VGP Spotlight Features

VGP Spotlight, September 2014: Back to School Edition

Welcome to the new VGP Spotlight Features! Each month we will be bringing you a short piece from VGP scientists highlighting some aspect of their research, education, or outreach work in the field. Our inaugural edition is geared for the back-to-school theme, given that many of us are doing just that right now.

Spotlight Feature: Explosive Education!

By Karen Harpp¹ and Alison Koleszar²

1. Geology Department, Colgate University, Hamilton, NY kharpp@colgate.edu
2. Jackson School of Geosciences, University of Texas, Austin, TX akoleszar@jsg.utexas.edu

Ever find yourself teaching volcanology but struggling to convey the power of an eruption to your students? Sure, we can show videos all day long, but it just isn’t the same as witnessing an eruption. What we’re about to explain isn’t the same as being at an eruption either, but it’s a pretty simple, very effective way to inject some volcanic excitement into a class or lab (or just to get a bunch of kids really excited about science)! Below, we present an overview of how to carry out the safe demonstration of an explosive volcanic eruption.

The short version of the demonstration is this: Liquid nitrogen is poured into a soda bottle, which is capped and sunk into a trashcan filled with water (with some alacrity, mind you…!).

Highly skilled volcanology students pour liquid nitrogen into the weighted bottle. We use a metal weight custom-made for us by our machine shop technician, but three bricks duct-taped around the bottle work just fine. After pouring in about an inch thick layer of liquid nitrogen, they will cap the bottle and place it gently into the trashcan full of water (all while wearing safety glasses, of course).

Within 10-15 seconds, the force of the expanding nitrogen propels a column of water 10 or more meters into the air, making a spectacular eruption column. You can watch a (very amateur) version of this here.

Students filling the sealed soda bottle with liquid nitrogen. The bottle is duct taped to a heavy metal weight to ensure that it sinks in the trashcan full of water (photo credit: Matt Loewen).

Classic eruption column (photo credit: Larry Talbert).

Sometimes the force of the explosion propels the trashcan off the ground (especially if the explosion is performed on concrete instead of grass). Photo credit: Matt Loewen.

Sometimes the force of the explosion propels the trashcan off the ground (especially if the explosion is performed on concrete instead of grass). Photo credit: Oregon State University CEOAS

You can find all the instructions for performing this demonstration in the PDF supplied below (from the Journal of Geoscience Education), as well as here.

The beauty of this demonstration is that it can be geared for a wide range of audiences, depending on how they engage with the experiment and how you guide them in their investigations:

1. Of course this is a great demonstration for large introductory classes, when discussing volcanic eruptions.
   
   
2. Young audiences are energized by it, so it makes an excellent ending to an outreach program.
   Styrofoam peanuts generate excellent tephra distribution patterns, but are a hassle to clean up. See another amateur version of a demo with tephra.
   
   
3. In our volcanology classes, we have students perform a range of experiments, varying the amount of liquid nitrogen in the bottles, the bottle style and strength, the amount of water in the can, and even the types of tephra that are placed in the water. Students can then measure heights of eruption columns, calculate ejection velocities, calculate pressure propelling the water column, and make isopach and isopleth maps. The complexity of these assignments can be tailored to the group, from new volcanology students to advanced graduate students. If you are interested in these details, contact the authors for examples of lab handouts.
   The end of the useful life of a trashcan. The upside, however, is that you get a great lateral blast, just like Mount St. Helens in 1980 (ok, much smaller, but it’s a great calculation for students to perform!).

We have a few additional tips that have emerged over the years that we’ve been doing this experiment:

1. If you do the demonstration in a wading pool, you generate a great example of a base surge.
2. Even though it’s tempting to use other containers, many of them don’t handle this explosion well: metal trashcans inevitably fail along the welds; cheap trashcans fail everywhere, really (but especially along decorative seams on the sides, generating lateral blasts but rarely surviving more than one demo); big agricultural feed troughs crack really easily…
3. It helps to place a gallon Ziploc bag of sand or soil in the indentation at the base of the contractors’ trashcans; the plastic often fails there first.
4. You can do a fabulous version of this indoors if you have a large enough room and substitute ping pong balls for water (see here for a much more professional video than ours). You must add a few inches of water in the base of the can, though, to encourage evaporation of the liquid nitrogen. Otherwise, the explosion can take up to 15-20 minutes, and that’s never good for all sorts of reasons. Also, don’t buy old ping pong balls. They shatter into a million little bits. It’s a mess.
5. Avoid using very old trashcans; if they’re brittle, they have been known to shatter into some nasty, sharp pieces that get propelled by the force of the explosion.
6. If you do this as a demonstration for a large group, perform it at the end of the session. It’s pretty tough to reign people’s attention back in after one of these explosions.
7. Whenever possible, do at least two iterations of the experiment. Usually, audiences are taken aback by the first explosion and don’t have much chance to register what they saw. The second one allows for them to absorb the event much better and make more meaningful observations.
8. This whole experiment is considerably more fun when the weather is warm.
9. It’s tough to get a good seal on the cap if it’s way below freezing out. Plus see (1).
10. Be sure to warn Campus Safety if you are going to do this experiment. They don’t react well to surprise reports of explosions on campus, for fairly obvious reasons.
11. Admissions Offices really like it when you do this experiment on days when prospective students are visiting. Really.
12. People teaching classes or having meetings near where you are exploding trashcans, however, are often less enthusiastic about it.Should you have any questions, please do not hesitate to contact us anytime! We are always happy to help. Enjoy!