Geology and History for Mount Jefferson
Mount Jefferson is a prominent feature of the landscape seen from highways east and west of the Cascades. The volcano has erupted repeatedly for hundreds of thousands of years beginning about 300,000 years ago. The largest explosive eruption from Mount Jefferson occurred in the late Pleistocene, and caused ash to fall as far away as the present-day town or Arco in southwest Idaho. Unfortunately, a precise time frame for that eruption has not been determined using modern dating methods. Mount Jefferson's most recent eruptive episode occurred during the last major glaciation, which culminated about 15,000 years ago.
Two types of volcanoes are found in the Mount Jefferson region: composite and monogenetic. Composite cones erupt over tens to hundreds of thousands of years and can display a wide range of eruption styles. Mount Jefferson is an example of a composite cone. Monogenetic volcanoes typically erupt for only brief intervals of times—weeks to perhaps centuries—and generally display a narrower range in eruptive behavior. Over a time span of hundreds of thousands of years, these monogenetic volcanoes have built a broad upland area (hundreds to thousands of square kilometers/miles) of dacite domes and andesite to basalt shields, lava flows, and small cones.