Mount Saint Helens explosive eruption on July 22, 1980. Events 1099 - First Crusade: Godfrey of Bouillon is elected the first Defender of the Holy Sepulchre of The Kingdom of Year 1980 ( MCMLXXX) was a Leap year starting on Tuesday (link displays the 1980 Gregorian calendar)

An explosive eruption is a volcanic term to describe a violent, explosive type of eruption. Plate tectonics and hotspots Divergent plate boundaries At the Mount St. Helens in 1980 was a good example of an explosive eruption. Mount St Helens is an active Stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United States Year 1980 ( MCMLXXX) was a Leap year starting on Tuesday (link displays the 1980 Gregorian calendar) Such an eruption is driven by gas including water vapour accumulating under great pressure. General properties of water vapor Evaporation/sublimation Whenever a water molecule leaves a surface it is said to have evaporated Driven by the hot rising magma as it interacts with the ground water the pressure increases until it bursts violently through the overmantle of rock. Magma (Plurals magmas and magmata) is molten rock that sometimes forms beneath the surface of the Earth (or any other Terrestrial planet Groundwater is Water located beneath the Ground surface in Soil pore spaces and in the Fractures of lithologic formations This is merely the beginning. In many cases the rising magma will have vast quantities of gas dispersed through it, partially dissolved; held only by the enormous pressure. Sometimes there is a lava plug blocking the conduit to the summit, and when this occurs, eruptions are even more violent. A volcanic plug, also called a volcanic neck or lava neck, is a volcanic landform created when Lava hardens within a Vent on With the sudden release of pressure following the initial explosion this gas resumes its gaseous form, violently and explosively. This secondary explosion is often far more violent than the first one; the rocks, dust, gas and pyroclastic material may be blown 20 km into the atmosphere at rate of up to 100,000 tonnes per second, travelling at several hundred meters per second. Pyroclastic rocks or pyroclastics (derived from the Greek πῦρ, meaning fire and κλαστός, meaning broken are Clastic rocks

Sooner or later this cloud collapses, almost as violently, creating a pyroclastic flow, the killer cloud of hot volcanic matter. A pyroclastic flow (also known as a pyroclastic density current) is a common and devastating result of some volcanic eruptions.

See also effusive eruptions, the gentler kind of volcano. Effusive eruptions are a Volcanic phenomenon in some ways the opposite of Explosive eruptions An effusive eruption is characterized by an outpouring of low Viscosity