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Types of Magma
These are determined by the chemical composition of the magma. Three general types are
Basaltic magma the solidified rock is Basalt. It is 45-55 SiO2% high in Fe, Mg and Ca and
low in K and Na. Temperature: 1000-1200 degrees. Gas content: low.
Andesitic magma the solidified rock is Andesite. It is 55-65 SiO2% and intermediate in Fe,
Mg, Ca, Na and K. Temperature: 800-1000 degrees. Gas content: intermediate.
Rhyolitic magma solidified rock is rhyolite. It has 65-75 siO2% and is low in Fe, Mg, Ca,
high in K and Na. Temperature 650-800 degrees Gas content: high.
Gases in Magmas
At depth in the Earth nearly all magmas contain gas dissolved in the liquid, but the gas forms a
separate vapour phase when pressure is decreased as magma rises toward the surface. This is
similar to carbonated beverages which are bottled at high pressure. The high pressure keeps the gas
in solution in the liquid, but when pressure is decreased, like when you open the can or bottle, the
gas comes out of solution and forms a separate gas phase that you see as bubbles. Gas gives
magmas their explosive character, because volume of gas expands as pressure is reduced. The
composition of the gases in magma is:
Mostly H2O (water vapor) with some CO2 (carbon dioxide)
Minor amounts of Sulfur, Chlorine, and Fluorine gases
The amount of gas in magma is also related to the chemical composition of the magma. Rhyolitic
magmas usually have higher dissolved gas contents than basaltic magmas.