A Pahoehoe Lava Flow

Volcanic Eruptions

Factors that determine the violence of an eruption
Composition of the magma
Temperature of the magma
Dissolved gases in the magma
Viscosity of magma
Viscosity is a measure of a material’s resistance to flow

Viscosity of magma
Factors affecting viscosity
Temperature (hotter magmas are less viscous)
Composition (silica content)
High silica – high viscosity (e.g., rhyolitic lava)
Low silica – more fluid (e.g., basaltic lava)
Dissolved gases (volatiles)
Mainly water vapor and carbon dioxide
Gases expand near the surface

Viscosity of magma
Factors affecting viscosity
Dissolved gases (volatiles)
Provide the force to extrude lava
Violence of an eruption is related to how easily gases escape from magma
Easy escape from fluid magma
Viscous magma produces a more violent eruption

Materials Associated with Volcanic Eruptions

Lava flows
Basaltic lavas are more fluid
Types of lava
Pahoehoe lava (resembles braids in ropes)
Aa lava (rough, jagged blocks)
Gases
One to five percent of magma by weight
Mainly water vapor and carbon dioxide