The earliest volcanism began about 30 Ma and consisted mostly of intermediate composition magmas (andesite and dacite) with small amounts of higher-silica rhyolite erupted from numerous vents. Some thin rhyolite tuffs are attributed to more distant eruptions in the San Juan volcanic field. The precaldera rocks are mostly metaluminous (moderate in aluminum content) with a silica content ranging from 53% (basaltic andesite) to 67% (quartz latite). The rhyolites likely formed from fractionation of the original intermediate magmas. The beginnings of regional extension at 26 Ma coincided with a transition to alkaline volcanism.