Belknap volcano is a Holocene example of the process which built the High Cascade Platform during the Pleistocene. It is a small shield volcano with a capping cinder cone.
A number of other eruptions also occurred in the general area during roughly the same time period. About 1,300 years ago (1,330 14C years B.P.) a series of small spatter cones formed between Belknap and Blue Lake. North of these, a phreatic explosion blasted out the crater which is today occupied by Blue Lake. To the south are several cinder cones and associated lava flows, including Collier, Four-in-One, and Yapoah cones. Four-in-One Cone has been dated at about 1,900 years old (1,980 14C years B.P.) while Collier is dated at about 1,500 years old (1,600 14C years B.P.)