The discharged eruption cloud contains volcanic gases like water vapour, carbon dioxide or sulphur dioxide. It also contains many ash particles with sizes between fractions of a millimetre and up to 2 mm. Even larger particles, so called Lapilli, can rise in the cloud. This type of eruption cloud is called Plinian cloud and at some volcanoes it rises up to 30 km or more into the stratosphere.
If such a cloud collapses under its own weight it can generate the both faszinating and very dangerous pyroclastic clouds. These "fire clouds" can travel with several hundred kilometres per hour away from the volcano and destroy all objects in their path.