Pyroclastic flows (nuee ardente) are fast moving high density flows of hot rock framents,ash and dust which flow down the slopes of volcanoes. They appear as a ground huggin billowing grey cloud that surges forward from the vent of a volcano. They can occur either as a result of dome collapse, collapse of an eruption column or a sudden explosion from a volacano. They consist of two parts a basal flow of rock fragments which hugs the ground and a billowing ash cloud that moves over the top.
Although the flow looks grey and fluffy it is lethal and will destroy all in its path. Temperatures inside the flow can reach 1000C and it can reach speeds of 80-100Km/hr and the flow will normally follow existing river valleys.
Pyroclastic deposits can be up to 200m deep and form unconsolidated slopes which are prone to lahar formation. Flows can also erode and melt glaciers then mix in with the meltwater to form lahars instantly.Pyroclastic surges are less dense and able to flow small hills they are colder at around 250C.
Montserrat 1995 to present
The southern 2/3 rds of the Caribbean Island of Montserrat has been devastated by the eruption of the Soufriere hils volcano. Pyroclastic flows caused by dome collapse frequently raced down the surrounding valleys destroying the once verdant land sape,the capital Plymouth and the airport. 19 people were killed in June 1997.