What Makes an Eruption Explosive?
On May 18, 1980, Mount St. Helens underwent a Plinian eruption—the largest and most violent type of volcanic eruption—expelling a large volume of gas, volcanic ash and pumice rock into the environment, causing widespread devastation.Copyright 1980 U.S. Geological Survey http://volcanoes.usgs.gov/volcanoes/st_helens/st_helens_geo_hist_99.html
Students learn about the underlying factors that can contribute to Plinian eruptions (which eject large amounts of pumice, gas and volcanic ash, and can result in significant death and destruction in the surrounding environment), versus more gentle, effusive eruptions. Students explore two concepts related to the explosiveness of volcanic eruptions, viscosity and the rate of degassing, by modelling the concepts with the use of simple materials. They experiment with three fluids of varying viscosities, and explore the concept of degassing as it relates to eruptions through experimentation with carbonated beverage cans. Finally, students reflect on how the scientific concepts covered in the activity connect to useful engineering applications, such as community evacuation planning and implementation, and mapping of safe living zones near volcanoes. A PowerPoint® presentation and student worksheet are provided.
Volcanologists are geologists who study volcanic processes and eruptions. Though volcanologists focus on the fluid dynamics, geology, earth processes and other related concepts around volcanoes, their findings can provide valuable insights for engineering innovations, such as in the field of geochemical engineering or for technologies that involve fluids. Understanding volcanic eruptions can help people in nearby communities to stay safe in the event of an eruption and help avoid triggering eruptions, as some events involving gas drilling have been linked to setting off volcanic activity. Engineers must have a thorough understanding of volcanoes to pursue advances in hydrocarbon recovery (such as gas lift techniques in porous reservoirs) or the use of magma for geothermal applications (exploiting the high temperatures of magma bodies to provide heat to geothermal systems for heat and energy).
After this activity, students should be able to:
- Define and describe viscosity and explain how it relates to the amount of pressure exerted on bubbles rising through magma.
- Describe how the viscosity of magma relates to the rise speed of bubbles, as well as the ability of a bubble to expand in magma.
- Describe “low” and “high” viscosity fluids.
- Explain how magma viscosity relates to the explosiveness of volcanic eruptions.
- Explain how the rate of pressure change or degassing time can affect how effusive or explosive eruptions are.