Modern Cascade arc rocks, young
Modern Cascade arc rocks, young
late(?) to middle Pleistocene
Consists of basalt to rhyodacite lava flows, lava domes, cinder cones, and two small shield volcanoes, distributed across the northwestern map area, south of Lassen Peak.
Compositions include basalt, basaltic andesite, andesite, and one rhyodacite; lavas are variably porphyritic with olivine ± augite ± hypersthene ± pyroxene; typically erupted from cinder cones and/or formed lava domes and constructed at least two small shield volcanoes (Clynne and Muffler, 2010).
Represents the southernmost extent of young Cascade arc volcanism. Post-dates deposition of the Tuscan Formation (map unit tus). Youngest dated eruptive unit is 167 ka; oldest is 667 ka; however, undated units are estimated to range from ~75 to ~725 ka (Clynne and Muffler, 2010).