Most schools in the U.S. are far from active volcanoes, and few students have the opportunity to witness eruptions. Simulations of eruptive processes, or analog models, provide ways for students to visualize eruptive processes and apply basic physical principles when field observations are not feasible. In this paper, we describe a safe simulation of violent volcanic explosions, one that can be carried out simply and easily as a demonstration for specialized volcanology classes, introductory classes, and science outreach programs.
Volcanic eruptions are fundamentally gas-driven phenomena. Depressurization of volatiles dissolved in magma during ascent is the driving force behind most explosive eruptions. Furthermore, phreatomagmatic eruptions result from the conversion of water to steam during magma-water interaction. During an eruption, the exsolution and expansion of gas causes ascent velocity of the magma to increase. When the volume fraction of bubbles exceeds about 80%, the magma fragments explosively and is carried by a gas stream (e.g. Sparks, 1978).
We have developed a demonstration whereby the instructor can initiate a gas-driven eruption, which produces a dramatic but safe explosion and eruptive column. First, one pours liquid nitrogen into a weighted, plastic soda bottle, which is then sealed and placed into a trashcan filled with water. As the liquid nitrogen boils, the pressure inside the bottle increases until it fails, resulting in an explosion. The expansive force propels a column of water vertically, to 10 or more meters. Because liquid nitrogen is thermodynamically unstable at room temperature (boiling point at 1 atmosphere: -195.8°C), its boiling provides the pressure necessary to cause an explosion, illustrating an important process that drives real volcanic eruptions (e.g., Francis, 1993).
As with most simulations, this one is imperfect. Unlike magma, the gas does not exsolve from the liquid, and consequently it is not dispersed throughout the water prior to explosive expansion. Also, there is no real transfer of heat during the eruption. Nevertheless, it is an exceedingly effective demonstration of gas-driven liquid explosions and one that is safe if done properly.
Most of the supplies necessary for this demonstration are common items; liquid nitrogen is the only material that may be difficult to obtain, but it is frequently used in science demonstrations (e.g., Nolan and Gish, 1996; McRae et al., 2002) and can be obtained on most college campuses or from gas supply companies at relatively low cost. Here we describe the details of the procedure (Table 1 includes further equipment details):
IMPORTANT SAFETY ISSUES:
Be sure to inform all participants of the potential dangers of contact with liquid nitrogen (e.g., Young, 2003) and the risks of sealing liquid nitrogen in closed containers. The Material Safety Data Sheet on liquid nitrogen is available at http://msds.ehs.cornell.edu/msds/msdsdod/a97/m48467.htm;
Metal garbage cans are inappropriate for this demonstration, because they simply rupture along the welded seams. In most cases, we use a high-grade, plastic contractor’s trashcan (Table 1).
Only two people should carry out the demonstration. They must have clothing covering their legs, arms, and feet and they must wear safety glasses and gloves throughout the process.
Prior to handling any liquid nitrogen, the demonstrators should do a practice run of the entire procedure. We recommend that the two participants rehearse their roles several times until everyone is confident of their part in the demonstration process, without hesitation.
Make sure the audience members remain at least 5 meters from the trashcan at all times.
Fill the trashcan about 80% full of water. This is about 95 L (25 gallons), nearly 100 kg, thus the trashcan should be filled at the exact site where the explosion is to take place. The demonstration is more impressive if the trashcan is located on a horizontal paved surface, not grass, and never on a cart.
Secure a 0.5 to 1 L plastic soda bottle to 1-2 bricks using duct tape around the middle of the bottle. Align the bottom of the brick(s) with the bottom of the soda bottle, so that the brick-bottle apparatus stands freely. Ensure that the weight of the brick(s) is sufficient to keep the bottle submerged by performing a practice run with an empty bottle. The sealed bottle and brick must submerge completely.
Set the open bottle and attached brick upright on the ground with the funnel in it. Have one individual hold the cap, so that it can be placed on the bottle quickly.
One person holds the funnel far enough out of the bottle opening to make an air outlet for the gas, otherwise boiling liquid nitrogen splashes out, and the bottle will not fill efficiently.
The second person pours the liquid nitrogen through the funnel into the bottle until the bottle contains approximately 20-30 mL of liquid nitrogen (i.e., 2-5 cm depth from the bottom). The amount need not be precise, and much of the nitrogen boils away as it is poured.
Time is of the essence at this point. The person holding the funnel should toss it aside and cap the bottle tightly. Be sure not to cross-thread the cap. It is critical that the cap be finger tight, so the bottle is well sealed.
Within no more than 5 seconds, one of the demonstrators should pick up the bottle/brick apparatus and drop it gently in the garbage can. Try to put the bottle in the center of the can and not against a side, where it is more likely to rupture the plastic walls.
After immersion of the N2(l)-filled bottle and brick, the two people performing the demonstration should walk away quickly and wait, at least 5 meters from the can. There is a delay of approximately 10-30 seconds before the explosion, so they do not need to run. The liquid nitrogen undergoes a phase change into gas. In the absence of the confining bottle, the small amount of liquid in the soda bottle would expand (at standard temperature and pressure) to well over 20 L. Because they are built to sustain overpressures from carbonated drinks, the bottle resists the expansion while pressure builds. Eventually, the bottle ruptures, and the force of the gas expansion passes into the water, resulting in an eruption column of water several meters high. The garbage can often makes an impressive jump as well.
If the bottle does not explode within the anticipated 10-30 seconds, do not approach it; we have seen the rare event where the bottle is particularly strong and resists for close to a minute. If you see vapor bubbling out of the can (which is just escaping nitrogen), then there is a leak in the cap or bottle, and it may or may not explode. There is a tendency for impatience at this point, but under no circumstance should anyone approach the trashcan until they are positive that the bottle has failed and all of the N2(l) has boiled away, and then only the demonstrators (with covered shoes and safety glasses) should go to the trash can. In our experience, the most common mistake is that the cap is not put on properly, which is not a dangerous situation.
Pedagogical Uses of the Eruption Column Demonstration
Introductory Level and Community Groups
At the simplest level, the eruption column demonstration illustrates gas-driven eruptions, dispelling the common misconception that there is some kind of fire or hot explosive involved in volcanic eruptions. Because the blast is short-lived, it is best to explain the procedure and its significance first, then carry out two detonations, so people can watch the second more carefully after the surprise of the first explosion. We have performed this experiment for groups as large as 500 people, provided there is an outdoor amphitheater of some kind. We routinely carry out the demonstration for students of all ages.
A modification of the demonstration simulates the famous lateral blast at Mt. St. Helens on May 18, 1980. An inexpensive trashcan will usually fail along one of the molded seams, propelling some of the water laterally.
At both Colgate and the University of Idaho, we have expanded this demonstration into a series of guided inquiry investigations (e.g., Smith et al., 1995; Beiersdorfer and Beiersdorfer, 1995) with a small class (20-35 students) of advanced students. Each group of 3-5 students has a trashcan full of water, a selection of bottles, material to simulate tephra (Table 1), and a measuring tape. They go through a detailed safety orientation covering liquid nitrogen safety prior to embarking on the exercise. The groups then design their own experiments to address the following problems, or others they generate in the investigative process:
a. By measuring the height of the eruption column, students can use basic physics to calculate the ejection velocity of the water propelled from the trashcan and compare the results to the velocities observed at actual volcanic eruptions.
b. Using their calculated ejection velocities, students can then determine the pressure needed to propel the water column using the modified Bernoulli equation: ½ U2 = (Pi – Ps) / S, where U is the ejection velocity in m/s, Pi is the reservoir pressure (in Pascals, Pa), Ps is atmospheric pressure (Pa), and S is the magma density (the density of water in kg/m3). The students can once again compare their results to observations from real volcanic eruptions.
c. Students can use the estimated volume of gaseous nitrogen in the soda bottle (via the ideal gas law) and the volume of water in the trashcan to calculate the average vesicularity prior to eruption, which they can then compare to theoretical estimates of fragmentation (Sparks, 1978). The density of N2(l) is ~0.807 g/cm3 under standard conditions.
d. Styrofoam peanuts or spheres of different sizes, such as tennis balls and apples can be used to simulate ballistic pyroclasts. After the eruption, students can construct isopleth maps of the clasts. The different densities and aerodynamic shapes of the “clasts” reproduce the distributions of volcanic bombs, blocks, and cinders.
e. Different vessels for the demonstration affect eruption style as well. For instance, a small rigid-sided wading pool generates a base surge and a shorter eruption column. As mentioned above, a cheap soft-sided trashcan is likely to fail along its seams, yielding a lateral blast.
This simulation operates on physics similar to that which drives volcanic eruptions, but on a scale accessible to students. Furthermore, the simulation permits interaction and experimentation with the driving forces behind eruptions, more so than can be accomplished by narrative or video footage. This demonstration has repeatedly proven to impress students of volcanology ranging from first-graders to professors (including a group of high school and college physics instructors at a national physics conference). In addition to being visually impressive, it provides a safe means to illustrate and explore explosive volcanic phenomena.
Table 1. Supplies for Eruption Column Demonstration
Basic Version of Demonstration
¨ Industrial strength plastic garbage can (a “contractor’s” trashcan, usually ~$35-40 at most home supply stores); these are generally re-usable. Avoid small buckets or other light plastic containers, as well as metal cans; all of these will be destroyed in the blast and the metal cans will split along their seams and possibly send out shrapnel.
¨ Soda bottles (16-20 ounce size is most convenient, but the 2 L bottles are equally effective) with their caps. Water bottles generally do not yield the same magnitude eruption, because they are not as strong as soda bottles and fail before pressure builds up significantly. One bottle is needed per demonstration, and several back ups should be on hand.
¨ Duct tape
¨ 2 bricks or a large cinder block (must prevent the soda bottle from floating)
¨ Hose from faucet to site of detonation (a full trashcan is difficult to move to a new location)
¨ Safety glasses for all participating individuals (usually 2)
¨ Insulated gloves for liquid nitrogen protection (two pair per demonstration set-up)
¨ The people performing the demonstration must wear closed-toe shoes, long pants, and long-sleeved shirts
¨ Liquid nitrogen (~20 mL per explosion) in a Dewar
¨ Funnel for pouring the nitrogen, preferably plastic
Lateral Blast (e.g., Mount St. Helens)
¨ Use an inexpensive plastic, residential curbside trashcan with indentations in the side ($8-12; e.g., Falcon 32-gallon molded black plastic “heavy duty” can); these generally fail in the explosion, resulting in a lateral blast, but the can is obviously not reusable. Stronger cans that have been damaged or have cracks in them also work for this version of the demonstration.
Distribution of Tephra
¨ Various floating objects that are neither sharp nor hard (apples, half-filled water balloons, Styrofoam packing peanuts or other non-water soluble packing material, tennis balls, etc.), placed in the can prior to detonation
¨ Tape measure
Beiersdorfer, R.E. and Beiersdorfer, S.I., 1995, Collaborative learning in an advanced environmental-geology course, Journal of Geoscience Education, v. 43, p. 346-351.
Francis, P., Volcanoes: A Planetary Perspective (1993), Hong Kong, Oxford University Press, 443 p.
Liquid Nitrogen Haiku, http://www.goer.org/Journal/2002/Feb/
McRae, R., Rahn, J.A., Beamer, T.W., and LeBret, N., 2002, The Liquid Nitrogen Fountain, Journal of Chemical Education, v. 79, no. 10, p. 1220-1221.
Nolan, W.T. and Gish, T.J., 1996, The Joys of Liquid Nitrogen, Journal of Chemical Education, v. 73, no. 7, p. 651-653.
Smith, D.L, Hoersch, A.L., Gordon, P.R., 1995, Problem-based learning in the undergraduate geology classroom, Journal of Geoscience Education, v. 43, p. 385-390.
Sparks, R.S.J., 1978, The dynamics of bubble formation and growth in magmas: A review and analysis, Journal of Volcanology and Geothermal Research, v. 3, p. 1-37.
Young, J.A., 2003, Liquid Nitrogen, Journal of Chemical Education, v. 80, no. 10, p. 1133.
We would like to thank the Colgate University “Volcano Cowboys” for their unerring dedication to refining the demonstration method, including (but not entirely limited to): David Kolodney, Adam Skarke, Jay Barr, Evan LeBon, Scott Annan, Nathan Rollins, Ashley Nagle, and Vanessa Simpson. DJG would like to thank the forgiving Secret Service agent who visited him when he performed the demonstration on the same day a supreme court justice visited campus.