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.