Silicic lavas of large volume and relatively great areal extent are not uncommon in association with the tracks of mantle plume hotspots across the continents. These units are commonly eroded and/or poorly exposed, and their interpretation as lavas has been controversial. The morphology of a 15 km-3 calc-alkalic rhyolitic unit (75 to 77 wt% SiO2) on the Owyhee Plateau of southwestern Idaho shows that it is a true lava flow emplaced by effusion. The eruption tapped a zoned magma chamber with a small volume of nearly aphyric magma underlain by a much larger volume of less-evolved, crystal-rich magma. Magmatic temperatures were about 830 ° C and H2O contents were about 2 to 2.75 wt%. Eruptive activity first involved explosive venting of the nearly aphyric magma, followed by effusion of the crystal-rich magma. Movement of the lava over and around the tephra deposits formed large, discrete lava lobes; one flow lobe reached 9 km from the vent. Though the unit is a true lava flow, a diverse suite of fragmental textures, including rock that closely resembles welded tuff, formed by lava flow processes during the time the flow was active.