Geology and History

The cluster of glaciated stratovolcanoes called the Three Sisters extends for 20 km (12 mi) along the crest of the Cascade Range in Oregon, 35 km (20 mi) west of the city of Bend and 100 km (60 mi) east of Eugene.

South Sister is the southernmost and tallest of the Three Sisters. Early eruptions began between 50 to 45 ka, when South Sister produced numerous rhyolite lava flows and domes. Between 37 and 30 ka, the broad cone was built of rhyolite to andesite domes and flows, and construction of the cone ended with an explosive eruption that left pyroclastic deposi

Broken Top is a long extinct volcano that was active in the middle Pleistocene between about 300 and 150 ka.

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.

Middle Sister formed between 40 and 14 ka, but mostly between 25 and 18 ka, closely in time with South Sister. It has erupted basaltic andesite, andesite, and dacite lavas from its central-vent area, as well as dacites from six flank vents and andesites from three more.

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.