The late Holocene Belknap shield volcano complex is one of several mafic eruption centers near central Oregon's McKenzie Pass. It is a typical example of one type of volcanism responsible for construction of the High Cascade volcanic arc. The Belknap complex comprises many lava flows and tephra layers erupted from a central vent and from several flank vents. The lava flows cover about 100 km2 (about 40 mi2). The main Belknap shield has a diameter of approximately 8 km (5 mi).
Eruptions from this area took place from about 3,000 to 1,500 years ago as a few different phases. The first eruptions produced tephra that spread over a broad area to the northeast and southeast as basaltic lava flows travelled eastward for 10 km (6 mi) from a growing shield. About 2,900 years ago, a second phase produced a smaller shield known as Little Belknap. The third phase produced the remaining bulk of the volcanic complex, which erupted basaltic andesite lavas from the central vent (Belknap Crater, about 1,500 years ago) and from a vent 2 km (just over one mile) to the south (South Belknap cone, about 1,700 years ago). The final eruptions from the base of Belknap Crater sent lavas 15 km (9 mi) west into the McKenzie River valley.
It is not likely that Belknap itself will erupt again, but eruptions similar to the type that formed Belknap could occur anywhere in the surrounding area. Such eruptions could sever major highways that cross this region of the Cascades (U.S. 20; Oregon highways 22 and 242). Tephra from such eruptions could affect nearby communities, especially to the east in Central Oregon.