Post-Mazama Eruption Products from <7,700 years ago
About 7,700 years ago, winds blew volcanic ash from the massive eruption that created Crater Lake directly northeast over Newberry, 75 miles away. This Mazama ash covered Newberry Volcano in a blanket of dusty ash and pumice up to 2 m (6 ft) thick on the south flank. The ash forms a time marker, clearly separating rocks older than 7,700 years from the many younger rocks and indicating that Newberry Volcano has seen many eruptions in just the last 7+ millennia.
The first eruption after Mazama ash fell on the volcano occurred within Newberry caldera. It consisted of viscous (pasty) rhyolite from a fissure that opened through the middle of the caldera. The eruption formed a ridge that divides East Lake from Paulina Lake. Along the fissure are the Inter Lake Flow and Central Pumice Cone. The cone rises 215 m (700 ft) above East Lake. It was explosively erupted when hot magma shattered upon encountering lake water.
Shortly after the young caldera rhyolite eruptions, more fluid lava (basalt and basaltic andesite) erupted from numerous, less explosive cinder and spatter cones. About 7,000 years ago, a 20-mile-long fissure system extending northwest from the caldera opened up to form the Northwest Rift Zone. Eruptions issued from numerous vents including 500-foot high Lava Butte, which spawned a lava flow that temporarily dammed the Deschutes River. In places, fluid lavas surrounded trees, which burned and left behind hollow molds including those found along the trail at Lava Cast Forest.
The most recent eruption at Newberry, about 1,300 years ago, took place within the caldera. Explosive plumes of tephra and pyroclastic flows were followed by slower effusion of lava that created the spectacular Big Obsidian Flow. Not only did more than 10 feet of ash and volcanic bombs fall inside the eastern part of the caldera, but the ash can be found as far east as Idaho.