2. Bedrock
3. Conduit
4. Base
5. Sill
6. Dike
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

A volcano is an opening, or rupture, in a planet's surface or crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...

, which allows hot magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...

, volcanic ash
Volcanic ash
Volcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions, less than in diameter. There are three mechanisms of volcanic ash formation: gas release under decompression causing magmatic eruptions; thermal contraction from chilling on contact...

 and gases to escape from below the surface.

Volcanoes are generally found where tectonic plates
Tectonic Plates
Tectonic Plates is a 1992 independent Canadian film directed by Peter Mettler. Mettler also wrote the screenplay based on the play by Robert Lepage. The film stars Marie Gignac, Céline Bonnier and Robert Lepage.-Plot summary:...

 are diverging
Divergent boundary
In plate tectonics, a divergent boundary or divergent plate boundary is a linear feature that exists between two tectonic plates that are moving away from each other. Divergent boundaries within continents initially produce rifts which produce rift valleys...

 or converging
Convergent boundary
In plate tectonics, a convergent boundary, also known as a destructive plate boundary , is an actively deforming region where two tectonic plates or fragments of lithosphere move toward one another and collide...


1783    The volcano Laki, in Iceland, begins an eight-month eruption which kills over 9,000 people and starts a seven-year famine.

1883    Krakatoa begins to erupt. The volcano's final and most notable explosion occurs on August 26.

1973    A volcanic eruption devastates Heimaey in the Vestmannaeyjar chain of islands off the south coast of Iceland.

1986    Carbon dioxide gas erupts from volcanic Lake Nyos in Cameroon, killing up to 1,800 people within a 20-kilometer range.