Lava poured 12 miles to the west and ash was ejected from the northernmost of two summit craters. The main bulk of Belknap ash, which has been traced over an area exceeding 100 square miles, was ejected earlier from a larger south crater. Still earlier lavas were basaltic and moved eastward 7 miles from their vents. A subsidiary shield volcano, built 2,900 years ago on the east flank of the larger Belknap shield . Glaciated remnant of a large basaltic andesite composite volcano.
Central plug is flanked by north-south swarm of dikes in summit cone.