Eruptions and the VEI

The 1912 eruption of Novarupta in Alaska put out about 30 times as much magma as Mount St. Helens did in 1980.

Two ways to erupt

Eruptions range from gas escaping quietly, to lava flows that people can watch safely from a distance, to explosions that blow apart mountains. Geologists group them as effusive and explosive. In an effusive eruption lava flows out of the vent; Hawaiian eruptions are the classic example. In an explosive eruption the gas in the magma expands so violently that it shreds the magma into tephra and ash and blasts it upward in a column that can rise more than 20 km. Many eruptions mix both styles, and one volcano can change style during a single eruption.

A scale for explosions

Earthquakes have magnitude, but what should a scale for eruptions measure? The volume of erupted material, the height of the plume, the area covered by ash, the length of the eruption? The Volcanic Explosivity Index, VEI, combines several of them. It is based mainly on the volume of tephra thrown out, with the height of the eruption column and descriptions ranging from gentle to mega-colossal. It runs from 0 to 8. VEI 0 is a non-explosive eruption with less than 10,000 cubic metres of tephra; VEI 8 is a mega-colossal eruption with more than 1,000 cubic kilometres and a column over 20 km high. From VEI 2 upward each step means ten times more material. Lava does not count, which is why quiet Hawaiian eruptions that pour out a lot of lava still rate 0 or 1.

The scale has limits. The USGS Yellowstone Volcano Observatory points out that modern eruptions are rated from deposit volumes plus what people saw, while prehistoric ones are known only from their deposits. A long, gentle eruption can produce as much ash as a short, violent one and still be rated lower, because duration and violence are part of the judgement.

Mount St. Helens in 1980 shows how an eruption builds. USGS was watching the volcano when, after several much smaller earthquakes from 16 March, a magnitude 4.2 earthquake on 20 March gave the first clear sign that it was waking, followed by hundreds more earthquakes and small steam and ash eruptions from 27 March. A bulge grew on the north flank. On 18 May an earthquake under that flank set off a huge landslide, which suddenly released the pressure on the magma inside and let it blast sideways. At least 17 pyroclastic flows raced down the slopes that day. The USGS volcanologist David Johnston was killed at his observation post.

Examples

The 15 June 1991 eruption of Mount Pinatubo in the Philippines was the second largest of the 20th century and by far the largest to affect a densely populated area. It produced avalanches of hot ash and gas, giant mudflows and an ash cloud hundreds of kilometres across. Scientists from the Philippine Institute of Volcanology and Seismology and USGS forecast the climactic eruption, which saved at least 5,000 lives. Ash and gas went so high that cooler than normal temperatures were recorded around the world afterwards. Small eruptions are far more common than large ones; there are many VEI 2 eruptions for every VEI 7.