Volcanic blast occurs when magma rises asymmetrically into the cone, making one sector of the volcano bulge outwards and become unstable.
In response to an earthquake or to the pressure of rising magma the flank collapses, causing debris avalanches and/or landslides.
Because of the debris avalanches, landslides and explosions the magma chamber
is revealed to the open.
The sudden reduction in pressure causes hot gases in the magma to blast out sideways in a huge explosion which is followed immediately by nuées ardentes.
At Mount St.Helens, the very hot (200C) blast devastated an area of 600sqkm! Trees and buildings were razed to the ground and scorched.