Eruptive activity began at an eruptive vent by Diamond Peak's northern summit, and shortly after, eruptions commenced at a second, slightly higher vent closer to the southern summit. Though Diamond Peak's eruptions always produced olivine-rich basaltic andesite, they likely grew more rich in silicon dioxide over time, ranging from 55 to 58 percent silicon dioxide. Lava flows from the volcano vary from in thickness, rich in porphyritic rocks like olivine, plagioclase, and clinopyroxene; there is no silicic rock (rich in silica) in the Diamond Peak vicinity. Mazama Ash, including pumice up to thick as well as plagioclase, pyroxene, and hornblende crystals, can be found throughout the Diamond Peak wilderness area and represents the newest volcanic rock deposited in the region. In total, eruptive output probably totaled in volume.