Using a Desktop Explosive Volcano Model to Explore Eruptions
Mattox, Stephen, Jenerou, Kimberly, Lenzo, Hilary, Science Scope
The 2010 volcanic eruption in Iceland had Ta dramatic impact in Europe, grounding 100,000 flights and causing over $1.7 billion in economic losses. Similar and larger eruptions occur every few decades. Understanding these explosive eruptions is critical to mitigating their impacts and protecting air travelers. After a review of volcano basics (provided below), this lesson explores the factors that influence the distribution of tephra, models explosive eruptions in the classroom, and guides students to demonstrate understanding by assessing tephra hazards on the island of Hawaii. The lesson is sufficiently generic to be applied to Cascade or Alaskan volcanoes or the next eruption that causes significant disruption. The lesson requires approximately one to two hours for the teacher to gather materials, half an hour to assemble and test the model, and one hour to present the lesson to students.
Tephra, synonymous with pyroclastics, is a general term for all volcanic rock material ejected (thrown) from a volcano. In general, tephra is classified by size (Figure 1). Tephra can be introduced to students using hand samples, slides, or photographs found on-line. Volcanic eruptions are either effusive, with the gentle outpouring of lava, or explosive, with tephra thrown through the air. Most explosive eruptions are caused by the violent expansion of dissolved gas in the magma. As the magma approaches the surface, the gas bubbles expand until they burst, fragmenting the magma in the throat of the volcano and generating tephra.
FIGURE 1 Terms and grain size for tephra Grain size Epiclastic Pyroclastic (mm) fragments fragments > 256 Boulders Coarse Blocks (and and blocks) bombs 64-256 Cobble Fine 1/16-2 Sand Coarse 1/256-1/16 Silt Fine Ash < Clay 1/256
Since early this century, eruptions have been classified by their resemblance to specific volcanoes, where certain types of activity are common (Williams and McBirney 1979).
Hawaiian eruptions are characterized by the gentle effusion of very fluid basaltic lava from fissures or central vents to produce stacks of thin flows. Lava fountains develop during some central vent eruptions. A lava fountain is a jet of lava shot in the air by pressure from expanding gas bubbles. Fountains usually range from 10 to 100 m in height, but occasionally reach heights of 450 m. Lava-fountain eruptions produce cinder cones, cone-shaped hills formed by the accumulation of cinder and other types of tephra. The early episodes of the ongoing eruption of Kilauea produced a cinder cone.
Strombolian eruptions are characterized by weak to violent explosive activity. This activity consists of intermittent to semi-continuous bursts of solidified and partly solidified blocks, bombs, cinder, and ash in ballistic trajectories. Strombolian eruptions commonly build cinder cones. Recent activity at Stromboli and Arenal volcanoes featured this type of eruption.
Plinian eruptions are characterized by the continuous violent explosions of ash to great heights. Plinian eruptions last for several hours and spread ash over large areas. The 1980 eruption of Mount St. Helens carried ash to a height of 26 km. The eruption of Mount Pinatubo in 1991 sent ash to an altitude of 30 km. Plinian eruptions are commonly associated with stratovolcanoes.
Factors that influence the distribution of tephra
Several factors influence where tephra goes during an eruption and how thick a deposit it creates. These factors include column height, size of materials, wind speed, and wind direction. Column height is influenced by gas thrust, convective thrust, and spreading in the umbrella region (Figure 2).
Gas pressure ejects the tephra from the vent. The heat of the tephra helps to carry it higher relative to the cooler adjacent air. …