Feb 11, 2016
FujiKao
Mayan
Bali
Pinatubo
Pinatubo
Cinder Cones Stratovolcanoes
Fountains: shields and plateaus
Domes
GasContent
Viscosity
Classification of Igneous Rocks
Figure 2-5. Classification of the pyroclastic rocks. a. Based on type of material. After Pettijohn (1975) Sedimentary Rocks, Harper & Row, and Schmid (1981) Geology, 9, 40-43. b. Based on the size of the material. After Fisher (1966) Earth Sci. Rev., 1, 287-298.
Wt. %
oxides
Wt. % SiO2
45 75
FeO , MgO, CaO
Na2O, K2O
H2O
Watercontent(wt. %)
CompositionBasalt Andesite Rhyolite
Water Saturation
Exsolution ofa vapor
of amagma
Fractional Crystallization
Basalt Andesite Rhyolite
low H2O Explosive volcanism
50 % SiO2 75% SiO2
?
Water content of a rhyolitic magma ????
Cinder Cones Stratovolcanoes
Fountains: shields and plateaus
Domes
GasContent
Viscosity
Mt. St. Helens
Granites
Origin of Andesites
• Fractional crystallization of basaltic magma
• Contamination of basaltic magma by continental crust
• Both of the above
• Partial melting of “special (wet ?) manlte”
Origin of Granite/ Rhyolite
• Fractional crystallization of basaltic magma ( volume problem for some areas)
• Partial melting of continental crust