The geography of Indonesia is dominated by volcanoes that are formed due to subduction zones between the Eurasian plate and the Indo-Australian plate. Some of the volcanoes are notable for their eruptions, for instance, Krakatau for its global effects in 1883, Lake Toba for its supervolcanic eruption estimated to have occurred 74,000 Before Present which was responsible for six years of volcanic winter, and Mount Tambora for the most violent eruption in recorded history in 1815.
The Volcanic Explosivity Index (VEI) was devised by Chris Newhall of the U.S. Geological Survey and Steve Self at the University of Hawaii in 1982 to provide a relative measure of the explosiveness of volcanic eruptions.
Volume of products, eruption cloud height, and qualitative observations (using terms ranging from "gentle" to "mega-colossal") are used to determine the explosivity value. The scale is open-ended with the largest volcanoes in history given magnitude 8. A value of 0 is given for non-explosive eruptions (less than 104 cubic metres of tephra ejected) with 8 representing a mega-colossal explosive eruption that can eject 1012 cubic metres of tephra and have a cloud column height of over 25 km. Each interval on the scale represents a tenfold increase in observed eruption criteria.
Note that ash, volcanic bombs, and ignimbrite are all treated alike - this is due to taking into account the vesicularity (gas bubbling) of the volcanic products in question and the DRE (Dense-Rock Equivalent) is calculated to give the actual amount of magma erupted. One weakness of the VEI is that it does not take into account the magnitude of power output of an eruption. This, of course, is extremely difficult to detect with prehistoric or unobserved eruptions.
Classification
VEI
Classification
Description
Plume
Ejecta volume
Frequency
Occurrences *
0
Hawaiian Eruption
non-explosive
< 100 m
< 10,000 m³
daily
many
1
Strombolian Eruption
gentle
100-1000 m
> 10,000 m³
daily
many
2
Vulcanian Eruption
explosive
1-5 km
> 1,000,000 m³
weekly
3477
3
Pelean Eruption
severe
3-15 km
> 10,000,000 m³
yearly
868
4
Plinian
cataclysmic
10-25 km
> 0.1 km³
≥ 10 yrs
278
5
Plinian Eruption
paroxysmal
> 25 km
> 1 km³
≥ 50 yrs
84
6
Ultra-Plinian
colossal
> 25 km
> 10 km³
≥ 100 yrs
39
7
Ultra-Plinian
super-colossal
> 25 km
> 100 km³
≥ 1000 yrs
4
8
Ultra-Plinian
mega-colossal
> 25 km
> 1,000 km³
≥ 10,000 yrs
none
*) Count of eruptions in the last 10,000 years based on 1994 figures maintained by the Global Volcanism Program of the Smithsonian Institution.
