Page "Volcanic ash" Paragraph 9

The types of minerals present in volcanic ash are dependent on the chemistry of the magma from which it was erupted.

Considering that the most abundant elements found in magma are silica ( SiO < sub > 2 </ sub >) and oxygen, the various types of magma ( and therefore ash ) produced during volcanic eruptions are most commonly explained in terms of their silica content.

Low energy eruptions of basalt produce a characteristically dark coloured ash containing ~ 45-55 % silica that is generally rich in iron ( Fe ) and magnesium ( Mg ).

The most explosive rhyolite eruptions produce a felsic ash that is high in silica (> 69 %) while other types of ash with an intermediate composition ( e. g. andesite or dacite ) have a silica content between 55-69 %.