Using effervescent heartburn tablets, model the action of volcanoes to measure the intensity of the explosions and create your own measurement scale.
Mount St Helens, Pinatubo, Vesuvius, Pelee, Krakatoa. These volcanoes have become famous, or infamous, for significant eruptions that caused havoc to the land, ecosystems and planetary atmosphere. It would therefore seem logical that measuring and comparing the intensity and destructiveness of volcanic eruptions would be a task for geologists. Employing basic models of volcanoes that use effervescent heartburn tablets, students can measure the intensity of their explosions and create their own scale in experiments taking 80–90 minutes.
Well-known rating scales exist for describing and classifying natural phenomena, such as the Richter scale for earthquakes, the enhanced Fujita scale for tornadoes, and the Saffir-Simpson scale for hurricanes. Students will therefore not be surprised to hear that a scale exists for measuring volcanic eruptions; it’s just that it is nowhere near as well known as the others mentioned.
Volcanologists have developed a logarithmic scale called the volcanic explosivity index (VEI) to measure the intensity of an eruption. Eruptive episodes are rated from 0 to 8, however, because the scale is logarithmic, an eruption classified as a 2 on the VEI is ten times more explosive than an eruption rated as a 1, while a VEI of 3 is 100 times more explosive than a VEI of 1. Logarithmic scales are fairly common throughout science, and representative examples that students should become familiar with are the Richter scale, the pH scale and the Hertzsprung–Russell diagram. The logarithmic aspect of explosivity is based on the volume of tephra that is expelled during an eruption (tephra is the material, such as lava, ash, and rock, that is ejected from the volcano). Volcanologists also look at the plume height, or how high the erupting cloud of ash reaches into the atmosphere during an eruption. How far into the surrounding area the erupting gas and tephra are blown is also considered.
Comparing the 1980 Mount St Helens (a VEI-5) disaster to a VEI-8 eruption is like comparing a firecracker to a briefcase of C-4 explosive. Although Mount St Helens caused a significant level of damage to the surrounding region, the explosion produced by a VEI-8 eruption would be 1000 times greater, causing widespread destruction for hundreds or even thousands of miles. On the other end of the spectrum, volcanic activity at Kilauea in Hawaii is non-explosive, churning out lots of lava and tephra without explosive fanfare. So if Mount St Helens is a firecracker, then Kilauea resembles a melted chocolate bar oozing out of its packaging.
Why do volcanic eruptions behave so differently? Although each behaves in a unique way as the result of many variables, it is possible to make some generalisations about explosivity based on the nature of the magma – the molten rock and other materials underneath the surface of the volcano. Magmas that are silica-rich are often very viscous, with large amounts of trapped gas, and they tend to erupt violently because the gases cannot escape easily. The more viscous the magma, the larger the energy needed to expel gases within it. These trapped gases build up pressure until they achieve enough force to blast out of their confinement and are released in a single, explosive event. Low explosivity occurs when magma has a low viscosity. Gases escape readily from such magmas, and fluid lava burbles out periodically and non-explosively. As an analogy for students, try blowing through a straw into a glass of milk, and notice how easily bubbles form and rise to the top and burst: this is analogous to the low-silica, low-viscosity mafic lava. Then repeat using a milkshake. But wear a towel! It is much more difficult to blow a bubble, but when you do, the milkshake explodes outward, causing a large upheaval.
This activity allows students to create pressurised gas within closed plastic bottles. Before doing this activity with students, collect several 500 ml plastic bottles and remove the labels. Indigestion or heartburn tablets such as Alka-Seltzer® are the source of gas as these tablets contain both citric acid and sodium bicarbonate, which react to form carbon dioxide once they are mixed with water. Each group of students should start with six tablets, although be sure to keep a large quantity on hand for errors and failed eruptions. The groups will measure how much material each ‘volcano’ produces by catching the ‘lava’ in a flat container and then measuring the volume collected.
Increasing the amount of Alka-Seltzer changes the pressure inside the model volcanoes and thus the magnitude and style of eruption. Changing the temperature of the water will also affect the eruption. Students can then determine the resulting effect on the eruption and create their own VEI rating.
Based on a combination of their data and observations, students can assess the eruptions and create a VEI scale of their own. They should be able to support or reject their hypotheses, and describe patterns underlying how and why things happened the way they did, and explain their VEI scale. A more formal lab report, as per the individual teacher’s format, can also be completed, but be sure to have students re-state the purpose of the activity and the relevance and application of VEI and studying volcanoes to science and society in general.