Middle Sister

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.

Pyroclastic flows can form in two ways: from the collapse of a tephra plume during an explosive eruption or from the collapse of the front of a steep lava flow or dome on a volcano's slope.

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

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.