Basic Geology/Volcanism

There are three types of lavas formed by volcanoes — Mafic, Intermediate, and Felsic. But it is believed that the magma present at the magma chamber, means at the asthenosphere is of basic composition. When it moves towards the surface of the earth, the broken fragments of country rocks digested within the magma and its composition gradually changes from basic to acidic. Mafic (Basaltic) %SiO2: < 50% %FeMg: 4% Temp: up to 1500C Viscosity: Low Eruptive Behavior: gentle Distribution: divergent plate boundaries, hot spots, convergent plate boundaries Intermediate (Andesitic) %SiO2: ~60% %FeMg: ~3% Temp: ~1000C Viscosity: Intermediate Eruptive Behavior: explosive Distribution: convergent plate boundaries Felsic (Rhyolitic) %SiO2: >70% %FeMg: 2% Temp: 700C Viscosity: High Eruptive Behavior: explosive Distribution: hot spots in continental crusts (Yellowstone National Park), convergent plate boundaries