Note: This article originally appeared on our old website: TeamUV.org

Volcanic Eruptions

On Wednesday April 22nd, 2015 the Calbuco volcano in southern Chile erupted for the first time in 40 years, triggering the evacuation of 4,000 people in a 20km (~12.43mi) radius while disrupting air traffic over Chile, Argentina, and Uruguay.  The eruption went on for an hour and a half and resulted in a plume of volcanic ash reaching 10km (~6.21mi) upwards.

At Team UV [EAF], our best wishes go out to those affected by this disaster and we are going to use today’s post to explore the science behind volcanic eruptions, in the hope of promoting a deeper understanding among our readers as to why and how these eruptions occur.

First off, we will be skipping over the science regarding the initial formation of volcanoes (plate tectonics, subduction, convection cells, etc.) in order to facilitate a more concise post/article; we will purely be looking at the eruption itself.  So where do we begin?  The answer is deep within the Earth’s mantle, far below the surface of the volcano.

Building Pressure

The magma that exists deep in the upper mantle is under an enormous amount of pressure, and this pressure keeps the gases contained within the magma [i.e. water vapor (steam), carbon dioxide, sulfur dioxide, hydrogen sulfide, hydrogen chloride, and some additional strong acids] completely dissolved within the magma, meaning the gas molecules are distributed throughout the magma and are essentially embedded within the magma, much like the CO2 bubbles that are dissolved in your soda, that give it its carbonation, and which you do not see until after you have opened your soda, relieving the pressure within the container.  Why is this the case that these gases are dissolved within the magma?  Because the vapor pressure of the gases (the pressure exerted by molecules that escape from a liquid to form a separate vapor phase above the liquid surface; think of this as the pressure at which the molecules boil or evaporate out of the liquid) is lower than the surrounding confining pressure (the pressure exerted on the gases by the surrounding magma and rock, which is highly pressurized).

Okay, so we have now discussed the starting point, but what happens next?  The next thing to occur will be a disruption in the balance between confining and vapor pressure.  As the magma rises, it decreases in depth from the surface of the volcano, which means that it has less and less material stacked on top of it, which translates to a decrease in the confining pressure (the same way water hydrostatic pressure decreases with decreasing depth in the ocean).  As the confining pressure decreases, eventually the confining pressure will drop below the vapor pressure, allowing the dissolved gases to begin to exsolve (emerge out from their dissolved state within the magma) and nucleate/form gas bubbles, or vesciles within the magma (just like when you relieve the pressure in your soda by opening the container, at which point the CO2 bubbles emerge).  As this gas forms and expands, it effectively decreases the density of the gas-magma mixture, because for the same amount of weight as before, the gas-magma mixture takes up more volume than the magma-dissolved gas mixture of before, and density is defined as mass divided by volume.  Since this new magma-gas mixture has a lower density than the magma-dissolved gas mixture, it floats (so to speak) to the top, allowing the bubbles to grow more and more as the magma rises further and experiences lower and lower confining pressures.  In this way, the hot magma-gas mixture begins to rise faster and faster, accelerating as it moves upward.

Side Note: It is important to at least note that there are many causes for the magma to begin to rise in the first place (i.e. convection currents, insertion of new magma to pressurize the chamber further, etc.) and many considerations such as the increase in vapor pressure due to gas-content enrichment with crystallization of cooling magma, that we are not discussing here as they are not vital for understanding the key fundamentals in a concise manner.

Breaking on Through

When the magma, which is moving quite quickly now, reaches the surface cap of the volcano, it can break on through if the cap is not strong enough.  This cap strength is, in part, a reason why volcanoes do not erupt repeatedly day after day (in most cases), as once a volcano erupts, the magma becomes lava, which can solidify the cap once again, potentially trapping the magma and gases for another few decades until the cap has been worn away at and the volcano has built pressure back up & is ready to burst again.  Having said this, a high cap strength can also produce a much more violent eruption, as the amount and pressure of the gases contained in the volcano will be much higher prior to eruption if they have been held back by a strong cap, without any side vents.

So, assuming the cap breaks, and the magma and its gases burst forth and produce lava, and ash (mostly frozen bits of gas and lava due to the transition from inner-volcano temperatures and pressures to the much lower atmospheric temperature and pressure), what decides the type of eruption?  This comes down to viscosity.  Now, you have probably heard the word viscosity before; this describes the resistance to shear strain/deformation in a material (usually liquid) resulting from an applied shear stress.  Viscosity is often described (although, somewhat improperly) as the molecular thickness of a fluid/plasma; so syrup is highly viscous, while water is much less viscous.  The viscosity of the magma is largely determined by the percentage of silica in the mixture, and this viscosity has a direct effect on the type of eruption.

Viscous Dependence

A high percentage of silica translates to a high viscosity for the magma; if the magma is very viscous, it is harder for the gases to escape out from the magma.  In this case, when the volcano does erupt, the gases explode out of the magma with violent force, blowing bits of ash, rock and jets of lava into the air.

If the magma contains less silica and is more basaltic (containing more basaltic crust, which is essentially that found in the Earth’s crust beneath the ocean, which has a lower silica content), the lava will ooze out, as the gases are not strongly trapped within the magma.  This is to say, the gases may freely exit from the magma, which is one reason why these eruptions tend to be much less violent, tend to ooze rather than burst, and tend to bubble more.

Side Note: In case you weren’t sure, the distinction between magma and lava is just essentially that lava is magma that is now external to the volcano.

Volcanic Lightning

So now that we’ve discussed volcanic eruptions, let us just take a quick second to figure out what’s going on with all the lightning shown in the first photo of this post; this lightning is termed volcanic lightning.  There are multiple theories as to why this occurs, but almost all of them accept the following:  as material is ejected out of the volcano, it breaks apart, either due to the explosive power of the eruption or due to collisions.  As the material breaks into pieces, it is thought that differences in the aerodynamics of the positively charged vs. the negatively charged particles causes the two, differently charged particles to separate from each other.  At this point, mother nature does what she usually does when there is a region of negatively charged particles separated from a region of positively charged particles: she bridges the gap by normalizing the charge imbalance with an electrostatic discharge…lightning.  This seems to add another dimension of stunningly intimidating violence to the already destructive eruption.