Volcanoes in the Classroom: A Simulation of an Eruption Column
by Karen S. Harpp, Alison M. Koleszar, and L. Dennis Geist
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.
Pedagogical Uses of the Eruption Column Demonstration
Introductory Level and Community Groups
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.
Table 1. Supplies for Eruption Column 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
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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.