Illustrated Volcano Glossary

A vulcanian (note the different term from "volcanic") eruption is an intermediately violent type of explosive eruption, stronger than strombolian explosions but much weaker than Plinian eruptions.
During vulcanian eruptions, a relatively large solid plug is ejected when magmatic gas pressure that had build up beneath it overcomes the strength of the plug.
In the same way as Stromboli island for strombolian eruptions, the term vulcanian was coined after the neighboring island of Vulcano in the Eolian Islands, where such eruptions had been observed during the eruption in 1888.

Vulcanian eruptions can follow longer or shorter intervals of repose during which gasses in the magma beneath the plug slowly build up. The smallest vulcanian explosions typically produce eruption columns of approx 1-2 km height, while larger ones approaching sub-plinian size can erupt columns reaching several to more than 10 kilometers height.
Often, the eruptions are accompanied by violent gun-shot noises and eject a large proportion of volcanic ash, bread-crust bombs and solid blocks. The magma involved in vulcanian eruptions (as opposed to strombolian eruptions which involve fluid, basaltic magma) is usually intermediately viscous, typically andesitic or dacitic in composition. This factor facilitates the formation of a solid plug and hinders that gasses contained in the magma escape easily, and can build up considerable pressure first.

A volcano that is famous for producing many vulcanian explosions is Sakurajima in Japan.