Illustrated Volcano Glossary

Explosive eruptions occur, when the erupting magma is ejected as fragments into the air, as opposed to effusive eruptions producing lava flows.

Explosive eruptions are called so, when the erupting magma is fragmented when exiting the conduit. The reason are expanding gasses from the magma itself or external water e.g. from an aquifer. The resulting fragments of the magma itself are called tephra, and consist in small and large pieces: Ash, lapilli and bombs are the typical products of explosive eruptions.
The reason for explosive eruptions are usually a high viscosity of the magma combined with high cas content. Volcanoes that often have explosive eruptions are also called "gray volcanoes", because the ash clouds they produce look gray. Most gray volcanoes are found in subduction zones, especially around the circumpacific Ring of Fire.
The opposite is effusive eruptions at the so-called "red volcanoes".
The range of explosive eruptions encompasses an incredibly large spectrum of orders of magmitude. The smalles explosive eruptions are spattering, when single lava fragments are ejected usually a few meters only from the vent. The next larger type of eruptions are lava fountains and strombolian eruptions, followed by vulcanian eruptions when a larger plug is exploded by pressurized gas-rich beneath it. The largest and most dangerous type of explosive eruptions are Plinian eruptions, which erupt several cubic kilometers of magma and produce ash columns that can reach 30-40 km of height!