ERS120H5 Lecture Notes - Lecture 10: Rhyolite, Viscosity, Mafic
LECTURE 10: VOLCANOES
1. Define viscosity and describe how it is influenced by both temperature and composition.
- Viscosity is the rate at which a substance can flow, or its thickness.
- The hotter a substance, the faster it flows, but the more concentrated the molecules are (the strong
the bonds), the slower the flow rate become.
- Water flows quickly (low viscosity) when compared to honey (high viscosity). Heated honey flows
faster compared to room temperature honey.
2. Explain which conditions control whether an
eruption is effusive or eruptive
- Effusive Volcanoes:
+ Lower viscosity
+ High melting temperature
+ Crystallizes first
+ Mafic
+ Darker mineral content
- Eruptive Volcanoes
+ Volatile (H2O, CO2) content
+ Low melting temperature
+ Silicic
+ Higher viscosity
+ Crystallizes last
+ Lighter mineral content
3. Compare the viscosity of basaltic magma with that of a silicic magma like rhyolite. Describe how
viscosity is related to temperature and silica content.
- Basaltic Magma
+ 40-52% silica content
+ 1 300ºC eruption temperature
+ Low viscosity (resistance to flow – thin, runny lava)
+ Effusive
- Rhyolite Magma
