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The Ascent of Magma

Introduction

Ok, so you know where magma comes from: deep inside the earth…but where? How deep? It’s obvious that volcanoes are erupting, but why? Much of Earth’s interior is solid, so why do we get these pockets of rising fluid and how do they reach the surface? You know that lava forms new earth at the surface, but how does the magma itself form? We know many volcanoes are old, but there aren’t any four billion year old volcanoes; when did the processes begin, and what started them? How do they end?

Questions you’ve never thought of? Maybe one or two have struck you, but most people don’t even think of these things. We see the erupting volcano and we take them for what they are…but often we forget to ask the less obvious questions; the tougher ones. This is what we’re talking about here: the hardcore science of why volcanoes exist. We’re going to look at the very processes that allow for life on Earth to exist, so if you’re ready to get your hands dirty, and waist-deep in science, then let’s get started.

A million questions with less-than-obvious answers

The first question we ought to address is just where this magma comes from and how it initially forms. We know there has to be a specific reason for its formation, magma doesn’t just spontaneously generate.

As you should know from the tectonics article, lava reaches the surface in three very different situations: over mantle plumes, over spreading centers (rift zones), and over subduction zones. Each of these locations have conditions that are unique to it. We know that magma is always generated through one or a combination of these three methods: decompressional melting, heating, and chemical reaction. The heating method is most obvious, but really does not apply. This is because almost all of our geophysical modeling of the Earth’s interior shows that there is very little variation in temperature within the mantle (laterally, not with depth). This rules out the heating method, although it will reappear later in the ascent of our magma. The second method is decompressional melting. This is thought to be one of the most common methods for the formation of magma. As density increases, so does the melting point of all materials. The relationship goes the other way, as well. Thus, rock deep in the mantle that is already somewhat plastic (almost fluid, but not) can become fluid when something causes it to expand (usually the removal of overlying material). Where is this most common? Under continental rift zones, where basalts come to the surface. The process is started through the stretching of the crust between plates. This causes it to become thinner, taking some weight off of the underlying mantle rocks. This causes the pressure to decrease, and allows the rock to expand. This decompression lowers the melting point of the rock to the point at which it becomes fluid. Here we have the formation of our magma for continental rift zones.

But that’s just one of three situations. We have far more happening at subduction zones. Here, one plate is being pushed beneath another. There is some frictional heating, but generally not enough to produce magma. Things do get warmer as you go deeper, however, and the rocks are heated. Here we see the chemical reaction method of formation. Water is carried into the earth with the subducting plate. It is brought between mineral grains (remember that it used to be under the ocean) or as parts of minerals; lawsonite, chloritoids, and amphiboles carry large quantities of water in their mineral structures. As the rock is heated, the minerals break down and the water is released. While water acts to cool the rocks, more importantly, it lowers their melting point…just like decompression! The already heated rocks now have the very thing they need to become magma…water. Further chemical reactions can affect the formation and travel of magma, but that will occur later in its ascent.

Finally, we have magma plumes. These plumes can be easy to identify because they are almost completely stationary; that is, they remain in the same place while the continental plates move overhead. How these plumes form to begin with, however, is highly subject to debate. Everything from breaks in the outer core to asteroid impacts have been proposed. Most likely, in my opinion, is a deep origin: at the D-zone. The D-zone is a boundary between the dense nickel-iron outercore and the much-less-dense inner mantle. Some theories and hypotheses suggest that the D-zone operates much like our crust between the mantle and the atmosphere, with plate tectonics and perhaps even iron volcanoes! I’m not about to begin speculating on this, however, as science has done little to support or discredit these claims. I do believe, though, that mantle plumes probably begin at or near the D-zone, and are heavily influenced by the outer core. Perhaps a break in the D-zone initiates heating, or even another unseen mechanism causes the plastic mantle rocks to become fluid and rise. As the plume rises, it is difficult to know what influences the process most: heating, decompression, or chemical reactions. All of these likely play some sort of role.

So we know where the magma comes from and roughly how it forms.

Ascent through the mantle and asthenosphere

Most diagrams of volcanoes beneath the surface show pockets of magma rising upward like bubbles reaching toward the top of a lake. Is this really the way it happens, though? The answer is yes and no. Yes in that the structure of it is probably rather bubble-shaped. No, however, in that the “bubble” is probably not composed of pure magma (in the mantle anyways). For most of the magma’s trip through the mantle and asthenosphere, it travels like water through the ground: between grains and through cracks. On a microscopic level, the magma makes its way up by traveling between grains and by granular compaction. Both processes are similar; when traveling between grains, the grains of mantle rock/mineral remain stationary but loosely packed, allowing magma to flow between them. Granular compaction means that there is much more fluid moving up. Here, the grains are moving downward as they compact together. As the grains move down, the magma is forced up. As the grains compact, magma is squeezed out from between the spaces. Thus, lava makes it’s way up on a microscopic scale once more.

On a larger scale, magma can flow through cracks in the mantle rock. Because much of the mantle and asthenosphere is plastic (bendable) there are probably very few such cracks for magma to flow through. Thus, while it’s a possible mode of transport, travel through fractures is probably minimal or nonexistent in most cases.

The driving force for the magma’s ascent from this point onward will be its lower density. Because it is a fluid, surrounded by heavier rock, it will make its way upward until it cools and hardens, or reaches the surface. As it approaches the crust, however, the surrounding rocks become cooler and the process is forced to change in order to survive.

Ascent through the crust and chemical differentiation

As the magma moves toward the surface, the cooling rock becomes rigid. Mineral grains are cemented together, meaning that intergranular flow is now restricted, if not totally cut off. This also means that other mechanisms for the transport of magma must come to the forefront. The previously discussed fracture flow will become more important since the rigid crust is capable of breaking along planes of movement. While this is probably significant, its importance depends, to a large degree, on the style of volcanism. At spreading centers, cracks and zones of weakness within the crust play an enormous role; spreading at the surface creates numerous cracks and weak zones through which the magma travels. Following the path of least resistance, it now has to push and melt its way up. The process of “melting” its way is a complicated one, however, and requires a bit of chemistry.

Rocks in the crust are not at all homogeneous; that is, they are composed of many different types of minerals, which all have different melting points and flow behavior. As the ascending lava works its way upward, the newly melted top is incorporated with the rest of the body. Now we really do see the bubble or tear-shaped pockets of magma rising. The newly melted material usually has a higher melting temperature than the rising magma, so it makes things a bit messy (the reason for this will be discussed at the end of this section since it is a cyclic pattern). The old rock is melted and mixed with the rising magma body. This action has now changed the chemical composition of the body as well as the temperature (the magma is cooling). As the body rises, the temperature drops below the melting temperature of some types of minerals. These begin to crystallize and start a bi-directional movement; that is, the crystalline bodies move both up and down. They move up relative to the crust, pushed by the momentum of the rising magma. They move down, however, relative to the magma body itself. This is because the crystallized minerals are more dense (they’re solid) than the surrounding the liquid. In short, the new minerals are still moving upward, but not as quickly as the rest of the surrounding magma.

Perhaps the most important impact of this process is the addition of silica to the mix. The melted rocks often contain large quantities of silica, which is readily dissolved into the magma. Other minerals, with higher solidus temperatures, will precipitate and drop toward the base of the rising body. The additional silica, however, raises the viscosity, which is important for later.

That’s not very important for continental rift volcanism, however. The magma follows cracks in the Earth and rises through the thinnest layers of crust. While the magma body must melt its way up, like the other situations, the effects on the eruption are far less dramatic. Lava erupted from rifts can be remarkably unaltered relative to their original mantle source. We do see an important change in eruption style, however. When the volcano first forms (as seen in Paricutin), an intial explosive/silicic eruption transpires. This is because the top of the magma body has been melting its way toward the surface, through silica. Hence, the top is relatively silica rich and erupts violently. As the eruption progresses, however, the style transforms to an effusive mode. This is due to the middle section of the body making its way up, relatively unaltered by the trip. During this time, you may have phenocrysts coming up with the lava. Minerals already precipitated, are erupted with the fluid lava; this can include a range of minerals, but is most frequently olivine (as this has a high solidus temperature). Also affecting magma chemistry, however, is the exsolution of gasses. In basaltic mixes, bubbles are free to migrate due to the low resistance to flow. In more viscous mixes, however, the exsolution of gas (opposite of dissolving/dissolution) can have a large impact on the chemical processes within the body (these are discussed in greater detail in the volcanic gasses article).

A similar situation exists for hotspot volcanism. Over oceans, the activity is always basaltic in composition. This is due to the massive magma body pushing its way through thin crust. The silicic beginning to the activity went completely unnoticed, in all likelihood, as it occurred deep under the ocean. On land, however, hotspot volcanism can be very explosive (as seen in the historical eruptions of Yellowstone) due to plate thickness.

Subduction zones are where the truly complicated action happens. We first had our magma rising like bubbles and tears off of the deeply buried plate. Where this activity begins is up to debate, but it likely occurs deep…possibly near the d-zone even. We traced the magma’s path upward, and it is now reaching the bottom of the crust. At this point, the composition of the magma probably does not differ too significantly from rift and hotspot magma. Here, however, the magma begins melting its way up through continental crust that is two or three times thicker than that of the oceans. Moreover, while the ocean basins are all basaltic in composition, the continents contain many types of rock including sedimentary, metamorphic, and older basaltic or silicic lavas. This means that the chemical input via melting is much more diverse and will probably involve much more silica. The trip upward takes a great deal longer, as well. The boundary between hot mantle and cold surface is much thicker now, and many rising magma bodies simply never make it to the surface. These are called plutons…potential volcanoes that never surfaced. As the magma works its way upward, it continues cooling dramatically. Silicic magmas are almost always much colder than their basaltic counterparts; this is because the silicic magmas spend more time climbing through the cold crust to reach the surface. Ironically, however, this is the exact same reason that the magma is silicic to begin with!

Chemical differentiation is extremely dramatic in this situation. While it may be a scary sounding word if you haven’t worked with it, the definition is pretty simple. This is the separating and grouping of various chemicals within a mixture. By this, we refer to the growing and precipitation of minerals in the magma body. With the diverse mineral input, along with the dramatic cooling, not all chemicals can stay fluid. As molecules interact, they combine to form distinct structures called minerals. If this mineral assemblage is below its solidus temperature (the temperature at which the substance becomes solid) then it hardens to form a permanent/semi-permanent mineral. As the particle floats through the sea of magma, it grows through nearby molecules latching onto it. This process is repeating itself billions of times over, and producing clouds of these minerals, which slowly drop toward the bottom of the magma body. Different minerals have different solidus temperatures, which means they don’t all drop out at once. They occur at distinct intervals so you actually have zoning. This can be seen in plutons, where zones of minerals layer the entire magma body. For a description of the minerals, you’ll have to read the section on ores and mining. As the minerals precipitate out, however, they have a bonus effect. By removing themselves from the mix, they have made the magma less viscous, allowing it to travel more freely. This system of energy conservation is one of the only reasons magma can actually reach the surface through continental plates!

As the body continues to rise, more minerals precipitate until there’s not much left before silica. As the silica cools, it begins to dramatically raise the viscosity of the entire mix. If it becomes too viscous, then the body has sealed its own fate and will become a pluton. If it reaches the surface just prior to this event, however, the magma will by a rhyolite. This type of magma can be very explosive due to its high gas content, but usually not. What does-in the explosion is the high viscosity of the lava. While you need the bursting of strong bubble walls to produce an explosion, rhyolite is usually too viscous to erupt; that is, the bubble walls have become so strong that they simply will not rupture. Rhyolite can be squeezed out like toothpaste, however, and form unstable structures. When these collapse you can have landslides or an explosion. Where the magma has punched through earlier, you can have dacite, which can be extraordinarily explosive…or docile (for the same reasons as rhyolite). If the crust is a bit thinner, you have andesite…like in the Andes. There is often a progression in eruption style, as well. This is similar to what happens in new basaltic volcanoes, where the extra-silicic top gives way to the more unaltered interior of the magma body. Any more detail than this, however, becomes site-specific and extremely complicated.

Conclusion

So we’ve covered quite a bit in this article. We’ve talked about the formation of magmas deep within the earth. We’ve looked at their ascent through the mantle, followed by their ascent through the crust. It’s not necessarily a direct trip, however; for information on magma chambers, you’ll have to read the article about them. Hopefully you have a better understanding for the mechanics of volcanoes, and the very reason for which they exist. For additional information on the subject, I would suggest consulting scientific journals and textbooks. I hope you enjoyed the article.