Between about 500 and 400 ka in the present-day Crater Lake region, rhyodacite lava flows and domes erupted in a few short-lived episodes, and Mount Mazama is defined as the andesite-dacite edifice built upon these >400 ka silicic lavas. Mount Mazama was formed as a succession of overlapping cones and shields during a period of relatively continuous volcanic activity that lasted nearly half a million years, from about 420 to 40 ka. The first eruptions built Mount Scott, located just east of Crater Lake. Over the next several hundred thousand years, Mount Scott and other nearby volcanoes became extinct, while new volcanoes grew to the west. Layers of lava flows from these volcanoes are visible in the caldera walls and in landmarks along the south rim of Crater Lake, including Applegate and Garfield Peaks. The cross-sectional views of the layers of lava flows and pyroclastic deposits reveal that most of Mount Mazama was built up by lava that was fed from low- level lava fountains. During the existence of Mount Mazama, glaciers repeatedly carved out classic U- shaped valleys on the volcano flanks. Some were filled with lava from later eruptions, while others, such as Kerr Notch and Sun Notch, were not. At its maximum elevation, the 3,700 m (12,000 ft) summit of Mount Mazama was located above the south-central part of Crater Lake.
By about 30,000 years ago, Mount Mazama began to generate increasingly explosive eruptions and thick flows of silica-rich rhyodacite lava. The change to more explosive and silica-rich eruptions was an outward sign that a large volume of silicic magma had begun to accumulate deep beneath the volcano. Grouse Hill and Redcloud Cliff are thick lava flows erupted between 30,000 and 27,000 years ago. Another high-silica eruption about 7,900 years ago formed a white layer of pumice and ash and the thick lava flow of Llao Rock. Within 200 years of the eruption at Llao Rock, more pumice and another thick lava flow erupted near present-day Cleetwood Cove. These eruptions culminated 7,700 years ago in the largest explosive eruption in the Cascades during the past 1 million years, and one of the Earth’s largest eruptions in the past 12,000 years (Holocene Epoch).
Mount Mazama's climactic eruption produced the caldera where Crater Lake exists today.