The volcano explosivity index, abbreviated VEI, is a scale used by vulcanologists to rate volcanic eruptions. This rating is similar to the Richter scale used to rank earthquakes. It was developed by Chris Newhall of the U.S. Geological Survey and Steve Self of the University of Hawaii in 1982. The VEI measures many aspects of an eruption, including explosive power, duration of eruption, the volume of ash released, and the height of the ash plume. Interestingly, volcanoes that release a large amount of lava may have a low VEI ranking if they do not also release a large amount of ash. The scale ranges from 0 for gentle, non-explosive eruptions to 8 for huge, catastrophic eruptions.
VEI rankings with examples:
VEI 0-1: “Non-explosive” to “gentle” eruptions, with a plume height not exceeding 1000 m. Roughly thousands to tens of thousands of cubic meters of ash deposited. Eruptions occur daily.
The eruptions from Kilauea in Hawaii are an example of a VEI 0 to VEI 1. Even though it produces an almost continual flow of lava it ejects very little ash, and is therefore low on the VEI scale.
VEI 2: “Explosive” eruptions, with a plume 1 to 5 km high. Roughly millions of cubic meters of ash deposited. Eruptions occur weekly.
Eruptions from the Stromboli volcano in Italy tend to be classified as VEI 2. Stromboli produces more violent eruptions than Kilauea but still only releases a minimal amount of ash.
VEI 3: “Severe” eruptions, with a plume 3-15 km high. Roughly tends of millions of cubic meters of ash deposited. Eruptions occur yearly.
The eruptions of Unzen volcano in Japan are rated VEI 3. They started in 1990 and continued for four years. Even though the volcano was near the town of Unzen there were minimal casualties. However, the eruptions did create over 10,000 ash flows.
VEI 4: “Cataclysmic” eruptions, with a plume 10-25 km high. Roughly hundreds of millions of cubic meters of ash deposited. Eruptions occur every ten years.
One example of a VEI 4 eruption is the 1965 eruption of Taal Volcano in the Philippines. This eruption killed about 200 people but caused only minor damage.
VEI 5: “Paroxysmal” eruptions, with a plume over 25 km high (all VEI rankings above this also have an ash plume over 25 km high). Roughly one cubic kilometer of ash deposited. Eruptions occur every hundred years.
The most famous VEI 5 eruption is Mount St. Helens in Washington State that erupted on May 18, 1980. The eruption produced a huge ash plume and destroyed one third of the mountain by landslides and lahars. Human casualties were minimal, however hundreds of acres of forest were destroyed.
VEI 6: “Colossal” eruption. Roughly tens of cubic kilometers of ash deposited. Eruptions occur every hundred years.
Krakatoa volcano on Krakatoa island had a VEI 6 eruption in 1883. This was the strongest marine eruption in recorded human history with the force of 100 megatons (in comparison, current nuclear bombs have a strength of around 100 kilotons (thanks Transitional Man)). The explosion could be heard 3000 miles away. Two thirds of the island was destroyed in the eruption and a massive tsunami was created that reached almost 4000 miles away. The resulting ash is thought to have blocked the sun and caused a decrease in global temperatures by 1 ° C. On the positive side, both scientists and painters of the time noted particularily spectacular sunsets due to the ash in the atmosphere. (thanks shannonhubbell)
VEI 7: “Super colossal” eruption. Roughly hundreds of cubic kilometers of ash deposited. Eruptions occur every thousand years.
Tambora volcano in Indonesia had a VEI 7 eruption in 1815, the strongest volcanic eruption in recorded human history. Over 10,000 people died from the explosion and an estimated 80,000 died later from starvation as the resulting ash blocked the sun and destroyed crops. 1816 was known as "the year without a summer" because of the Tambora eruption a year before. The ash cloud was thought to lower temperatures globally by 3 ° C and layers of ash from the volcano were found as far away as Greenland.
VEI 8: “Mega colossal’ eruption. Roughly thousands of cubic kilometers of ash deposited. Eruptions occur every ten thousand years.
These are bad, very bad. Fortunately, there have been no VEI 8 eruptions in recorded human history, but there are two prominent examples that occured earlier. The first is the Toba volcano in Sumatra that erupted roughly 70,000 years ago. The Toba eruption was thought to cause a six year nuclear winter followed by an Ice Age that lasted thousands of years. Most plants and animals would not have survived, and it is thought that the human race was reduced to roughly ten thousand individuals. The second example is the supervolcano present in Yellowstone National Park in Wyoming that erupted roughly 600,000 years ago (and is supposedly due for another eruption). If it erupts it would also deposit enough ash to plunge the world into the equivalent of a 5 year nuclear winter and potential Ice Age. Animals and plants would die, people would starve, and civilization as we know it would likely end. Scary stuff.
http://www.iomoon.com/VEI/vei.html
http://en.wikipedia.org/wiki/Volcanic_Explosivity_Index