Volcanoes/Basics of Volcanoes

Volcanoes periodically erupt with volcanic gases, lava, and ash. There is a scale used in judging a volcanic eruption's size, called the Volcanic Explosivity Index, or VEI, scale. A table with some information included is right below.

Classification

Scientists measure how powerful volcanic eruptions are using the VEI. It records how much volcanic material is thrown out, how high the eruption goes, and how long it lasts. The scale goes from 0 to 8. A score of 1 is 10 times more powerful than a score of 0; this is a logarithmic scale, like decibels or the Richter scale.

VEI	Classification	Description	Plume	Ejecta volume	Frequency	Example	Occurrences *
0	Hawaiian Eruption	non-explosive	< 100 m	< 10,000 m³	daily	Mauna Loa	many
1	Strombolian	gentle	100-1000 m	> 10,000 m³	daily	Stromboli	many
2	Strombolian/Vulcanian	explosive	1-5 km	> 1,000,000 m³	weekly	Galeras (1993)	3477*
3	Vulcanian/Pelean	severe	3-15 km	> 10,000,000 m³	yearly	Nevado del Ruiz (1985)	868
4	Peléan eruption/Plinian	cataclysmic	10-25 km	> 0.1 km³	≥ 10 yrs	Soufrière Hills (1995)	278
5	Plinian	paroxysmal	> 25 km	> 1 km³	≥ 50 yrs	Mount St. Helens (1980)	84
6	Plinian/Ultra-Plinian	colossal	> 25 km	> 10 km³	≥ 100 yrs	Mount Pinatubo (1991)	39
7	Plinian/Ultra-Plinian	super-colossal	> 25 km	> 100 km³	≥ 1000 yrs	Mount Tambora (1815)	5 (+2 suspected)
8	Ultra-Plinian	mega-colossal	> 25 km	> 1,000 km³	≥ 10,000 yrs	Lake Toba (73,000 Before present)	1
9	Ultra-Ultra-Plinian	fortunately	> 25 km	> 10,000 km³	≥ 10,000,000	Possibly the Siberian Traps	0

* Count of eruptions in the last 10,000 years based on 1994 figures maintained by the Global Volcanism Program of the Smithsonian Institution

Note that there is a discontinuity in the definition of the VEI between indices 1 and 2. The lower border of the volume of ejecta jumps by a factor of 100 from 10,000 to 1,000,000 m³ while the factor is 10 between all higher indices.

Introduction

Cross-section through a stratovolcano:
1. Large magma chamber 2. Bedrock 3. Conduit (pipe) 4. Base 5. Sill 6. Branch pipe 7. Layers of ash emitted by the volcano 8. Flank	9. Layers of lava emitted by the volcano 10. Throat 11. Parasitic cone 12. Lava flow 13. Vent 14. Crater 15. Ash cloud

Cleveland Volcano in the Aleutian Islands of Alaska photographed from the International Space Station

A volcano is an opening, or rupture, in a planet's surface or crust, which allows hot, molten rock, ash, and gases to escape from below the surface. Volcanic activity involving the extrusion of rock tends to form mountains or features like mountains over a period of time.

Volcanoes are generally found where tectonic plates are pulled apart or come together. A mid-oceanic ridge, for example the Mid-Atlantic Ridge and mid-oceanic ridge, has examples of volcanoes caused by "divergent tectonic plates" pulling apart; the Pacific Ring of Fire has examples of volcanoes caused by "convergent tectonic plates" coming together. By contrast, volcanoes are usually not created where two tectonic plates slide past one another. Volcanoes can also form where there is stretching and thinning of the Earth's crust (called "non-hotspot intraplate volcanism"), such as in the African Rift Valley, the Wells Gray-Clearwater Volcanic Field and the Rio Grande Rift in North America and the European Rhine Graben with its Eifel volcanoes.

Volcanoes can be caused by "mantle plumes". These so-called "hotspots" , for example at Hawaii, can occur far from plate boundaries. Hotspot volcanoes are also found elsewhere in the solar system, especially on rocky planets and moons.