Commencing about 400,000 years ago, Newberry Volcano erupted primarily basalt and grew into a broad shield shape. The Deschutes River, to the west of Newberry, was sometimes shifted further westward as lava flows originating at Newberry diverted the river. At other times, eruptions from volcanoes in the Cascades shifted the channel back to the east. About 350,000 years ago, a massive lava flow from Newberry moved over 65 km (40 mi) north until it reached Smith Rock, where it filled the channel of the Crooked River and flowed downstream.
Indenting the top of the broad shield-shaped edifice is the volcanic basin of Newberry caldera, created by a major explosion and collapse event about 75,000 years ago. This was the most recent of at least three caldera-forming eruptions that lofted pumice and ash high into the air and spread pyroclastic flows across the volcano's surface. An earlier caldera likely formed about 300,000 years ago, the approximate age of several mapped rhyolitic to dacitic ash-flow tuffs on the eastern side of the volcano.
After the most recent caldera formed, eruptions began to fill the large crater with lava flows while numerous cinder cones erupted on the north rim. Newberry Volcano erupted at least a dozen times between the end of the last Ice Age (about 12,000 years ago) and 7,700 years ago, which is when Mount Mazama erupted to form Crater Lake.
Winds blew a thick blanket of volcanic ash and pumice (up to 2 m or 6 ft on the south flank) from the massive Mazama eruption directly northeast over Newberry, 75 miles away. In Newberry caldera, evidence of seasonal hunting camps was discovered underneath this layer of ash, suggesting that the eruption disrupted an ancient human occupation. Without the Mazama ash cover, many of Newberry's flanking lava flows would present a much starker appearance than the muted landscape seen on the volcano's flanks.
Lava flows, and tuff and cinder cones that erupted after deposition of the Mazama ash stand out due to their youthful appearance. These volcanic features are younger than 7,700 years old and are concentrated along the Northwest Rift Zone and within the caldera. Most are located within Newberry National Volcanic Monument.
The first post-Mazama eruption products formed viscous rhyolite lava flows and cones within Newberry caldera. These include the Inter Lake Flow, Game Hut Flow, and Central Pumice Cone, which were erupted from a fissure.
After the caldera rhyolite eruptions, the Northwest Rift Zone formed as a line of fissures stretching from the caldera to the northwest. Lava flows and cinder cones erupted from these vents about 7,000 years ago.
The most recent eruption at Newberry, about 1,300 years ago, produced explosive plumes of tephra and pyroclastic flows followed by the slower effusion of Big Obsidian Flow within the caldera. Not only did more than 10 feet of ash and volcanic bombs fall inside the caldera, but the tephra can be found to the east as far as Idaho.