North Sister

South, Middle, and North Sister, as well as Broken Top are high, steep-sided peaks that could produce debris avalanches.

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.

The Three Sisters region has hosted volcanic eruptions for hundreds of thousands of years, and future eruptions are a certainty.

In 2001, scientists discovered a broad area of uplift west of South Sister that had been rising a few cm (inches) per year since the 1990s. Uplift rates have varied, with a new pulse starting in 2020–2021. Geochemistry data indicate that magmatic activity in the region predates the past few decades.

The high stratovolcanoes support only about one dozen small glaciers, chiefly in cirques, and numerous patches of perennial snow and ice.