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