火山喷发是指火山口或火山裂缝排出熔岩、火山喷发碎屑（火山灰、火山砾、火山弹和火山块）以及各种气体的过程。根据火山学家定义，火山喷发可分为数种类型，通常以该模式中著名的火山命名。部分火山在一次活跃期间只呈现单一喷发特征，也有他种火山在一次喷发过程表现多种喷发类型。

火山喷发依据产物可区分为三种主要类型：最普遍可见的岩浆喷发（magmatic eruption），即地底深处的气体降压、推动岩浆上升而喷发；蒸气岩浆喷发（phreatomagmatic eruption），是由压缩气体推动岩浆，过程与岩浆喷发正好相反；蒸气喷发（phreatic eruption）则是地下水或地表水因为与岩浆接触，转为过热的蒸气而后喷发。上述三种喷发类型通常不会伴随岩浆释放，而是促成原有岩石粒化。

The term vulcanian was first used by Giuseppe Mercalli, witnessing the 1888-1890 eruptions on the island of Vulcano. His description of the eruption style is now used all over the world for eruptions characterised by a dense cloud of ash-laden gas exploding from the crater and rising high above the peak. Mercalli described vulcanian eruptions as "...Explosions like cannon fire at irregular intervals..." Their explosive nature is due to increased silica content of the magma. Almost all types of magma can be involved, but magma with about 55% or more silica (e.g. basaltic andesite) is most common. Increasing silica levels increase the viscosity of the magma which means increased explosiveness. They usually commence with phreatomagmatic eruptions which can be extremely noisy due the rising magma heating water in the ground. This is usually followed by the explosive clearing of the vent and the eruption column is dirty grey to black as old weathered rocks are blasted out of the vent. As the vent clears, further ash clouds become grey-white and creamy in colour, with convolutions of the ash similar to those of plinian eruptions.