This year marks the 40^th anniversary of the May 18, 1980 eruption of Mt. St. Helens. This eruption and associated landslide, one of the more significant in U.S. history, killed 57 people, removed 2.5km³ of the mountain (enough to fill almost a million Olympic swimming pools), and decreased its height by nearly 400m. See Figure 1.¹ The eruption sent ash as high as 19km (11 miles taking it into the stratosphere) and scattered 5.2 x 10⁸ tons of ash across an area of 5.7 x 10⁴ km² of the U.S., an area slightly smaller than the size of West Virginia.¹⁶

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Figure 1. Mt. St. Helens viewed in 2008 from the memorial at Johnston Ridge. (photo by author)

This and similar events provide opportunities for discussion of the science associated with volcanic eruptions. Many people are familiar with the classic vinegar-baking soda demonstration. In this demonstration, acetic acid solution and sodium hydrogen carbonate react to produce carbon dioxide gas, which bubbles up to erupt simulated lava from the container in which they were mixed.

H⁺(aq) + HCO₃^-(aq)  →  CO₂(g) + H₂O(l)

Lesson plans and science kits have been developed based on this vinegar-baking soda demonstration.^2,3 Reaction of 2.5cm³ (level ½ teaspoon) of powdered baking soda with the acids in tomato ketchup from a condiment packet produces a red, bubbly mixture reminiscent of more viscous lava (more about lava viscosity below). A reaction between citric acid and sodium hydrogen carbonate in effervescent antacid tablets produces gaseous carbon dioxide to pop the lid from a film canister and scatter glitter to represent an ashfall.⁴

Before continuing let's clarify some geological terms. Magma is melted rock underground and lava is melted rock above ground after erupting.¹ Volcanic “ash” is not produced from combustion, but is ejected rock fragments from the volcano (tephra) that have sizes smaller than 2mm in diameter.¹ Figure 2 below shows ash collected at various distances from Mt. St. Helens.

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Figure 2. (LEFT) Ash collected at various distances from Mt. St. Helens. (top) 400 km (middle) 35 km (bottom) 8 km (Scale marks are in mm). (RIGHT) Two lava samples from the Mt. St. Helens area. Both contain bubbles from trapped volcanic gases, but the pumice at top contains so much trapped gas that it floats on water. (photos by author)

Flows comprised of rock fragments combined with water are referred to as lahars; flows comprised of rock fragments combined with hot gas are referred to as pyroclastic (fire-broken) density currents.¹

Gas plays an integral part in most volcanic eruptions, and the gas output from volcanoes have been monitored in attempts to predict eruptions.^3,5,6 However, these gases are not produced by an acid-base reaction like in the vinegar-baking soda demonstration or in the catalytic decomposition of hydrogen peroxide. Rather these gases are volatile species outgassing from the liquid magma as the downward pressure on the magma decreases. This is much like carbonated soda in a closed pressurized container- can or plastic bottle- releasing carbon dioxide gas bubbles when opened. In a closed carbonated soda bottle, high gas pressure in the headspace above the liquid keeps the CO₂ dissolved in the water, much like the pressure deep below a volcano keeps volatiles dissolved in molten rock. When the soda bottle cap is opened, the decrease in headspace pressure above the liquid enables outgassing of the dissolved CO₂ to take place, CO₂(aq) → CO₂(g). This is Henry's gas law.

Both magma and soda outgassing are examples of Henry’s law, which describes how gas solubility in a liquid decreases as the pressure above the liquid decreases. Though magma compositions vary widely, the three main gases dissolved in liquid magma under pressure and then outgassed are typically water vapor, followed by carbon dioxide and sulfur dioxide.^5,7 In a continental-margin volcano such as Mt. St. Helens, some of the water in the magma likely came from under the ocean, as oceanic portions of the Earth’s crust are forced under continents (subducted) by continental drift. Liquid water released from subducted minerals depress the rock's melting temperature, forming liquid magma, which rises as it is less dense than solid rock.⁵ This solution chemistry is an example of freezing/melting point depression with liquid water as the solute and liquid magma as the solvent. When liquid magma moves upward under the volcano, the pressure on the magma decreases and outgassing occurs. Carbon dioxide outgasses more at lower (deeper) depths with higher pressures, whereas water vapor outgasses more at shallower depths with lower pressures.⁸ The pressures involved are enormous, with simulations predicting that outgassing can occur even at 1500 atm.⁸ Many samples of hardened lava contain bubbles of various sizes from those volcanic gases (see Figure 2).

Let's take a moment to talk about two volcanic gases. First, CO₂, a well-known greenhouse gas. Confusion sometimes exists in the mind of the public as to how the atmospheric concentrations of CO₂ have fluctuated in the past when humans were not around to emit CO₂ via fossil fuel emissions. Some might seize upon this misunderstanding to cast doubt on climate science. Therefore, a discussion of processes that contributed to past CO₂ fluctuations is beneficial. The demonstrations discussed in this article can be used as a platform or springboard to such a discussion, noting that volcanic emissions have long been a source of atmospheric CO₂ over geological time scales. Furthermore, the chemical reaction below can be used to describe how atmospheric CO₂ is removed from the atmosphere and 'locked' as a solid into the lithosphere:⁹

CaSiO₃(s) + CO₂(g) → CaCO₃(s) + SiO₂(s)

Sulfur dioxide produced by volcanic eruptions also impact climate, though on shorter time scales. The sulfur dioxide produced by the catastrophic 1991 eruption of Mt. Pinatubo in the Philippines cooled the Earth by about 0.6^oC for about 15 months.¹⁰  

In addition to gas composition and degree of saturation, liquid magma varies in composition of dissolved nonvolatiles, density, temperature, and viscosity. The physical property viscosity (= resistance to flow) can have a significant impact on a volcano's eruptive behavior, and itself can depend on chemical composition, temperature, the presence of undissolved crystals, and the presence, size, and deformability of gas bubbles.⁵ Liquid magma is quite viscous; for example, at 1400K some forms can be a billion times more viscous than water.⁵ Magma viscosity increases as temperature decreases, like molasses or motor oil becoming more difficult to pour at lower compared to higher temperatures. Silica (SiO₂) content infuences molten rock viscosity. Magma with less than 52 wt% silica is classified as basic and tends to have low viscosity, yielding lava like that produced by the volcanoes in Hawai’i.⁵ Solidified lava flows of this type, about 2,000 years old, have also been found at Mt. St. Helens.¹ To illustrate, if a short, wide diameter candle is slowly heated from below and near one edge on a hot plate or electric stovetop, the melted wax, which has no gas dissolved in it to aid its buoyancy, rises up due to thermal expansion- another physical property- from a sort of vent and upon reaching the vent's end at the surface flows across the candle's top like low-viscosity lava. Figure 3. below shows this.⁴ 

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Figure 3. (LEFT) Candle wax heated from below, right side. The less dense liquid wax rises and flows across the candle top to resemble a low-viscosity lava flow. (RIGHT) As the wax cools it solidifies and contracts back into its vent. Demonstration adapted from Reference 4. (photos by author)

When hot liquid magma has a high silica content, silicon atoms can connect with each other more via covalent bonding to oxygen atoms to form a network of atoms less flexible than without bonding. This covalent bonding network restricts flexibility and as a result imparts a greater viscosity to the magma. These lavas, classified as intermediate or acidic, can be pasty and can pile up to form lava domes, like the ones that have arisen in the crater of Mt. St. Helens after the May 18, 1980 eruption.⁵ Violent eruptions can be produced when silica-rich, volatile-rich magma rises up in a volcano, forming bubbles which can dramatically increase the magma viscosity still further. The magma comprised of hot liquid with bubbles can eventually break up into gas carrying pyroclastic lava fragments, which spew from the volcano at supersonic speed.⁵ This can produce towering columns of ash seen in eruptions at Mt. St. Helens and other volcanoes and can be demonstrated using the demonstrations described below.

As noted above, outgassing from molten rock in a volcano bears some resemblance to outgassing from carbonated soda. Promoting nucleation of gas bubbles in a bottle of soda by shaking it, or dropping Mentos candies into the bottle of soda have been described as ways to demonstrate volcanic eruptions.^7,11 These relatively safe and inexpensive demonstrations are suitable for illustrating volcanic eruption concepts to an all-ages “pre-K through gray” audience. What follows are a few ideas using soda demonstrations that could provide additional understanding of volcano behavior.

Demo 1: Low-Viscosity Lava Flow Effects Using Carbonated Soda

The release of liquid carbonated soda from a closed container can represent the release of lava in a volcanic eruption. It is easy to place the carbonated soda source in a cone shaped structure to represent a volcano. See Figure 4. In this case there does not have to be a spectacularly large soda eruption; the soda foam can simply well up out from of the source (perhaps accompanied by a small fountain) and flow down the cone. The cone can be made from common materials such as sand or snow. Even the top of a soda bottle is roughly shaped like a volcanic cone. Soda from a variety of sources, (e.g., plastic bottles, glass bottles, even aluminum cans) can be used. Initiating the eruption could be performed by shaking the soda before opening the container, but obviously one cannot shake the soda once it is in the cone. Another option is to add a bubble nucleation source to the soda after opening the container. For plastic bottles, Mentos candies are well known to nucleate bubbles.^12-14 For smaller, oozing eruptions, fewer Mentos candies are needed. Soda cooled in a refrigerator or that is a little old and flat (= less carbonated) can also produce flows down the cone's sides. The video link for an example of this demo is https://www.youtube.com/watch?v=_laceLVdf4U

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Figure 4. (TOP) Pouring sand into an aluminum soda can placed in a cone of snow and the resulting “lava” flow (BOTTOM) Adding Mentos candies into a 2L soda bottle placed in a cone of sand and the resulting “lava” flow. (photos by author)

If the soda container 'mouth' is too small for whole Mentos candies, smaller pieces or other nucleating agents such as sand can be used. Sand is sufficiently dense to sink into the soda and generate nucleation of many bubbles.¹⁵ The sand can be added using a paper or plastic tube (e.g. a bubble tea straw) and if the cone is made from sand, then there should be plenty readily available. Again, in these examples, the soda represents liquid lava flows. Soda flowing down a sandy cone leaves behind a muddy deposit, which somewhat resembles a lahar. Caution is advised for this analogy since it is based on water in erupted soda mixing with sand in the cone, whereas real lahars often result from tephra erupted by the volcano mixing with water from other sources. Large soda fountains coming from a relatively small volcano cone model are not really scaled correctly for demonstrating most low viscosity lava eruptions, as lava fountains are not typically several times the height of a volcano. However, the larger soda fountains do have an application in the ash examples in the following demo.

Demo 2: Pyroclastic Effects Using Carbonated Soda

The release of soda from a closed container can represent the movement of pyroclastic materials, rather than fluid lava, in a volcanic eruption. Vigorous soda foam fountains represent Plinian eruptions. These violent eruptions are named after Pliny the Elder, who was killed during the eruption of this type at Mt. Vesuvius in 79 CE that buried Pompeii.⁵ In a Plinian eruption, a large ash column is thrust high into the atmosphere, initially by the force of gas and pyroclastic material leaving the vent in the crater, but also due to the buoyancy of the hot gas. When plume density equals atmospheric density, the ash cloud spreads out into an umbrella shape.

If the column loses its upward thrust, the pyroclastic cloud collapses downward and fan out down the volcano in pyroclastic density current called a pyroclastic surge.⁵ This is similar to base surges (debris clouds produced at the base of some nuclear explosions) and downburst phenomena associated with thunderstorms. Cloud motions simulating these surges can be produced when liquid nitrogen is added to a warm, narrow mouth container.⁴ With the soda demonstration, the eventual collapse of the fountain represents the collapse of the eruption column, with soda flowing down the bottle and outward representing the pyroclastic surge.

As in Demo 1 above, the fountain demonstration is initiated by bubble nucleating disturbances, using Mentos candies or dense powders like sand, to well-carbonated soda. In the demonstration shown in Figure 5, nearly 400mL of the original 590mL of carbonated soda erupted back out with the addition of sand. Sand not only initiates a fountain, but some is carried back out of the bottle during the eruption. This sand can, in addition to the soda itself, represent tephra produced in an explosive eruption. The sand from the soda fountain can be captured on an absorbent white towel (use colorless soda to best see the deposit). The sand deposit can represent the tephra distribution around an eruption. See Figure 5. The deposit can sometimes appear to be thicker and have larger particles closer to the bottle than further away, again similar to tephra deposits from volcanic eruptions. The May 18, 1980 eruption of Mt. St. Helens sent ash as high as 19km and scattered 5.2 x 10⁸ tons of ash across 5.7 x 10⁴ km² of the U.S.¹⁶ The following link is to a video showing this demo, https://www.youtube.com/watch?v=sZ88DvhYIao

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Figure 5. (LEFT) Pouring sand from a bubble tea straw into a 500 mL soda bottle placed on a white beach towel. (MIDDLE) Fountain produced by eruption of the soda that in part collapses down the sides of the bottle. (RIGHT) Deposit of sand left behind after the soda soaks into the towel. (photos and associated video by author and Kathryn Campbell)

Conclusion:  The science of volcanology is a complex and fascinating discipline, which has many connections to chemistry topics taught in a typical general or AP chemistry course. Connections range from simple concepts like density, viscosity, Henry's law, and melting temperature depression to more complex topics related to inorganic, atmospheric, and environmental chemistry. Volcanoes vary widely in their properties and eruptions. A composite volcano like Mt. St. Helens can have varying eruption behavior from one eruption event to the next.¹ There is a large amount of information available about the May 18, 1980 eruption of Mt. St. Helens that was not covered here (including the massive avalanche and the lateral blast). It is highly recommended that you safely visit this and/or other volcanoes if you get the chance. Although soda eruptions are not the same as actually being at a volcano, it is hoped that the demonstrations described above will provide a deeper appreciation of some of the aspects of volcano behavior, all at much safer scales than real eruptions. And the 1993 eruption of Galeras in Columbia that killed and injured several scientists has been used for presentations about scientific ethics in chemistry courses.⁶

Safety:  Proper personal protective equipment (PPE) such as goggles are recommended, ESPECIALLY if vertically rising fountains contain entrained particles like sand. Soda containing pigments can stain ceiling tiles, carpeting, and other surfaces. It is recommended that a tray or plastic sheet be placed under the demonstrations to help contain spilled materials and aid in fast cleanup.

Acknowledgments:  This work is dedicated to the 57 people killed and other people adversely impacted by the May 18, 1980 eruption at Mt. St. Helens. This work was supported by Bradley University and the Mund-Lagowski Department of Chemistry and Biochemistry with additional support from the Illinois Heartland Section of the American Chemical Society and the Illinois Space Grant Consortium. Thanks very much to Thomas Kuntzleman in the Department of Chemistry at Spring Arbor University and Scott Donnelly in the Department of Chemistry at Arizona Western College for lots of helpful discussions, especially on the connection of these experiments to climate change, general chemistry connections, and technical assistance. Thanks also to Kathryn Campbell for assistance in photographing and making movies of the demonstrations. The author also thanks his family for putting up with his lifelong fascination with volcanoes (Figure 6 below).

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Figure 6. (LEFT) Picture of a volcano (and some cacti?) that the author drew in kindergarten. The yellow is supposed to represent a conduit and magma chamber. (RIGHT) View into the crater of Mt. St. Helens early September  2004 not long before the beginning of a new eruption cycle. View is looking north toward a lava dome with surrounding glacier, Spirit Lake, and Mt. Rainier (upper right).  

References:

1.   Pringle, P. T. Roadside Geology of Mt. St. Helens National Volcanic Monument and Vicinity, 2nd ed. Division of Geology and Earth Resources Information Circular 88; Washington Department of Natural Resources, Olympia, WA, Firefly Books, Inc.: Buffalo, NY, 2002.

2.   Balicki, S. Making a (big) eruption with chemistry. ChemEd Xchange. https://www.chemedx.org/blog/making-big-eruption-chemistry (accessed May 2020).

3.   Smithsonian Giant Volcano Kit Instructions. Smithsonian Institution Natural Science Industries, LTD.: West Hempstead, NY, 1997.

4.  Campbell, D. J. Demonstrations Page 9 – Geology. http://bradley.bradley.edu/~campbell/demopix9.html (accessed May 2020).

5.   Rosi, M.; Papale, P.; Lupi, L.; Stoppato, M. Volcanoes. Firefly Books, Inc.: Buffalo, NY, 2003.

6.   Bruce, V. No Apparent Danger: The True Story of Volcanic Disaster at Galeras and Nevado Del Ruiz. HaperCollins Publishers Inc.: New York, 2001.

7.   U. S. Geological Survey. Soda Bottle Volcano. Living with a Volcano in Your Backyard- An Educator’s Guide. https://pubs.usgs.gov/gip/19/downloads/Chapter_1/Activities/SodaBottle_v... (accessed May 2020).

8.  Chiodini, G.; Paonita, A.; Aiuppa, A.; Costa, A.; Caliro, S.; DeMartino, P.; Acocella, V.; Vandemeulebrouck, J. Magmas near the critical degassing pressure drive volcanic unrest towards a critical state. Nat. Commun., 2016, 7, 13712. doi: 10.1038/ncomms13712.

9.  Dressler, A. Introduction to Modern Climate Change, 2nd ed.; Cambridge University Press, Cambridge, 2016.

10.  NASA Earth Observatory. Global Effects of Mount Pinatubo. https://earthobservatory.nasa.gov/images/1510/global-effects-of-mount-pi... (accessed May 2020).

11.  Weather Wiz Kids. Soda Bottle Volcano. https://www.weatherwizkids.com/experiments-volcano-soda-bottle.htm (accessed May 2020).

12.  Kuntzleman et. al, Kinetic Modeling of and Effect of Candy Additives on the Candy-Cola Soda Geyser: Experiments for Elementary School Science through Physical Chemistry J. Chem. Educ., 2020, 97, 283–288.

13.  Kuntzleman, T. S.; Davenport, L. S.; Cothran, V. I.; Kuntzleman, J. T.; Campbell, D. New Demonstrations and New Insights on the Mechanism of the Candy-Cola Soda Geyser J. Chem. Educ. 2017, 94, 569–576.

14.  Kuntzleman, T. S.; Nydegger, M. W.; Shadley, B.; Doctor, N.; Campbell, D. J. Tribonucleation: A New Mechanism for Generating the Soda Geyser. J. Chem. Educ., 2018, 95, 1345-1349.

15.  Liljeholm, A. “Diet Soda and Iron Filings.” Am. J. Phys., 2003, 77, 293.

16.  Brantley, S. R. Volcanoes of the United States. U.S. Government Printing Office: Washington, DC, 1994.