Sherrod and his associates described an ancient basaltic andesite deposit from the Pleistocene, which displays reverse polarity and is older than 0.78 million years. This deposit includes lava flows from Black Butte, which are slightly to moderately porphyritic in texture and exhibit 5-10 percent plagioclase phenocrysts, about in length. They also have 3-5 percent olivine phenocrysts, which are about across. Sherrod et al. also described two previously identified, reverse-polarized basaltic andesite lava deposits at Black Butte, with ages of 0.46 ± 0.30 and 0.48 ± 0.2 million years, respectively. The volcano's last known eruption was during the Pleistocene. However, some eruptive activity has continued in the High Cascades; there is one lava flow in the McKenzie Pass area to the west of Black Butte, which is only 1,500 years old.