Part of a mostly mafic chain of volcanoes between the Three Sisters and Mount Jefferson, Hogg Rock has magma with an intermediate composition. A tuya, Hogg Rock is also considered a small lava dome with the flat top typically observed for tuya volcanoes. Unlike most lava domes in the Cascades, which are made of dacite or rhyodacite, Hogg Rock is comprised by andesitic lava. It is thick with steep sides that have glassy margins between in thickness,[a] and the rock forming the dome is heavily jointed. Andesite forming Hogg Rock has an aphyric, only slightly porphyritic texture, with plagioclase, orthopyroxene, and olivine phenocrysts. Along with Hayrick Butte, the silica content for Hogg Rock measures about 59-60 percent.