Eruptions at Belknap Crater built up the main shield volcano over repeated activity from vents surrounding a composite summit cone. The lava had a fluid character, leading to inundation of an area that encompassed more than . Rather than forming extensive streams, the lava settled into shorter channels that intersected, leaving complicated drainage patterns. The volcano sits on top of the thick and extensive lava deposits left behind. Belknap Crater has been the focal point for Holocene volcanism producing basaltic and basaltic andesitic lava in its vicinity, which was complex and sustained over a long period of time.