Belknap is well-preserved and serves as a good example of Holocene-era activity in the High Cascades. Its core consists of cinder, which is surrounded by a broad shield. It marks an intermediate scale between cinder cones that produced small lava flows such as Twin Craters or Yapoah and larger volcanoes like Mount Washington or Three Fingered Jack. Along with lava flows from the Sand Mountain Volcanic field and other young volcanic centers in the Santiam and McKenzie Pass area, many of the lavas produced by Belknap have crusts with slag or volcanic blocks, with a smaller number having surfaces with ropy, pāhoehoe appearances. The deposits consist of piles of basaltic andesite shards arranged in piles as a result of collapsed lava tubes and breakage of lava crusts and interior lava flows that continued moving while their exteriors cooled; Taylor (1965) wrote that Belknap is only a pile of cinders on the summit of a vast shield of recent lava. The estimated volume of cinders at Belknap Crater is .