The different magma types (and the solid rock forms that they make) are listed in order of decreasing iron and magnesium, and increasing silica, with ultramafic rocks as the most iron and magnesium ri...The different magma types (and the solid rock forms that they make) are listed in order of decreasing iron and magnesium, and increasing silica, with ultramafic rocks as the most iron and magnesium rich and silica poor, and felsic rocks as the most deficient in iron and magnesium but containing the most silica.