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Magma Chambers

So why do we need a whole article on magma chambers, you ask. It’s just magma collecting in a buried chamber waiting to reach the surface. My, have we got a lot to cover.

First of all, why would we want to discuss magma chambers at all? Well, the simple fact is that magma doesn’t just “sit” there; it is extremely active. The composition of incoming magma is changing dramatically as it broils within the chamber, which directly influences the eruption at the surface. Moreover, the chamber walls are changing and one weak point can mean the eruption shifts to several miles away. Oh yes, it’s waiting to reach the surface…but that’s not all it’s doing...

To begin, magma chambers can be a variety of shapes and sizes. Most important is the fact that they are not all beautiful little spheres sitting under the volcano. Magma chambers may be dome shaped, spherical, or completely different. In many places, your “chamber” is actually a series of broad interconnecting dike structures that are growing or shrinking beneath the volcano. The chamber may be very shallow…a kilometer or two beneath the surface, or it may be very deep: a dozen or more kilometers deep. In some situations, you may not have just one chamber, but even a succession coming from depth (though this is difficult to prove). Chamber dimensions may be on the order of hundreds of meters to hundreds of kilometers (in some rare instances).

So two paragraphs into the article we’ve discovered that magma chambers can be small, huge, and anywhere in between. We’ve found that they can be virtually any shape, occur at any depth, and there may be more than one. This is going to be fun!

The basics

So, before we get into the dirty specifics, we ought to cover the basic processes going on within the chamber. First we have the older, already emplaced, magma simmering away. Gas is being exsolved from this magma and is traveling toward the surface. The magma, albeit slowly, is also cooling. This cooling causes minerals to grow and precipitate out of the mixture, changing the chemistry and the way different mineral components interact. We have differential cooling going on, which causes movement of magma within the chamber while new magma may be coming into it, causing further chemical changes. Chamber walls may be expanding or contracting, and producing underground landslides. We may also have escape of lava at the top of the chamber, through a surface-reaching conduit or a dike system. All of these processes, working together, make up the story of a magma chamber. Here we will discuss all of them.

Creation of the Chamber

It’s not known, with any certainty, just why a chamber forms where it does. The location can vary dramatically and involve multiple connected chambers of various sizes. The most likely explanation is that they begin forming below a cap of highly resistant rock, which causes the magma to slow and pool beneath it. Another possibility is an encounter with the opposite: a layer that is susceptible to chemical and thermal erosion. This layer may fracture and dissolve, allowing magma to collect. The shape of the chamber probably depends upon the composition of the surrounding rocks the chamber is in; for example, if the rock is easily eroded, it would melt to create the most economical space for the magma- a spherical, or roughly so, chamber. If the wall rock is resistant to erosion, however, you may have the chamber occupying cracks to form dike-style chambers. This is all highly subject to interpretation, however, and should not be thought of as unquestionably fact by any stretch of the imagination. There are probably many factors that contribute to the shape of the chamber. The size depends on a couple of things: how much magma is being injected to it and how much is escaping through dikes or a surface-reaching conduit. Obviously, if you have a great deal of magma entering with little escaping, the chamber will grow to accommodate it. Where the opposite occurs, the chamber may cool resulting in a sort of shrinking effect. It should also be noted that the size of the chamber may influence the shape: for example, if you have a dike-style chamber that continues to expand, dikes may join together to form a spherical/subspherical arrangement.

Chamber wall interactions

From talking about the formation of a magma chamber, we can easily progress to a discussion of how that magma interacts with the surrounding rocks. The process is no simple matter, as seems to be the case in every aspect of science today. Where magma contacts the surrounding rock, we have what is called a “solidification front.” Think of it as something similar to a boundary between armies or even a weather front. The army analogy gives you the sense of these two bodies battling each other, with one always losing to the other. The second analogy gives a more accurate perception of the physical process…it’s always either a cold front or a warm front. Solidification fronts are rarely, if ever, stationary. Dynamic in nature, this boundary between solid rock and liquid magma is constantly changing.

I want you to think for a moment what the boundary between the wall rock and the magma would look like. Have you got an image in your head? Think about it. Now we’ll check to see how close you were.

When surface lava flows over bare rock, there is little occurring because the surface lava quickly cools and hardens. In magma chambers, however, the magma is not quickly cooled at all. This means that the liquid rock and wall rock must coexist for long periods of time and cannot simply be “pressed” against each other for any stretch of time. The big question is: is the wall rock cooling the magma, or is the magma melting the wall rock? In most situations, both are the case. Both are influencing each other and creating a gradual boundary between the two. As we move from the center of the chamber to the wall, we encounter the solidification front. As we approach, we enter a zone where minerals are beginning to float about. As the magma cools, minerals drop below their solidus temperature (the temperature at which a mineral crystallizes from the magma; example, 32oF/0oC would be the solidus temperature of water). As we enter the front, the number of minerals we encounter increases. Soon we have a “cloud” of minerals floating everywhere, and at some point (~35% minerals) something changes. There are too many minerals and they begin to interlock. This causes a huge jump in viscosity, making the mix a “mush layer.” This mush layer becomes much more rigid as we approach the cooler side of the front. This occurs as more minerals form and bind to each other and the amount of liquid magma consequently drops. Once we reach around 50-60% crystals, the magma has become rigid and can be called rock with magma pore-spaces. What you actually call “solid” depends on your definition. The magma begins to thicken as soon as crystals start forming. At the mush zone, the magma can still flow, but not very easily. In the more rigid zone, there can still be deformation, but the body is now very strong. Still, as we approach the wall rock, the crystallinity and viscosity increase dramatically…to a point most people wouldn’t call it a fluid at all. Finally, as we travel, we have solid rock with only a tiny amount of magma in it…this is the point we can finally call it true wall-rock.

Now that we’ve defined the contact zone, we can talk about movement of the front. If the magma is eroding the wall via melting and chemical reactions, we say that the front is retreating…after all, the solid part of the solidification front is being pushed farther back. If the magma is solidifying, however, then we say the front is advancing.

But how does the moving front influence the boundary, one might ask. The answer is simple: it really doesn’t. Picture all of those zones; they still exist, no matter what is happening. If the front is retreating, then that dilute zone of crystals is where the wall rock is finally disappearing. At the mush and rigid zones, the originally solid rock is heating to become fluid. At the solid end, small pockets of fluid magma are beginning to form in the rock.

Where we have an advancing front, we have the opposite. New minerals are being formed in the dilute zone, instead of disappearing. The mush is slowly turning into a rigid zone, the rigid zone is solidifying to wall rock, and the wall rock is, through this process, moving toward the center of the chamber. While this section may seem long winded, it is my intention to make these processes as clear as possible. They are important to understand.

No less important is the formation of those minerals we encountered and events surrounding them. Let’s dive in:

The minerals, as I mentioned earlier, form as a result of the cooling; they also form, however, due to changes in the chemical make-up of the magma. All minerals have a solidus temperature; this is the temperature at which a liquid becomes solid. For water/ice, the solidus temperature is 0 degrees Celsius. For magmatic minerals, the temperature is much higher. As the magma body cools, minerals begin to accumulate and drop toward the bottom of the chamber. These minerals, while fluid within the magma, have a certain solubility.

Solubility refers to the amount of a substance that can be held by a fluid. For example, NaCl (table salt) is highly soluble in water. It can dissolve into it almost completely. You can’t drop an infinite amount of salt into water, however. At some point, the water will no longer be able to hold the salt and it will begin to drop toward the bottom. Try it with a glass of water and watch it yourself. Moreover, note that hot water can hold more salt than cold water. Take a glass of extremely hot water and dissolve as much salt into it as you can. Now, let the glass cool and keep an eye on it; eventually salt will begin to crystallize and drop to the bottom of the glass. If you’re impatient, put the glass in the freezer and check it after about 20 minutes; you’ll see a little pile of salt at the bottom.

So how does this comparison relate to magma chambers? Well, the same process is occurring in the magma body. Substitute magma for water and a host of magmatic minerals for the salt. Each of these minerals precipitate out at different temperatures and so in solidified magma chambers (called plutons) we can see specific zoning of minerals; layer after layer, showing how the temperature of the chamber changed through the type of mineral deposited. The solubility of minerals also depend on the other minerals and elements present in the mix.

Let’s go back to our glass of water. We’ll use fictional numbers for demonstration purposes, but real numbers could just as easily be substituted. We’ll say our glass of water can hold 3 grams of NaCl before it begins to crystallize. Our glass can also hold 2 grams of KCl (another type of salt) before that type crystallizes. It can also hold 0.5 grams of baking powder before that begins to drop out. Does that mean the water can hold 5.5 grams of everything combined? The answer is no. The solubility of each depends on the amount of other chemicals in the mix. Perhaps the water can only hold a total of 3.5 grams of dissolved “stuff.” The water would then only be able to hold something like 2.5 grams NaCl, 0.75 grams KCl, and 0.25 grams baking powder. Anything extra will drop out. If we had a glass with 3 grams NaCl, and then added 2 grams of KCl, you would have 0.5 grams of NaCl drop out and 1 gram of KCl drop straight to the bottom. Hopefully you can see how the chemistry of the water affects what it can hold.

As you’ve seen, the solubility of minerals depends on how much other stuff is present in the mix. As some minerals drop out, other elements become more numerous. If you have 1000 Magnesium atoms and 1000 Barium atoms, you have a 1:1 ratio in the mix. If 800 magnesium atoms drop out as olivine, then the mix ratio is 1:5 and the barium may want to drop out. Here you will hopefully begin to understand the complicated business of mineral interactions and precipitation. Entire books have been dedicated to these subjects with fiercely complicated diagrams. If you’d like to learn more about the subject of mineral formation, then you’ll have to consult scientific journals and text books. It’s a complicated subject to learn.

Back at the solidification front, however, the process is simpler because the cooling is so much quicker. You don’t have time for zoning if all of the minerals begin to form at roughly the same time. Hence, this zone provides a bit of simplicity to the subject. Some respite from the complicated events occurring elsewhere.

Remember that the wall rocks in many situations are being eroded. This can cause severe stability issues. Rocks from the ceiling may become loose and break apart, collapsing toward the bottom of the chamber. Rocks and mush zones along the sides of the chamber can become unstable, sliding toward the bottom as large underground landslides. This can occur where the chamber is shrinking too, however. With new rock being formed, the chamber boundaries are not always perfectly smooth, leaving room for break-aparts and collapses.

During all of this, it is important to remember that the magma within the chamber is not stationary. Along those boundaries, the lava cools. For almost all substances, density increases as the temperature decreases. This means that the magma along the edges is heavier than the stuff in the interior. Consequently, stuff along the edge of the chamber drops downward. Meanwhile, material in the interior (including fresh magma from below) rises upward due to its lower density. Consequently, we have thermally driven motion. It’s difficult to know how important this is, since that sinking cold magma is being trapped by our solidification front! Thus, most of the motion is probably caused by collapsing chamber walls and exsolving gas bubbles. Of course, the gas can only exsolve from fresh magma if there is a large pressure change between the top and bottom of the chamber; this is provided by magma escaping as dikes or through a surface-reaching conduit. Oh yeah, it reaches the surface at some point. Almost forgot about that!

Influence on the Eruption

So we know about what goes on in these magma chambers, but the question of why it’s important still remains. The simple explanation is that activity in the magma chambers dictates what will happen at the surface. Take, for example, Kilauea volcano on the island of Hawaii. By studying gas emissions at the summit, we can predict what will happen there and along the east rift zone where the current activity is (at the time of this writing). These gasses come from the magma chamber below and, even though it is far from the erupting vent, we can use them to understand how the eruption might change.

Magma has three options when broiling in a magma chamber: escape through a conduit, escape through a dike, or solidify. The last will not be discussed here because it lies beyond the realm of volcanology. Dikes are discussed in another article and so will not be covered here. We will talk about behavior around a surface-reaching conduit, however. At actively erupting basalt volcanoes, fresh magma is inputted to the base of the magma chamber and rises upward. The new mix is hotter and more gas-rich, which allows it to rise as the rest of the chamber material descends. As the fresh magma ascends, it cools and mixes with the surrounding magma. This causes its chemistry to change and results in a loss of gas to the rest of the chamber. Regardless, however, the new stuff reaches the top of the chamber, still infused with magmatic gasses. Being that the volatiles are far lighter than the magma itself, these volatiles will tend to collect at the top of the chamber and around the conduit. Now we can have two types of activity here, which greatly influence the surface eruption. If the magma is a viscous basalt, gas can collect into large bubbles and escape into a conduit suddenly. It may be hard to imagine large bubbles building up next to the conduit without immediately escaping, but consider the viscosity of the magma and the influence of the mush zone it is pressed against. This escape results in a massive bubble of gas rising upward rapidly, pushing magma ahead of it and pulling magma from behind. At the surface, there is a loud explosion and lava is shot through the air by the exploding bubble, known as a slug. The gas explosion can last several seconds, or longer, and is either followed by more magma exiting the vent or by the lava flowing back into the conduit from the surface! This is called a Strombolian eruption and is discussed in another article. The other occurrence we may see is where magma is a very fluid basalt. This allows the gas to remain very diffuse and only small bubbles collect before escaping. Because the mix is less viscous, the bubbles can travel freely and aren’t stopped from reaching the exit. This results in the lava being brought up gently, or sometimes rocketed upward, to the surface. Here we have a Hawaiian eruption, which may be docile but can result in lava fountaining hundreds of meters high.

Where we have viscous lavas such as andesites, dacites, and rhyolites, we have something significantly different going on. Because the magma is not nearly so fluid, the wall-rock interactions discussed earlier are not so dramatic. This is not to say nothing unusual happens; there are still small mush zones, still wall collapses, still convection…but the effects are much less so due to resistance to flow and the fact that viscous magmas generally occur in smaller quantities than basalts.

In an actively erupting explosive volcano, we see a gas-rich layer at the top of the magma chamber. This bubble-rich region surrounding the chamber is being pulled upward by the high pressure-gradient: huge pressures at the bottom of the chamber compared to virtually none at the surface. The conduit, in this respect, acts like a vacuum hose in the chamber. The top bubble-rich material is first erupted, but then gas-poor magma below it may be pulled upward. This introduction of partially degassed magma can occur while adjacent top-level gas-rich magma is still being pulled into the conduit from the sides. This can result in the eruption of mixed magmas. In fact, the same event is thought to happen in large basaltic eruptions as well.

As the viscous bubble-rich material rises upward, the decreasing pressure causes the bubbles to expand. Here, the bubble walls aren’t fluid as we think of it, but more a glass. This means the bubbles explode upward, which in turn fuels the eruption. For more information on how viscous eruptions occur, see the similarly titled article.

Most of the time, however, active volcanoes are not erupting. The majority of the time, they quietly simmer for years or even decades. This is when it is most important to understand the magma chamber.

During an eruption, the influence of the magma chamber is good to understand…but more important is keeping people a safe distance and monitoring current activity to determine the likelihood of future events. During times of quiescence, an understanding of the chamber can lead to accurate predictions of when an eruption might occur. During this time, the conduit is almost always blocked.

This doesn’t mean that nothing is happening below, however. If the volcano is active, new material is probably still being inputted below and the processes we mentioned earlier may still be occurring. In situations like this, two types of geologic behavior are important to monitor: deformation and gas (seismic is much more important during or shortly before an eruption). Deformation is caused, in this instance, by the expanding or contracting magma chamber. Scientists measure the tilt of the volcano, to determine how things are moving at the surface and below. They also measure how the volcano may stretch or shrink in certain directions. Expansion usually indicates that new magma is entering the reservoir below…contraction indicates that magma may be solidifying or is being diverted elsewhere.

In my biased opinion, the study of escaping gasses is most important. Changes in the behavior of the magma chamber can be seen almost immediately at the surface; if new magma is entering the chamber, then we will see a jump in the amount of volcanic gasses exiting above it (gasses can escape the magma chamber through large networks of cracks in the overlying rock). If nothing is happening, we will see the volcanic gasses remain roughly the same and dropping slightly over a fairly long period of time. While deformation tells us about what is happening to the chamber as a whole, gasses tell us exactly what the magma is doing inside that chamber…the two methods work exceedingly well together (though are by no means perfect).

Conclusion

Somehow you’ve been able to work your way through this lengthy and long-winded article. Hopefully you now have a working understanding of how magma chambers behave. I wrote this article in greater detail than others because most people have little reason to care about magma chambers. Thus, you likely have some sort of special interest in them…as a learning volcanologist or an avid amateur geologist. Either way, you should now have a far better understanding of volcanology than you did before reading this article…at least I would hope so. We’ve talked about why magma chambers are important, how they form, and why they form; we’ve discussed their shapes, how they change with time, and the processes that dominate them; we’ve covered their influence on eruptions and how they are used to better-tune our predictions. That was quite a bit of ground to cover for a single article, but it’s been done. For more information on the subject, I would suggest consulting text books and scientific journals.