One gauge of progress in geology is the increasing number of numerical scales in use. Scales help scientists think about subjects by letting them see what happens when scale-ranked things are plugged into equations and models. Sometimes new laws of nature are revealed, sometimes new puzzles are revealed instead, but the general result is scientific progress.
Volcanic eruptions would seem to be hard to rank on a scale, but in 1982 Christopher Newhall and Stephen Self created a widely used magnitude scale called the volcanic explosivity index or VEI. David Pyle simplified it in 1995. It has worked so well that all kinds of people are using it, including another set of researchers who want to extend it to the very smallest volcanic explosions.
The original VEI of 1982 by Newhall and Self measured explosive eruptions in two different ways: by the amount of ash (tephra) they make and the height their clouds of ash rise in the atmosphere. Both of these quantities are important for human as well as geological purposes, and they're straightforward to measure.
Ash volume is important to all sorts of land managers, not just volcanologists. People who deal with air quality need to know how much volcanic dust will be blowing around and for how long. People who deal with rivers need to know how much volcanic sediment will enter the watersheds, how fast it will move downstream, how it might pollute the water and how it will affect dams and bridges and fisheries. People who deal with farms and forests need to know how soils and crops and trees will deal with the ash load. All of these people also advise the public, landowners and the authorities.
Geologically speaking, ash rather than lava is the primary material that eruptions make. (Lava flows make up a smaller volume; moreover, lava flows aren't explosive features.) Ash layers are common in the geologic record, where they serve as time markers of these instantaneous events. Finally, ash chemistry is our most abundant evidence of magma chemistry and the doings of the deeper part of the crust and mantle. The widespread interest in volcanic ash means that we have abundant data, both worldwide in extent and deep in geologic time.
The amount of ash an eruption makes is estimated by mapping the size and thickness of the blanket of tephra around the vent. Eruption blankets have a fairly predictable shape, as we can tell simply from the overall shapes of explosive volcanoesthose are the sum of hundreds of thousands of separate eruptions. Even ancient eruptions, preserved in the geologic record, can be measured this way.
Eruption Plume Height
A prime concern with volcanic eruptions is how their eruption plumes might affect air traffic. Tiny amounts of fine ash in the air can destroy jet engines and sandblast airplane windshields. Forecasters use basic data on a plume's size and content, starting with its height, to model where a high ash cloud will go next.
Another more global concern is that the largest eruptions inject volcanic gases into the stratosphere, where they linger for months and perturb the weather of the whole planet.
Geologically speaking, high eruption plumes are important because they can affect the whole world at once, or large parts of it. The very largest episodes of volcanism coincide with mass extinctions, which suggests that volcanic material carried in the atmosphere can knock world climate and the ocean itself off balance. Whatever we can learn about eruptions today helps us understand the much larger events in the past. We've been lucky enough to miss them, but they can occur again.
Plume height is easily monitored day and night by satellite observations from above, with help from seismic observations from below. But we have little global data on plume heights from the geologic past. The main proxy for plume height is levels of sulfur compounds recorded in cores of ancient ice, but that is not very exact because eruptions vary a lot in their sulfur output. We can estimate plume height from ash volume, but there is error involved here too.
Simplifying the VEI
The original VEI, by Newhall and Self, listed several different aspects of eruptions by the order of their reliability. Ash volume came first, then plume height. After that came one-word descriptions from "gentle" to "colossal," which are hardly reliable at all, and next the descriptions of what I might call an eruption's style: the five terms Hawaiian, Strombolian, Vulcanian, Plinian and Ultraplinian.
In a 1995 paper, David Pyle argued that of all these types of eruption measurements, ash volume is most important for long-term, large-scale science. For his purposeexploring the general problem of explosive volcanism and the amounts of energy and rock involved at the thousand-year scalehe defined the VEI strictly by ash volume. After all the energy that an eruption releases is not in the noise and smoke, but mostly in the heat of the magma it brings to the surface. The total heat is best defined by the mass of erupted rock, which in turn is best defined by the volume of ash.
Pyle's revised scale is more purely a magnitude scale of eruptions because, like the "Richter" magnitude scale for earthquakes, it expresses the total energy of the event rather than the particulars of how it happened. Pyle reasoned that plume height is more about an eruption's intensityhow fast it releases energythan its magnitude.
The VEI scale is becoming more widely known, especially for its use in extremely large eruptions. The "supervolcano" eruptions that science television loves to scare us with have the highest rank, VEI 8. We've never witnessed one. We've documented just one eruption of VEI 7, the 1815 eruption of Tambora. The 1980 Mount St. Helens eruption was a VEI 5 event, and the 1991 Pinatubo eruption was a VEI 6 event. The index helps us think about these things.
Revising and Extending the VEI
In 2013, Bruce Houghton and five coauthors brought up some problems with the VEI scale. The high end of the scale works well, but for their purposes the low endevents of VEI 1 and 0is useless. Houghton's research involved precisely measuring small explosive events in Hawaii using simple tools like buckets to collect ash fall or sweeping ash from road surfaces. These events have ash volumes that give them a VEI 0 rank, defined as "non-explosive" events with less than 10,000 cubic meters of ash. Nevertheless they are just as explosive as the larger kind. More than that, the little Hawaiian events are a measurable hazard to visitors, as is true at other volcanoes frequented by tourists.
Another problem was that while the higher VEI ranks are based on straightforward powers of ten, each rank being 10 times as great as the next lower one, VEI rank 1 lumps together ash deposits from 10,000 to 1 million cubic meters, a hundredfold range.
Houghton's team suggested that the powers-of-ten scheme be extended downward and that any talk of non-explosive versus explosive be removed. First they proposed that VEI 1 be split in two: VEI 1 for 0.1 to 1 million cubic meters and VEI 0 for 0.01 to 0.1 million (10,000 to 100,000) cubic meters. Second they proposed that smaller events would use negative VEI numbers. Houghton's team reported their own measurements of explosive eruptions that left ashfalls smaller than 10 cubic meters, or VEI 4 events.
Under the Houghton proposal, explosive eruptions of any size could be assigned a consistent VEI number with no artificial breaks in the scale, just like the Richter scale. The earthquake scale is useful to a wide range of scientists. Some are studying the giant M 9 events like the 2011 Tohoku earthquake, while others need to measure the tiny M 2 crackles that accompany underground mining and drilling activities. The continuous magnitude scale has made it possible for seismologists to make and test ideas that span the whole range of earthquake sizes. A similar scale for eruptions may make similar advances possible in volcanology.
Houghton et al., 2013, "Pushing the Volcanic Explosivity Index to its limit and beyond: Constraints from exceptionally weak explosive events at Kilauea in 2008," Geology v. 41, p. 627-630.
Newhall and Self, 1982, "The Volcanic Explosivity Index (VEI): An estimate of explosive magnitude for historical volcanism," Journal of Geophysical Research v. 87, p. 1231-1238.
Pyle, 1995, "Mass and energy budgets of explosive volcanic eruptions," Geophysical Research Letters v. 22, p. 563-566.