Volcanoes/Volcanism

Volcanism

There are three (3) major types of lavas formed by volcanoes: Mafic, Intermediate, and Felsic.
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)

An interesting exception to this classification is the carbonatite volcanoes. These volcanoes have a significant amount of calcium/magnesium carbonates in their magma. While low in Silica and with a low viscosity, which would place these lavas in the mafic class, they are also low in iron and vary in their magnesium. The origin of the CO (typically CO3) in these magmas is a geologic puzzle at present. Is it present in the original magma coming up from the mantle or has it been incorporated by the melting of rocks that the magma has passed through? Solving this puzzle is made harder since there is only one volcano known to have carbonitite lava eruptions in recent times, Ol Doinyo Lengai in the East African Rift Valley. This volcano has a gentle eruptive behavior today but in the past has been explosive. The fossil footprints of Laetoli, Kenya (showing bipedal hominids existing 3.7 million years ago) are preserved in a carbonitite ashfall.

An interesting, though very technical, article showing how geologists are using chemistry to try to determine the origins of carbonitite magmas is at [1]