Silicic Magma
Volcano Learning zone > Volcano Glossary
Silicic magmas are those with a high percentage of silica, over 65%, granite and rhyolite come into this category. Granitic plutons will crystallise slowly and larger crystals will grow.Continental crust is largely composed of silicic rocks. Indicative minerals are
- Quartz Si O2
- Alkali Feldspar KAlSi308
- Plagioclase Feldspar NaAlSi3O8
- Biotite (Mica) K(Mg,Fe)3AlSi3O10(F,OH)2
- Muscovite (Mica)KAl2(AlSi3O10)(F,OH)2
- Amphibole/Hornblende (Ca,Na)2–3(Mg,Fe,Al)5(Al,Si)8O22(OH,F)2.
Within the magma flow patterns caused by convection currents align the crystals. Silicic magmas occur in subduction zones beneath continental crust,continental collision where fractional crystallisation occurs. This where lighter minerals float to the top of the magma chamber and more dense Fe/Mg minerals sink to the bottom.