Magma contains large volumes of dissolved gases confined under high pressure. Once pressure drops, gases exolve with rapid expansion of volume, forcing magma to the surface. Lava with low viscosity enables gas to escape freely through vesicles, while more viscous lava traps the gas until sufficient pressure to blow lava apart. Lava blown out of the vents in fragments is called pyroclastic material. this material settles through the air and is called tephra. Words such as bomb, lapilli ad ash describe tephra of decreasing size.
Sometimes the highly viscous lava is pushed down the sides of volcano. The pyroclastic material and gas are mixed in a hot cloud called a nuee ardente.

Lava types:
basaltic: Free flowing and fast moving (low viscosity). Eruptions produce large volumes of lava with little pyroclastic material.
Andesite: more viscous, producing less lava and more pyroclastic material. Generally found at destructive margins.
Rhyolite: extremely viscous that it does not move away from the vent, and is forced into dome like mounds called tholoids.

The extremely viscous eruptions produce virtually no lava, with all products being in pyroclastic form.

	Erstellt von Will Welford vor fast 11 Jahre

Folienset am Plate tectonics and associated hazards, erstellt von Will Welford am 30/11/2015.