The volcanic cone of Mount Thielsen sits atop prior shield volcanoes, and has a volume of . The cone was built from basaltic andesite, a common component of other shield volcanoes in the Oregon Cascades, breccia, and tuff, and it is intruded by dikes. A coalesced volcanic cone, it formed as pyroclastics erupted and fountains spewed lava. Glaciers cut and deformed the cone, eroding its upper sector. This erosion opened the interior of Thielsen for observation. Within the cone, lava flows, pyroclastic flow deposits, and strata of tephra, and volcanic ash, are easily visible. Potassium-argon dating of deposits in the cone suggests that Thielsen is at least 290,000 years old. Since its eruption stopped about 250,000-100,000 years ago, the period of eruptive activity was short in time. The eruptions of the cone came in three phases: a period where lava flows built up its cone, one where more explosive pyroclastic eruptions took place, and the final period, in which pyroclastic and material of lava-based origin were erupted together forming a weak cone encircled by long deposits.