Volcanic Explosivity Index

Measuring Explosive Eruptions

Chris Newhall of the United States Geological Survey and Stephen Self of the University of Hawaii developed the Volcanic Explosivity Index (VEI) in 1982. It is a relative scale that enables explosive volcanic eruptions to be compared with one another. It is very valuable because it can be used for both recent eruptions that scientists have witnessed and historic eruptions that happened thousands to millions of years ago.

The primary eruption characteristic used to determine the volcanic explosivity index is the volume of pyroclastic material ejected by the volcano. Pyroclastic material includes volcanic ash, tephra, pyroclastic flows, and other types of ejecta. The height of the eruption column and the duration of the eruption are also considered in assigning a VEI level to an eruption.

Measurement Scales for Natural Events

Measuring the size or strength of natural events has always been a challenge for natural scientists. They developed the Richter Magnitude scale to estimate the amount of energy released by an earthquake, the Saffir-Simpson scale to estimate a hurricane's potential, and the Fujita scale for rating the intensity of hurricanes. These scales are valuable for comparing different events and for understanding the amount of damage that events of different size can cause.

Measuring the strength of a volcanic eruption is more challenging than collecting wind speed data or measuring ground motion with an instrument. Volcanic eruptions produce different types of products, have different durations and develop in different ways. There is also a problem that some eruptions are explosive (rock materials are blasted from the vent), while other eruptions are effusive (molten rock flows from the vent).