Rock in the area ages from the Upper Pliocene and Quaternary. Basalt and basaltic andesite comprise newer volcanoes atop the High Cascades: Major volcanic centers include Mount Hood, Three Sisters / Broken Top, Mount Mazama (Crater Lake), and Mount Jefferson. All have produced diverse eruptions, including both lava flows and pyroclastic eruptions, and variability in composition between dacite, basalt, and even rhyolite (except for Mount Hood, which is not known to have produced rhyolite). Thielsen is part of a series of extinct volcanoes in Oregon termed pyramidal peaks or the Matterhorns for their steep, spire-like summits; Thielsen is the highest at . Other Matterhorns include Mount Washington, Three Fingered Jack, Mount Bailey, and Diamond Peak. Unlike other mountains in the High Cascades, all these volcanoes became extinct 250,000-100,000 years ago, and their summits endured the last few ice ages, accounting for their distinct shapes.