THE MOUNT BACHELOR VOLCANIC CHAIN – A MODEL FOR FUTURE MAFIC ERUPTIONS IN THE CENTRAL OREGON HIGH CASCADES

The latest Pleistocene to early Holocene Mount Bachelor volcanic chain (MBVC) typifies one style of mafic volcanism found throughout central Oregon: effusive eruptions of lava from aligned vents over a geologically short time period. The 25-km-long chain consists of over 50 cinder cones and shield volcanoes aligned roughly N-S; eruptive products are dominantly lava flows and near vent scoria falls. Approximately 40 km3 of geochemically diverse basalt and basaltic andesite (49 to 57 wt.% SiO2) were emplaced in four eruptive episodes (I to IV, oldest to youngest) during the 8-10 kyr history of the chain.

The four eruptive episodes span a broad range in eruption complexity and duration --from eruption of a single cinder cone (IV) to a chain of cinder cones (II) to a single shield volcano (I) to a cluster of shield volcanoes (III). Episode duration, interpreted from secular-variation paleomagnetic data, ranges from perhaps weeks to several centuries. During episode I, calc-alkaline basaltic andesite lavas from two distinct magmas built the Sheridan Mountain shield near the center of the chain. Concurrently, calc-alkaline and tholeiitic basalt lavas were emplaced along the margins of the MBVC. After a hiatus of several centuries, activity resumed with eruptions of calc-alkaline basaltic-andesite on the flanks of the shield, and with calc-alkaline basalt flows from new vents along the margin of the chain. During episode II, calc-alkaline basalt lavas were erupted from numerous cinder cones at the southern end of the MBVC. Identical paleomagnetic directions for episode II lava flows suggests a short duration (months to years) of activity. More than half of the total MBVC volume was emplaced during episode III, with growth of the calc-alkaline basalt to basaltic andesite shield volcanoes of Kwohl Butte, Tot Mountain and Mount Bachelor. Volumetrically minor amounts of tholeiitic basaltic andesite lavas were also erupted during this episode that lasted at least several centuries. The last brief eruptive episode (IV) consisted of a single tholeiitic cinder cone, Egan cone, emplaced on the north end of the MBVC about 8000 years ago.

Most individual eruptive episodes were on time scales of human life spans and thus provide useful insights regarding the possible complexity and duration of mafic activity that may be seen in the future.