It produced largely intermediate composition lavas and breccias, together with ash flow tuffs reflecting differentiation of the original magma.

[3] The precaldera intermediate volcanic rocks include the Conejos Formation in the southeastern part of the field.

It was characterized by bimodal volcanism, producing both low-silica alkaline flows and high-silica rhyolites.

[3] The San Juan volcanic field has been a historically important mining district, producing lead, zinc, copper, gold, and silver.

Ore veins were concentrated in fractures associated with caldera activity and in postcaldera intrusive bodies.