which multiple glacial periods occurred and glacial advance eroded the mountains. The Three Sisters form the center of a region of closely grouped volcanic peaks. This is in contrast to the typical spacing between volcanoes elsewhere in the Cascades. Among the most active volcanic areas in the Cascades and one of the most densely populated volcanic centers in the world, the Three Sisters region includes nearby peaks such as Belknap Crater, Mount Washington, Black Butte, and Three Fingered Jack to the north, and Broken Top and Mount Bachelor to the south. Most of the surrounding volcanoes consist of mafic (basaltic) lavas; only South and Middle Sister have an abundance of silicic rocks such as andesite, dacite, and rhyodacite. Mafic magma is less viscous; it produces lava flows and is less prone to explosive eruptions than silicic magma. The region was active in the Pleistocene, with eruptions between about 650,000 and about 250,000 years ago from an explosively active complex known as the Tumalo volcanic center. This area features andesitic and mafic cinder cones such as Lava Butte, as well as rhyolite domes. Cinder cones accumulate from the airfall of many pyroclastic rock fragments of various sizes, while the viscous rhyolite domes extrude onto the surface like toothpaste. The Tumalo volcano spread ignimbrites and plinian deposits in ground eruptions across the area, similar to the eruption of Vesuvius that destroyed Pompeii. These deposits spread from Tumalo to the town of Bend. Basaltic lava flows from North Sister overlay the youngest Tumalo pyroclastic deposits, indicating that North Sister was active more recently than 260,000 years ago. North Sister North Sister, also known as "Faith", is the oldest and most highly eroded of the three, with zigzagging rock pinnacles between glaciers. It is a shield volcano that overlays a more ancient shield volcano named Little Brother. North Sister is wide, and its summit elevation is . Consisting primarily of basaltic andesite, it has a more mafic composition than the other two volcanoes. Its deposits are rich in palagonite and red and black cinders, and they are progressively more iron-rich the younger they are. North Sister's lava flows demonstrate similar compositions throughout the mountain's long eruptive history. The oldest lava flows on North Sister have been dated to roughly 311,000 years ago, though the oldest reliably dated deposits are approximately 119,000 years old. Estimates place the volcano's last eruption at 46,000 years ago, in