Eruptions can come from side branches or from the top of the volcano. Some eruptions are terrible explosions that throw out huge amounts of rock and ash and kill many people. Some are quiet outflows of hot lava. Several more complex types of volcanic eruptions have been described by volcanologists. These are often named after famous volcanoes where that type of eruption has been seen. Some volcanoes may show only one type of eruption during a period of activity, while others may show a range of types in a series.
Volcanic explosivity index
The volcanic explosivity index (commonly shortened VEI) is a scale, from 0 to 8, for measuring the strength of eruptions. It is used by the Smithsonian Institution's Global Volcanism Program in assessing the impact of historic and prehistoric lava flows. It operates in a way similar to the Richter scale for earthquakes, in that each interval in value represents a tenfold increasing in magnitude (it is logarithmic). Most volcanic eruptions are of VEIs between 0 and 2.
Volcanic eruptions by VEI index
|VEI||Plume height||Eruptive volume *||Eruption type||Frequency **||Example|
|0||<100 m (330 ft)||1,000 m3 (35,300 cu ft)||Hawaiian||Continuous||Kilauea|
|1||100–1,000 m (300–3,300 ft)||10,000 m3 (353,000 cu ft)||Hawaiian/Strombolian||Months||Stromboli|
|2||1–5 km (1–3 mi)||1,000,000 m3 (35,300,000 cu ft) †||Strombolian/Vulcanian||Months||Galeras (1992)|
|3||3–15 km (2–9 mi)||10,000,000 m3 (353,000,000 cu ft)||Vulcanian||Yearly||Nevado del Ruiz (1985)|
|4||10–25 km (6–16 mi)||100,000,000 m3 (0.024 cu mi)||Vulcanian/Peléan||Few years||Eyjafjallajökull (2010)|
|5||>25 km (16 mi)||1 km3 (0.24 cu mi)||Plinian||5–10 years||Mount St. Helens (1980)|
|6||>25 km (16 mi)||10 km3 (2 cu mi)||Plinian/Ultra Plinian||1,000 years||Krakatoa (1883)|
|7||>25 km (16 mi)||100 km3 (20 cu mi)||Ultra Plinian||10,000 years||Tambora (1815)|
|8||>25 km (16 mi)||1,000 km3 (200 cu mi)||Supervolcanic||100,000 years||Lake Toba (74 ka)|
| * This is the minimum eruptive volume necessary for the eruption to be considered within the category.
** Values are a rough estimate. Exceptions occur.
† There is a discontinuity between the 2nd and 3rd VEI level; instead of increasing by a magnitude of 10, the value increases by a magnitude of 100 (from 10,000 to 1,000,000).