How does a volcano erupt?
The earth's tectonic plates are composed of lithosphere, the rigid outer portion of the earth. The lithosphere is comprised of an upper layer of crust and a lower, more dense layer of the earth's upper mantle.
The asthenosphere is a hot, mobile layer of partially molten rock (magma) lying within the earth's upper mantle.
Volcanic eruptions above these lithospheric plates are driven by the ascent of magma (molten rock) from deep beneath the surface. For details of the pressure surfaces within the magma column that cause a volcanic eruption view the eruption model at how volcanoes work.
What are the types of volcanic eruption?
Volcanic eruptions range from relatively calm, non-violent extrusion of lava flows on the earth's surface to the highly explosive, with violent ejection of fragmented volcanic debris (called tephra) as high as 80 or 90 kilometres above the volcano.
One of the factors underlying the type of eruption is the composition of the magma (molten rock) underlying the volcano. How Volcanoes Work provides a detailed explanation on the different compositions, viscosity, temperatures and gas content of magma within the eruption dynamics controls section.
Frequency & size of volcanic eruptions
There are around 550 active volanoes on earth (i.e. that have erupted in historic times) plus 500+ dormant volcanoes (that erupted within the past 10,000 years). Both dormant and active volcanoes can erupt again. In fact, on any given day, there are about ten volcanoes actively erupting.
Eruptions vary in size and explosiveness.
Small volcanic eruptions (producing 0.001 to .01 cubic kilometers of volcanic material) are more frequent, occuring globally every few months. Kilauea and Unzen are good examples.
Very large eruptions (producing thousands of cubic kilometers of ash) only occur once every 100,000 years, due to the time taken to build up the necessary gas pressures. The eruption of the Toba volcano in Sumatra 74,000 years ago was the last massive eruption on earth, producing around 2,800 cubic kilometers of ash, more than 2000 times that generated by the 1980 eruption of Mt. St. Helens.
Read our volcanoes worldwide section for details of active volcanoes.