water sits down inside the hole.

There is no additional hydrostatic pressure in the water beyond that due to gravity, because if there were, the ice would have been slowly pushed up to a new equilibrium position fairly quickly.

If I extended this thinking to volcanism, I'd wonder why volcanos can sometimes have high velocity ejecta, and also build cones from liquid that rise high above the local surface. I am guessing that the height of the cone represents effects due to hydrostatic pressure *beyond that due to the weight of material above* (rock being less flexible than ice) but the high speed ejecta that falls far from the cone is accelerated by gas coming out of solution.

Is the momentum of a volcano's high-velocity ejecta mostly due to expanding gas?

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

**above:** Example of an ice-fishing hole, from [here](https://commons.wikimedia.org/wiki/File:Ministry_of_Defence_of_Russia_-_035.jpg).

**Tags:** volcanology


---

## 3 answers

### Answer by username_4 on 2017-02-05. Score: 10 | Accepted answer

The ejecta and their momentum appear to be entirely the product of the dissolved gasses in magma, according to this page on [Physicochemical Controls on Eruption Style](http://www.geology.sdsu.edu/how_volcanoes_work/Controls.html) on the How Volcanoes Work pages of the Geology Department of San Diego State University:

> **The amount of dissolved gas in the magma provides the driving force
> for explosive eruptions.** The viscosity of the magma, however, is also
> an important factor in determining whether an eruption will be
> explosive or nonexplosive. A low-viscosity magma, like basalt, will
> allow the escaping gases to migrate rapidly through the magma and
> escape to the surface. However, if the magma is viscous, like
> rhyolite, its high polymerization will impede the upward mobility of
> the gas bubbles. As gas continues to exsolve from the viscous melt,
> the bubbles will be prevented from rapid escape, thus increasing the
> overall pressure on the magma column until the gas ejects explosively
> from the volcano. As a general rule, therefore, nonexplosive eruptions
> are typical of basaltic-to-andesitic magmas which have low viscosities
> and low gas contents, whereas explosive eruptions are typical of
> andesitic-to-rhyolitic magmas which have high viscosities and high gas
> contents.

[A similar description is offered](http://www.tulane.edu/~sanelson/Natural_Disasters/volcan&magma.htm) by Stephen A. Nelson, associate professor in the Department of Earth & Environmental Sciences at Tulane University:

> At depth in the Earth nearly all magmas contain gas dissolved in the
> liquid, but the gas forms a separate vapor phase when pressure is
> decreased as magma rises toward the surface of the Earth. This is
> similar to carbonated beverages which are bottled at high pressure.
> The high pressure keeps the gas in solution in the liquid, but when
> pressure is decreased, like when you open the can or bottle, the gas
> comes out of solution and forms a separate gas phase that you see as
> bubbles. Gas gives magmas their explosive character, because volume
> of gas expands as pressure is reduced.

So the gross extrusion of magma from the earth is not innately gas-dependent (though gas influences the composition of the magma), but the violent processes are caused more by gas coming out of solution and forming pressurized bubbles than gas or magma being under pressure, if that makes sense.

> **Comments:**
> **username_0**: It makes perfect sense, thank you! This is really an excellent example of a stackexchange answer. It is both thoughtful and well supported. I learned a new word also - * exsolve*.  
> **username_0**: This seems to be quite an extreme, but very visual example of low-viscosity lava; https://www.wired.com/2014/03/strangest-magma-earth-carbonatites-oldoinyo-lengai/ direct link to the video *Oldoinyo Lengai Volcano - Amazing Eruptions of Silvery Carbonatite Lava* https://youtu.be/qputaVyn7TE  
> **username_4**: Thank you! My job involves science communication, but mostly in a support role, so Stackexchange gives me a productive outlet.  

---

### Answer by username_1 on 2021-02-16. Score: 7

You and @username_4 are right in saying that it is the exsolving gas that drives the eruption of magma at the surface and gives it momentum. However, I'd like to point out that in order for the gas to exsolve from the magma, it first needs to reach a certain depth in the conduit. See for instance the solubility curve for water, the most common volcanic gas:

[![enter image description here](https://i.sstatic.net/h55pE.png)](https://i.sstatic.net/h55pE.png)

*H2O solubility in basaltic and rhyolitic melts at typical magmatic temperatures based on experimental data. From [The Encyclopedia of Volcanoes (2nd edition, 2015)](https://doi.org/10.1016/C2015-0-00175-7)*

It shows that if you take a rhyolitic magma with a typical, initial content of 4 wt% H2O at depth (i.e., in the chamber), you need to rise it to ~4 km (1 kbar) to reach the saturation curve, thus to begin water exsolution. Which means that **gas exsolution usually happens in the conduit, while the magma has already started ascending**.

So, your question actually leads to another: Why does magma start ascending from the chamber in the first place? Magma can stay for decades in its chamber, at equilibrium with the surrounding country rock (we speak of "lithostatic" pressure rather than hydrostatic, but it's the same thing with a different density). So why does it decide to start ascending one day? It is indeed a matter of **overpressure, although not from above but from below**. Crustal magma reservoirs are sometimes recharged by inputs of "fresh" magma from deeper, generating an overpressure in the chamber that triggers the eruption. Some examples of this:

* [Effusive eruption of viscous silicic magma triggered and driven by recharge: a case study of the Cerro Chascon-Runtu Jarita Dome Complex in Southwest Bolivia](https://doi.org/10.1007/s004450050274)
* [Mafic magma injection triggers eruption at Ilopango Caldera, El Salvador, Central America](https://doi.org/10.1130/0-8137-2375-2.175)
* [Magma reservoir response to transient recharge events: The case of Santorini volcano (Greece)](https://doi.org/10.1130/G37333.1)

> **Comments:**
> **username_0**: Oh this provides *really helpful* insight, thank you! I'll give it some time to sink in that exsolution can begin many km below the surface.  
> **username_1**: If you'd like to know more on the topic of volatile solubility in magmas, here is a nice video at e-Volcano: https://evolcano.iavceivolcano.org/volatile-solubilities-in-magmas/  
> **username_0**: The constantly evolving hats in the video remind me of the winter hats in Stack Exchange :-) and are somewhat distracting me from what seems to be an interesting talk. Thanks for the link!  

---

### Answer by username_2 on 2021-02-12. Score: 0

Convection also plays an important part in making the eruption column rise further than it would just from its initial upward velocity alone.

> **Comments:**
> **username_0**: Welcome to Stack Exchange! Once you reach 50 reputation points you can leave comments under other people's posts. Right now your answer post seems to disagree with the other answer, but provides no support for this. Can you cadd some supporting links or citations to make this a proper Stack Exchange answer? *Thanks!*  
> **username_3**: I'm not sure if this disagrees or not (it's just not mentioned in other answers, but could still play an additional important part?).  But if you happen to come back, it'd be best to explain more, give reference/reasoning/experience to support the answer so those of us who now little on the subject can better process the information and validity! :-)  

---

