Measuring and Assessing Volcanic Activity

Volcanic Explosivity Index (VEI) 

Some volcanic emissions are thousands, or even a million times more unstable than others. 

Volcanic explosivity record 

Volcanic explosivity record: The circles in the representation above speak to the volume of ejected tephra for probably the most broadly known hazardous volcanic emissions. Albeit a great many people accept that Vesuvius (79 AD – the Pompeii ejection), Mount St. Helens (1980), and Mount Pinatubo (1991) were colossal, they are little contrasted with antiquated ejections, for example, Wah Springs, Toba, Yellowstone, or Long Valley Caldera. 

Volcanic explosivity index

Volcanic Explosivity Index: The volcanic explosivity record depends on the volume of tephra delivered during an emission. The circles right now a relative size correlation for each progression of the record. 

Estimation Scales for Natural Events 

Estimating the size or quality of casual occasions has consistently been a test for regular researchers. They built up the Richter Magnitude scale to assess the measure of vitality discharged by seismic tremor, the Saffir-Simpson scale to appraise a tropical storm’s latent capacity, and the Fujita scale for rating the force of typhoons. These scales are significant for contrasting various occasions and for understanding the measure of harm that occasions of various sizes can cause. 

Estimating the quality of a volcanic emission is more testing than gathering wind speed information or estimating ground movement with an instrument. Volcanic emissions produce various kinds of items, have various terms and create various manners. There is additionally an issue that a few ejections are unstable (rock materials are impacted from the vent), while different emissions are unrestrained (liquid stone streams from the vent). 

Redoubt ejection 

Redoubt ejection: Eruption cloud from Redoubt Volcano, as saw from the Kenai Peninsula. This emission kept going from December 14, 1989, until June 20, 1990. It was just a VEI 3. Toba was around multiple times increasingly hazardous. Photo by R. Clucas, April 21, 1990. USGS picture. Augment. More data. 

Estimating Explosive Eruptions 

Chris Newhall of the United States Geological Survey and Stephen Self of the University of Hawaii built up the Volcanic Explosivity Index (VEI) in 1982. It is a relative scale that empowers unstable volcanic ejections to be contrasted and each other. It is entirely essential because it very well may be utilized for both ongoing ejections that researchers have seen and notable emissions that happened thousands to a considerable number of years back. 

The essential emission trademark used to decide the volcanic explosivity list is the volume of pyroclastic material shot out by the volcano. Pyroclastic material incorporates volcanic debris, tephra, pyroclastic streams, and different kinds of ejecta. The stature of the ejection segment and the length of the emission are additionally considered in relegating a VEI level to an emission. 

Wah Springs: Eric Christiansen and Myron Best of Brigham Young University clarify the proof that underpins the Wah Springs ejection as one of the biggest, if not the biggest, unstable volcanic emissions known. 

Fish Canyon Tuff 

Fish Canyon Tuff: Another VEI 8 ejection that rivals Wah Springs happened around 28 million years back in what is presently southwestern Colorado. The emission at La Garita Caldera created the Fish Canyon Tuff, a dacitic ignimbrite, with a unique evaluated volume of around 5,000 cubic kilometers! Picture by USGS. Grow/picture source. 

Steps of the VEI Scale 

The VEI scale starts at 0 for emissions that produce under 0.0001 cubic kilometer of ejecta. The vast majority of these emissions are little in size. Be that as it may, some of them are “unrestrained” as opposed to being “unstable.” Effusive emissions are described by magma spilling out of the vent rather than ejecta being impacted from the vent. 

Emissions appraised at VEI 1 produce somewhere in the range of 0.0001 and 0.001 cubic kilometers of ejecta. Above VEI 1, the scale gets logarithmic, implying that each progression in the scale speaks to a 10X increment in the measure of material launched out. VEI 2 emissions produce somewhere in the range of 0.001 and 0.01 cubic kilometers of ejecta. VEI 3 emissions produce somewhere in the range of 0.01 and 0.1 cubic kilometers of ejecta. The movement of the scale from VEI 0 to VEI 8 appears in the graph on this page. 

With each progression in the scale speaking to an explosivity increment of 10X, a VEI 5 is about multiple times touchier than a VEI 4. Two stages of the scale are an expansion of 100X in explosivity. For instance, a VEI 6 is approximately multiple times touchier than a VEI 4. A VEI 8 is one million times touchier than a VEI 2. The entirety of this depends on the ejecta volume. 

Since each progression of the scale is a 10X increment in material shot out, there is a significant contrast in the size of emission on the low finish of a stage and an ejection on the high finish of a stage. Hence, a “+” is frequently added to ejections that are known to be on the top finish of their progression. For instance, the ejection of Katla in Southern Iceland on October 12, 1918, was evaluated at VEI 4+ because the emission was an extremely solid VEI 4. 

Toba Eruption Site 

Toba Eruption Site: About 73,000 years prior, a volcano known as “Toba” emitted on the island of Sumatra, Indonesia. It was one of the most significant volcanic emissions that can be reported with current proof. The impact is accepted to have deforested portions of India – around 3000 miles away – and catapulted around 2600 cubic kilometers of volcanic flotsam and jetsam. Today the pit is the world’s biggest volcanic lake – around 100 kilometers in length and 35 kilometers wide. Picture created utilizing Landsat Geo cover 2000 information from NASA. 

What Eruption Has the Highest VEI? 

Around fifty emissions have been appraised VEI 8 since they are thought to have created a stunning 1,000 cubic kilometers or a higher amount of ejecta. This would be a mass of uncompacted ejecta ten kilometers long, ten kilometers in width and ten kilometers down. Ejections at Toba (74,000 years back), Yellowstone (640,000 years prior), and Lake Taupo (26,500 years back) are three of the 47 VEI 8 destinations that have been recognized. 

The VEI 8 emission with the best volume of ejecta known is the Wah Springs emission that happened in what is presently the province of Utah, around 30 million years prior. It is evaluated to have created more than 5500 cubic kilometers of ejecta in about seven days. 

Eruption(s) at the Paraná and Etendeka traps volcanic territory had an eruptive volume of over 2.6 million cubic kilometers. Be that as it may, these are believed to be gushing emissions delivering liquid basalt magma as opposed to touchy emissions creating ejecta. The Paraná and Etendeka eruption(s) happened around 128 to 138 million years back. Their magma streams range from eastern Brazil onto the western bits of Namibia and Angola. They happened when Africa and South America were associated. 

Mount St. Helens ejection 

Mount St. Helen’s ejection: The May 18, 1980 emission at Mount St. Helens was considered by the vast majority to be a large ejection. The impact expelled the central 400 meters of the mountain, delivered flotsam and torrential jetsam slide that secured 62 square kilometers, and thumped down trees over a territory of around 600 square kilometers. This emission was a VEI 4. Toba, at a VEI 8, was around multiple times as unstable. Picture by USGS. 

The Frequency of Large Eruptions 

VEI Eruption Frequency 



0    frequent 

1    frequent 

2    tens every year 

3    several every year 

4    tens every decade 

5    one every decade 

6    several every century 

7    several every thousand years 

8    two per 100,000 years 

Information from USGS. 

Likewise, with most regular occasions, light volcanic emissions are incredibly standard, and massive ejections are uncommon. The information left from the United States Geological Survey outlines the general recurrence of ejections of different VEI appraisals. It unmistakably shows the uncommonness of high VEI emissions – yet exhibits that they are potential occasions. 

The structured presentation on this page condenses the recurrence of ejections with different VEI appraisals utilizing information from the Global Volcanism Program of the Smithsonian Institution for emissions that happened between around 10,000 years back and 1994. Just four emissions of VEI 7 have been archived; however, more than 3,000 VEI 2 occasions have happened. Luckily, massive ejections are uncommon occasions. 

VEI versus ejection recurrence 

VEI versus ejection recurrence: This graph shows how little, less touchy emissions are considerably more regular than significant emissions. The information used to set up the diagram is from the Global Volcanism Program database of the Smithsonian Institution. This database incorporates recorded and noteworthy ejections that happened between around 10,000 years prior and 1994. 

Evaluating Ejecta Volumes 

Debris Thickness at 16 km 



0    nil 

1    dusting 

2    a barely any centimeters 

3    several centimeters 

4    a barely any many centimeters 

5    about 1/2 meter 

6    about three meters 

7    at least a few meters 

Information from USGS. 

At the point when a touchy emission happens, the ejecta is spread by the power of the impact and by the breeze. It is commonly thickest close to the source and diminishes in thickness with separation. 

With present-day emissions, eyewitnesses can arrange debris thickness reports from a wide range of areas and make a shape guide of debris thickness. This information can be utilized to appraise the volume of ejecta. 

Accurate appraisals become progressively troublesome when the emission happens in a remote region and extremely troublesome when the ejection happens on an island that is a significant stretch from different islands or landmasses. In these circumstances, the size of the ejection cloud and the span of the emission can be joined with debris store information to dole out a VEI rating.

Similar estimation issues happen in ascertaining ejecta volumes for old emissions. Ejecta is effortlessly eroded and frequently secured by more youthful materials. In these circumstances, “best gauges” must be made. When appointing a VEI number is troublesome, a question mark is frequently added to the number to demonstrate the vulnerability. For instance, the Global Volcanism Project records the VEI of the October 24, 79 AD emission of Italy’s Vesuvius as “5?” because lacking information is accessible to be sure about the number. Why Does the Scale Stop at VEI 8? 

The biggest unstable ejections that have been archived to date have been appraised at VEI 8. Could emissions more significant than Toba, Yellowstone, and other VEI 8 occasions happen? Does Earth have the capacity to deliver a shoot fit for propelling the 10,000 cubic kilometers of ejecta expected to rate a VEI 9 emission? 

It is conceivable that proof for a VEI 9 ejection exists and is covered in the geologic record. Emissions that enormous would be extremely uncommon occasions; however, it is difficult to state that ejections that massive have never happened. On the off chance that an ejection that huge were to happen, later on, it would be a massive danger to life on Earth.


  • Volcanic Eruptions and Their Repose, Unrest, Precursors, and Timing (2017) . (n.d.). Retrieved from NAP.EDU:
  • Measuring and Assessing volcanic activity. (n.d.). Retrieved from ALevel Geography:
  • Measuring volcanic eruptions. (n.d.). Retrieved from JKGeography:
  • Volcanic Explosivity Index (VEI). (n.d.). Retrieved from
  • Volcanoes. (n.d.). Retrieved from BBC.Co:

Cite/Link to This Article

  • "Measuring and Assessing Volcanic Activity". Geography Revision. Accessed on April 19, 2021.

  • "Measuring and Assessing Volcanic Activity". Geography Revision, Accessed 19 April, 2021.

  • Measuring and Assessing Volcanic Activity. Geography Revision. Retrieved from