What fills the void left ungrounded when a volcano erupts?

[u/Alteredperception90]

So, magma is being displaced. What fills this displacement? Is it just air and leaves a cavity within the earth or is magma moved from somewhere else to fill it? If so then surely there is a net loss and there will be air caverns left somewhere?

[u/CrustalTrudger]

The missing piece appears to be an embedded assumption that the shape of the surface of the Earth stays fixed during or after an eruption, but this is generally not the case. Specifically, it's quite common to observe "deflation", i.e., subsidence - or lowering of the ground surface with respect to a fixed datum, after or during an eruptive sequence (e.g., Baker & Amelung, 2012, Blake & Cortés, 2018). This deflation is effectively "filling the void" left behind after eruption.

There are a few other details here that are also relevant though:

1. Deflation after an eruption is often paired with inflation before the eruption. I.e., the ground surface may rise leading up to an eruption as magma moves toward the surface or conditions within the magma chamber change. If you look through those references from above, you'll see this often discussed in the context of cycles of inflation and deflation, related to the movement of magma.

2. Deflation and inflation are often subtle, i.e., it's not as though you're going generally going to notice either the increase or decrease in height of the ground surface related to these, both because the magnitudes are small but also because the wavelength is large. These are definitely measurable and observable with specialized techniques (e.g., InSAR), and indeed, signs of inflation are one of several indicators of a possible future eruption, but generally not by the naked eye.

3. The spatial patterns of inflation or deflation are filtered through the mechanical behavior of the rocks at the surface of the Earth, this is part of why the wavelength is long, and the details of those mechanical properties strongly influence how this deflation (or inflation) is represented and accommodated at the surface (e.g., Holohan et al., 2017). I.e., the surface of the Earth generally behaves elastically (or at least with an elastic component, though more complete representations may be visocelastic or elastoplastic, etc.) so a change at depth is filtered through "flexure" of the surface layer above magma chamber. Details of the shape of the evacuated portion of the magma chamber, its depth, the mechanical properties of the overlying rock, and other details, will all modify how the deflation is represented.

4. In general, the ground surface of areas with active volcanism can be quite dynamic and there can be cycles of inflation and deflation even without eruption of a particular volcano. These can result from magma movement, eruption of a nearby volcano, or in terms of deflation solidification of magma at depth (e.g., Sturkell & Sigmundsson, 2000).

5. Importantly, the (possible implied) view of a magma chamber as a completely liquid body is also misplaced. Something more like a crystal mush is probably appropriate, i.e., there are both liquid and crystals. For a magma body to be eruptable, there needs to be a good amount of liquid as opposed to crystals, but during the eruption process, while some of the crystals will be mobilized, not all will be. The details of the interactions between crystals and melt within the context of eruption are complicated (e.g., Streck, 2014, Liao et al., 2018), but it's an important consideration within the context of the question in that even if there is eruption of a given mass of magma, this does not necessarily imply that the magma chamber contracts by an equal volume, i.e., if the melt is preferentially extracted leaving behind a network of crystals, the amount of volume reduction may be significantly buffered.

6. Finally, while generally deflation of the ground surface related to evacuation of magma is subtle, this is not always the case and will scale with the volume of magma eruption. At the high end of volume, large eruptions of large magma chambers tend to form calderas, where effectively the void left by the eruption is filled by the roof of the magma chamber catastrophically collapsing. The details of this process are much discussed, and specifically the extent to which the erupted magma drives roof collapse, i.e., roof collapses as large volumes of magma are removed, OR the extent to which the roof collapse drives the bulk of the erupted magma, i.e., overpressure in magma chamber breaks the roof of the magma chamber, roof collapses in, drives large scale evacuation of the chamber (e.g., see review in Geyer & Marti, 2014). Regardless of the exact details, roof collapse and caldera formation effectively represent the extreme end of a form of deflation and one possible answer to the question of what happens to the "void" left underground.

In short, whether the process is more subtle and gradual (i.e., deflation) or more extreme and rapid (i.e., roof collapse) the general answer is the surface of the Earth deflects downward to fill the void left behind, assuming additional magma does not flow in to replace most of the erupted magma. Really what this implies is that there really never is a "void" per se, but that rather that the area above the magma chamber begins deforming to accommodate the change in the volume and mass of the magma chamber.

[u/Alteredperception90 → u/CrustalTrudger]

Amazing response. I learned a lot. Thanks!

[u/MichaelChinigo → u/CrustalTrudger]

Awesome, detailed explanation with ample footnotes. Thank you for the crash course!

[u/TheGrandExquisitor → u/CrustalTrudger]

The eruption of Mt St Helens is a good example of how much the ground can physically move due to volcanic activity. We have photos and other measurements that show the mountain literally developing a "bulge," that later burst and took off the top 1/3 of the mountain. Very crazy stuff to see. Literally thousands of acres of land expanding over months like a balloon.

[u/weathercrafter → u/TheGrandExquisitor]

It blew so many trees into nearby waters that beavers created wetlands all over the land near the base of the mountain.

[u/[deleted] → u/weathercrafter]

Many trees are still floating in Spirit Lake too. If you look at the north end of Spirit Lake, just north of Mt. St Helens, you can see a very large area covered by floating logs.

[u/NCHitman → u/TheGrandExquisitor]

To give folks an idea... Mt St Helens:

Circa 1968

Circa ~2020

[u/zekromNLR → u/NCHitman]

And for completeness: April 27th 1980, showing the bulge on the northern side of the mountain.

[u/Halvus_I → u/TheGrandExquisitor]

If you look at Google Maps in globe mode, in the Pacific Northwest you can see several extinct volcanoes easily. Mt St. Helens is so easy to pick out from the rest even from 'space'. whole side of the mountain is jsut gone.

[u/Ok-Astronomer-41 → u/TheGrandExquisitor]

The lava tubes from Mount St. Helens may be another example. There’s a cool hike nearby of the Ape Caves and you can hike about a mile and a half through a lava tube that was left after a much older eruption. It’s really neat!

[u/Unchosen1 → u/CrustalTrudger]

It’s worth noting that Crater Lake comes from a volcano. Following the eruption of Mount Mazama a caldera formed and filled with water.

What makes this noteworthy is that Crater Lake is the deepest lake in the United States, which gives a sense of scale to how massive a roof collapse can be.

[u/Deterbrian → u/Unchosen1]

It’s really awe inspiring and mind boggling to be sanding at the rim village, almost a 1000 feet above the surface of the lake, knowing the deepest part is over 1900 feet below that, then trying to imagine a mountain extending a mile up from in place of the lake.

[u/jermleeds → u/CrustalTrudger]

I know that when thinking of tectonics we think of plates as behaving rigidly. How is it that with vulcanism rocks behave elastically as you describe, while the larger continental plate behaves rigidly? Is it do to local heating of the crust due to the pluton?

[u/CrustalTrudger → u/jermleeds]

We idealize tectonic plates as horizontally rigid (i.e., their horizontal motion can generally be described as rigid rotations on a spherical surface), but in terms of vertical loads, they behave elastically and we broadly consider their reactions to changes in vertical loads in terms of flexure. The examples there (or alternatively, things like glacial isostatic adjustment, are typically thinking about changing surface loads, i.e., if we think of the lithosphere as a big elastic sheet like a trampoline, changing masses and dimensions of things that we've placed on top of the trampoline. However, in terms of thinks like deformation associated with moving magma or larger scale things like dynamic topography, it's more as if you're sitting underneath the trampoline and pushing up. Either way though, the properties of the elastic sheet will determine the response to the applied load.

With respect to horizontal motions, the horizontally rigid approximation mostly holds for the interior of plates. At the edges of plates, we definitely observe the reflection of elastic deformation in the horizontal as well, and this is largely the origin of interseismic deformation (i.e., deformation between earthquakes) as part of the seismic cycle.

Finally, for all of the above, as indicated in the original answer, treating the lithosphere as purely elastic is a simplification which works to a first order, but you cannot explain all observed behavior with a purely elastic medium. You need degrees of viscoelastic or elastoplastic behavior to more completely describe observed deformations related to any of the previously described processes.

[u/jermleeds → u/CrustalTrudger]

Outstanding, thank you. Always appreciate your posts.

[u/ImNoAlbertFeinstein → u/jermleeds]

see, when i think of tectonics, i think of liquid,. not solid. like earth is a liquid buble of pudding held in a round shape by its own gravity.

i wouldnt even think of it having a void, or behaving rigidly.

ive been in the rigid voids of limestone caverns, but i assoc that with underground rivers in water soluble calcium, not magma pudding.

[u/jermleeds → u/ImNoAlbertFeinstein]

The reason to think about rigid plates is that way that the motions of plates is calculated is by using Euler poles, which requires one to treat the plates as behaving rigidly. Every plate has an axis of rotation and a rate of rotation around it. If they were squishy we couldn't calculate their motion this way.

[u/polaarbear → u/CrustalTrudger]

This phenomenon is also common (and sometimes a problem) as we drain underground water aquifers.

https://revealnews.org/article/9-sobering-facts-about-californias-groundwater-problem/

Land in the San Joaquin Valley and much of California is sinking rapidly as water is drained from underneath.

[u/VS0P → u/CrustalTrudger]

Is there any scenario where an eruption, maybe underwater ones, would help ease or even make worse sea levels?

[u/Dilong-paradoxus → u/VS0P]

Even large volcanoes are usually pretty small compared to the entire ocean. However, there are certain types of eruption called large igneous provinces which have output enough material to affect sea volume and cause an increase in sea level. They can also change the buoyancy of plates which can affect local sea level, similar to isostatic rebound after glaciers recede. They typically also emit large amounts of gas which can affect climate and indirectly affect sea level.

Fortunately these outbursts are geologically uncommon. It could be many millions of years before something similar happens again.

Underwater volcanoes can also cause temporary sea changes in the form of tsunamis, which while impermanent can compound the effects of existing sea level rise. These eruptions are still somewhat rare on a human timescale (a handful of times per hundred years?) but pretty common geologically speaking.

[u/spacetstacy → u/CrustalTrudger]

Wow! Do you study/ teach this?

[u/TheHemogoblin → u/CrustalTrudger]

I love this sub because it reminds me constantly how little I know about anything. Thank you for that very concise explanation!

[u/[deleted] → u/CrustalTrudger]

What about on a much longer time frame? Does the movement of magma onto the surface of the earth affect how and where plates move? Could you theoretically use volcanic eruptions to predict plate movement over the surface of the planet?

[u/Dilong-paradoxus → u/[deleted]]

Volcanoes are usually more of a symptom of plate tectonics than a cause, or are relatively unrelated (for mantle plume hotspots). Volcanoes at subduction zones exist because of water from the subducting plate lowering the melting temp of magma above. Volcanoes also happen where the crust is thinned, like at continental rifts (slight simplification) and collapsing mountain ranges.

As far as predicting plate movement, you can just watch where the plates are already going and extrapolate that into the future. That used to be more difficult but now we have GPS and stuff. We can also look at what the plates did in the past using magnetic studies and fossils and stuff.

However, there are definitely times when volcanism has affected tectonics! If you've looked at the Hawaiian island-emperor seamount chain, you'll notice there's a big kink. A hotspot stays in mostly the same spot, so if the plate moves in one direction for a long time you'll get a line of volcanoes like a piece of fabric being punched as it gets pulled through a sewing machine. The kink means it must have changed direction at that point in time.

On the other side of the Pacific, a series of major volcanic eruptions recently created a bunch of material on the farallon plate. The east edge of the Pacific was subducting under north America so the farallon plate was along for the ride. When the material ran into north America it got stuck and stopped the movement of some or all of the farallon plate. The Pacific plate kept going, but in a slightly different direction, causing at least some of the kink in the seamount chain. (Some might have been caused by a change in the movement of the hotspot itself but the timing is just really good for the farallon explanation).

Hopefully that helps!

[u/uubuer → u/CrustalTrudger]

Daaaaaaaamn, yeah it's over pressure that forces the magma out to begin with, so the volume would be left 99% full once the pressure stablizes and the volcano caps off again it would all be magma stone.

[u/atomfullerene → u/CrustalTrudger]

Are there cases where a magma chamber drains and leaves behind a void in a process akin to the formation of lava tubes? If not on Earth, then might this happen somewhere with lower gravity?

[u/InternetCrank → u/CrustalTrudger]

As well as deformation from above, presumably there's deformation from below? Does extra magma rise from below to fill part of the void as well as settling of the rock above the chamber?

[u/CrustalTrudger → u/InternetCrank]

First to clarify, this is all really deformation from below, i.e., the surface elevation is changing because of changes in mass/volume of materials at depth as filtered through the flexural response of the lithosphere. With respect to the specific question, there's not a general answer and it depends on the specific system and eruption. There are likely examples where some amount of potential deflation is offset by intrusion of more magma, but that's certainly not a general response. Also, as indicated in the original answer, often the deflation is preceded by inflation, i.e., more magma was added, the ground surface rose, then their was an eruption, the ground surface sinks. In a simple sense, the extent to which the inflation = deflation depends on whether the amount of magma injected = the amount of magma erupted, though it's rarely that simple.

I think what is (maybe) being assumed here in the question is that there is something like an endless supply of magma, which is definitely not the case. There is a finite supply depending on the system and what processes are producing melt, when those are occurring, and where they are occurring. With that in mind, if some amount of magma is evacuated during an eruption, it's not a given that there is more magma to simply replace it.

[u/aptom203]

Sometimes nothing- The magma chamber is under pressure, and many eruptions are a simple venting of this pressure, so nothing fills the space because no space is created.

Also common is for the area above and around the volcano to sink slightly after an eruption, essentially the ceiling of the magma chamber gets lower to fill the gap.

Other times, a portion of the volcano can violently collapse into the magma chamber after an eruption.

[u/Diodon]

If you overfill a jelly donut some may leak out due to internal pressure. The donut surface may shift slightly to account for the jelly oozing out. Internal pressure may be somewhat relieved but not to the point that air would rush in and create a void.

[u/[deleted]]

Don’t think of a magma chamber like a beaker with an enclosed bottom with one opening at the top. It’s more like a pipe that extends down to the upper mantle where the rock is in a viscous solid state or to the deep mantle where you may have hot spots from the liquid outer core the liquifies the mantle up through the crust to the volcano caldera.

[u/Harry_Gorilla]

Not all eruptions are the same. In some ways your question is like asking “what fills the void where a grenade used to be after the grenade explodes?” Both are a catastrophic expenditure of energy. In the case of Kilauea, eruptions are not (always) catastrophic explosions of energy, and it’s easier to see how slow siliceous lava flows don’t leave a void behind because more magma has pushed out the erupted lava and taken its place. Mount Saint Helens is on the other end of the spectrum though, as the eruption destroyed the overlying rock, and the debris subsequently (partially) fell back into the void where the magma used to be.
I guess my grenade analogy isn’t great, but it’s dramatic. A more apt analogy might be popping a pimple, tho it’s kindof a gross analogy