Magma
Physical and chemical properties of magma
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% Temperature: < 900°C Viscosity: High Eruptive behavior: explosive or effusive Distribution: common in hot spots in continental crust (Yellowstone National Park) and in continental rifts Temperatures of most magmas are in the range 700 °C to 1300 °C (or 1300 °F to 2400 °F), but very rare carbonatite magmas may be as cool as 490 °C, and komatiite magmas may have been as hot as 1600 °C. At any given pressure and for any given composition of rock, a rise in temperature past the solidus will cause melting. Within the solid earth, the temperature of a rock is controlled by the geothermal gradient and the radioactive decay within the rock. The geothermal gradient averages about 25 °C/km with a wide range from a low of 5–10 °C/km within oceanic trenches and subduction zones to 30–80 °C/km under mid-ocean ridges and volcanic arc environments.
Magma-6