The Tower Mountain caldera is the main feature of a late Oligocene bimodal volcanic field that was recently mapped in the La Grande 30’ x 60’ quadrangle (Ferns and others, 2001). Late Oligocene volcanism began in this region at about 30 Ma with eruption of channel-filling tholeiitic and alkalic olivine basalt lavas. Later eruptions of porphyritic dacite and andesite lavas produced a thick series of lava flows and lahars that flowed eastward from the central vent area at Tower Mountain. The subsequent climactic eruption resulted in a 100+ m thick outflow sheet of rhyolitic ash-flow tuff and a lithic tuff-filled caldera nearly 15 km in diameter. At about 28 Ma, a series of rhyolite domes extruded along arcuate ring fractures marginal to the tuff-filled core. Caldera-fill tuffs were subsequently intruded by porphyritic and aphyric dacite and rhyolite masses, forming the resurgent caldera core complex. Silicic volcanism ceased at about 22 Ma when dacite and andesite lavas erupted from vents located to the east of the caldera margin. Flood basalts of the Columbia River Basalt Group began burying the Tower Mountain caldera’s northern margin and outflow sheet at about 17 Ma.