North Sister is the oldest and longest active volcano of the Three Sisters, and its period of construction from about 120 to 45 ka occurred after the eruptions of Broken Top ended.
North Sister is a glacially dissected basaltic-andesite stratovolcano. It has been sufficiently eroded to expose the internal part of the cone, which shows pathways where magma flowed through the edifice in dikes and sills. It is the oldest and longest active volcano of the Three Sisters, and its period of construction from about 120 to 45 ka occurred after the eruptions of Broken Top ended. North Sister lavas show little chemical diversity over the lifetime of the volcano. A north-south chain of mafic vents around Matthieu Lakes erupted intermittently between 60 and 20 ka, cutting across the old North Sister edifice.
