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Lava behavior



The viscosity of lava is important because it determines how the lava will behave. Lavas with high viscosity are rhyolite, dacite, andesite and trachyte, with cooled basaltic lava also quite viscous; those with low viscosities are freshly erupted basalt, carbonatite and occasionally andesite.
Highly viscous lava shows the following behaviors:
tends to flow slowly form meniscus which resist flow
tends to entrap vesicles bubbles within the rock as they rise to the surface

correlates with explosive or phreatic eruptions and is associated with tuff and pyroclastic flows
Highly viscous lavas do not usually flow as liquid, and usually form explosive fragmental tephra. However, a degassed viscous lava or one which erupts somewhat hotter than usual may form a lava flow

Lava with low viscosity shows the following behaviors:
tends to flow easily, forming  channels, and rivers of molten rock

tends to easily release bubbles as they are formed
eruptions are rarely pyroclastic and are usually quiescent
volcanoes tend to form shields and steep cones
Lavas also may contain many other components, sometimes including solid crystals of various minerals, fragments of exotic rocks known as xenoliths 

correlates with explosive or phreatic eruptions and is associated with tuff and pyroclastic flows

Highly viscous lavas do not usually flow as liquid, and usually form explosive fragmental tephra. However, a degassed viscous lava or one which erupts somewhat hotter than usual may form a lava flow

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