Magma is the word used to describe melted or molten rock inside Earth. Magma is composed of elements, minerals, and gases that were present in the rock before it melted.
The major elements in magma are those present in Earth's crust: oxygen (O), silicon (Si), aluminum (Al), iron (Fe), calcium (Ca), sodium (Na), magnesium (Mg), and potassium (K). These elements combine to form minerals such as magnetite, hauynite, olivine, pyroxene, hornblende, plagioclase, potassium feldspar (k-feldspar), and quartz.
Magma also contains dissolved gases like water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2).
The composition of the magma determines the eruption style, rock type, and volcano shape. Variations in the chemical compositions and properties of the magma determine whether it will be classified as mafic, felsic, or intermediate.
Mafic
- High melting point, pH, density, and Mg and Fe content
- Low viscosity
- Form basanite and basalt rocks
- Typically effusive eruptions
- Form shield volcanoes, flood basalts, cinder cones, and fissure eruptions
Intermediate
- Intermediate melting point, viscosity, pH, density and mineral composition
- Form andesite rocks
- Typically explosive eruptions
- Commonly form composite volcanoes
Felsic
- Low melting point, pH, and density
- High viscosity and Si content
- Form rhyolite rocks
- Typically very explosive eruptions
- Form composite volcanoes, lava domes, and calderas
Source: NPS
|