Composition, Melt Structure and Properties
Silicate melts are composed mainly of silicon, oxygen, aluminium, alkalis (sodium, potassium, calcium), magnesium and iron. Silicon atoms are in tetrahedral coordination with oxygen, as in almost all silicate minerals, but in melts atomic order is preserved only over short distances. The physical behaviours of melts depend upon their atomic structures as well as upon temperature and pressure and composition.
Viscosity is a key melt property in understanding the behaviour of magmas. More silica-rich melts are typically more polymerized, with more linkage of silica tetrahedra, and so are more viscous. Dissolution of water drastically reduces melt viscosity. Higher-temperature melts are less viscous.
Generally speaking, more mafic magmas, such as those that form basalt, are hotter and less viscous than more silica-rich magmas, such as those that form rhyolite. Low viscosity leads to gentler, less explosive eruptions.
Characteristics of several different magma types are as follows:
- Ultramafic (picritic)
- SiO2 < 45%
- Fe–Mg > 8% up to 32%MgO
- Temperature: up to 1500°C
- Viscosity: Very Low
- Eruptive behavior: gentle or very explosive (kimberilites)
- Distribution: divergent plate boundaries, hot spots, convergent plate boundaries; komatiite and other ultramafic lavas are mostly Archean and were formed from a higher geothermal gradient and are unknown in the present
- Mafic (basaltic)
- SiO2 < 50%
- FeO and MgO typically < 10 wt%
- Temperature: up to ~1300°C
- Viscosity: Low
- Eruptive behavior: gentle
- Distribution: divergent plate boundaries, hot spots, convergent plate boundaries
- Intermediate (andesitic)
- SiO2 ~ 60%
- Fe–Mg: ~ 3%th
- Temperature: ~1000°C
- Viscosity: Intermediate
- Eruptive behavior: explosive or effusive
- Distribution: convergent plate boundaries, island arcs
- Felsic (rhyolitic)
- SiO2 > 70%
- Fe–Mg: ~ 2%
- Temp: < 900°C
- Viscosity: High
- Eruptive behavior: explosive or effusive
- Distribution: common in hot spots in continental crust (Yellowstone National Park) and in continental rifts
Read more about this topic: Magma
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