(seen on the right)|alt=The smaller Belknap formed on the lower slopes of the larger Mount Washington, which is larger in size and height The Cascade Range resulted from the
subduction of the
Juan de Fuca tectonic plate under the
North American tectonic plate, with the High Cascade subprovince in central Oregon forming about east of the margin of convergence. The High Cascade platform of the
Cascade Volcanic Arc consists of overlapping layers of
lava flows produced by
shield volcanoes within the past 2 million years. The Cascade shields are steeper and smaller than Hawaiian shield volcanoes, often featuring cinder cones at their summit. Belknap is the youngest shield volcano in the Cascade arc by far, with rugged, barren lava fields that contrast with the forested fields of older Cascade shields. It lies within the central Oregon segment of the Cascades near the Three Sisters area, which is marked by closely clustered volcanic centers that include, from south to north,
Mount Bachelor, the Three Sisters, and Belknap. Known as the Sisters Reach, the cluster abruptly broadens first to a width of then to as it approaches its southern end, running for about in length. There are at least 466
Quaternary volcanoes in the Sisters Reach, including several aligned segments of volcanic vents and rare eruptive units of
rhyolite (uncommon elsewhere in the Cascade arc). Between the North Sister and
Three Fingered Jack volcanoes, Holocene volcanism has been intense, with at least 125 volcanic centers becoming active between 4,000 and 1,300 years ago. Belknap was the last volcano to erupt in the Three Sisters area. Basaltic andesite dominates the eruptive material in the local mafic volcanoes, which range from early Pleistocene to Holocene age. Belknap is one of the larger mafic volcanoes in the Sisters Reach, more than 30 of which run continuously along the segment. As with other mafic
volcanic fields in and next to the Cascades, it does not exhibit a distinct pattern for compositional evolution over time like at the
Hawaii hotspot. The area by Belknap and Mount Bachelor is marked by
extensional tectonics, with a high density of mafic (rich in magnesium and iron) volcanic vents. There are clear trends among volcanic centers in the area including at Sand Mountain and Inaccessible Cone, suggesting underlying faults; according to Taylor (1965), Belknap's alignment with the Spatter Cone Chain and
Blue Lake Crater might similarly indicate underlying faults or fractures connecting the vents at depth, though there are some irregularities. One important distinction is that almost all vent patterns in the area except the supposed Belknap–Blue Lake alignment individually trend north–south, no matter the alignment of the aggregate trend. Belknap formed on the lower slopes of
Mount Washington, a highly eroded volcano; Mount Washington's
pinnacle lies about from Belknap.
Dissolved magmatic carbon dioxide flux at Mount Washington and Belknap Crater was calculated by James et al. (1999) to be per year. Belknap consists of a shield volcano and
pyroclastic cone, which form a late
Holocene shield volcano
complex. It is made up of basaltic and
basaltic andesite lava, which is
sub-alkaline. Mafic in composition, it is characteristic of High Cascade volcanism. In addition to its shield volcano vents, the Belknap complex also consists of lava flows and
tephra deposits erupted from one central vent and several other flank vents. Its lava flows encompass about . It has a
pyroclastic core. The more southern of the two craters on Belknap's summit cone has a depth of and width of more than at its rim. Belknap is one of three major lava fields in its area with Sand Mountain and Yapoah–Four-in-One–Collier. Of these, Belknap is the largest, encompassing Belknap Crater's shield, Little Belknap, and South Belknap cone. Lava flows in the area have a black and
glassy appearance with a
vesicular texture. The basalt and basaltic andesite lava flows from Belknap include small
plagioclase and
olivine phenocrysts, which run for across, as well as
glomerocrysts, which are sparse and run up to across. Some of the lava flows encased
charcoal. Lava flows range from olivine basalt to basaltic andesite and generally show few signs of
weathering or
mineral alteration, with solid cores and outer surfaces with jagged volcanic blocks and flow
breccia. The cores of lava deposits are inaccessible, except at
lava tubes, lava gutters, road cuts, and
quarries. 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 . Belknap's volcanic deposits have well-preserved
pressure ridges (tumuli) and
levees on their surfaces, and they were emplaced between 7,000 and 1,300 years ago. Tephra erupted from Sand Mountain Volcanic Field and Belknap forms a thick blanket across Santiam Pass and near McKenzie Pass, with radiocarbon dating ages of 3,440 ± 250 to 1,600 years B.P. for deposits from source vents, making them younger than
Mazama Ash deposits. Cinder cones near Inaccessible Cone have been nearly buried to completion by basalt and basaltic andesite erupted from Belknap. Belknap's ash deposits are recognizable by their dark color among road cuts and at near the base of lava flow margins.
Notable vents and subfeatures Little Belknap shield, formed about 3,000 years ago, sits to the right of a high scoria cone of the main Belknap Crater vent, with Mount Washington located to the right of both cones. It is about east of Belknap shield's summit craters. Erupted on the flank of the much more extensive Belknap Crater, which also has its own secondary eruptive vents, Little Belknap lacks scoria cones, potentially because it was fed by
magma that was
degassed prior to its eruption. Eruptions at Little Belknap were "quiet", but voluminous, creating the separate shield, which is topped with cinders and lava blocks with collapsed lava tubes radiating outward. A western tube forms a confluence with a vertical conduit that has a diameter of about . Little Belknap's lava flows extended to within of Windy Point to the east and McKenzie Pass to the southeast, forming deposits over ash from Belknap Crater and covered by lava flows from Yapoah Cone. Along the Skyline Trail, one of the lava flows from Little Belknap peeled back on itself as it cooled, creating overturned slabs of hardened lava with widths up to and lengths up to . The blocks are often parallel to the lava's flow direction; Taylor (1965) called them "lava curls". There is a small
lava cave system near the summit of Little Belknap, which has spatter material for a roof. The cave system is short with a number of lava tubes. At the bottom of the conduit lies another system of lava tubes, which served as a drain to create a lava tube system with two levels that are connected by this open conduit, which C. E. Skinner (1982) calls "quite unusual." The open vertical conduit cave has a diameter of , dropping between the upper and lower cave systems. It has an ovular shape and a remelted lining with
stalactites made from lava. South Belknap is a small volcano to the south of Belknap, which was breached on its southwestern side by lava flows that also extended over Belknap Crater's southern base. Early lava flows near South Belknap were covered by a later lava flow, which was produced by a vent located about northwestward. This lava flow also overlapped with the western part of Belknap Crater's lava, and then it reached Lake Valley, where it formed the northern shore of the local Hand Lake. A deposit of
alluvium abuts the margin of South Belknap's lava deposits. On top of Belknap Crater, there is a high cinder cone, which may be the surface extension of the inner pyroclastic core. Within it are three craters, all of different size, which align along a north–northwestern trend. The southern crater is large and produced most of the tephra that covers the surrounding area; the northernmost crater is smaller and erupted ash and a lava flow, which breached the rim of the crater. The Little Belknap volcano erupted lava flows that formed steptoes out of two hills by surrounding them with black, basalt. Its most recent eruptions filled its crater and created a mound of red rock with
clinkers; there are a number of collapsed lava tubes diverging radially from the crater. Besides the volcanic vents related to Belknap Crater, there are two volcanoes south of Belknap which were also recently active: Four-in-One Cone and Collier Cone. Another, unnamed cone at the northern end of Inaccessible Cone's alignment, which rises to a height of , was breached on its western and southwestern sides by gray basalt lava, which is older than Sand Mountain Volcanic Field and Belknap. There may have been a large lava field in the
glacier valley north of the Twin Craters cone, but any evidence is now buried under the Belknap shield. There is a glaciated steptoe (island) in the western part of the Belknap lava field. Deposits from Belknap buried older lava flows from the Sand Mountain Volcanic Field as well as a series of four cinder cones located about to the southwest of Mount Washington. There are also deposits from Belknap in the Lake Valley region between Belknap and both Sims Butte and
Mount Mazama. There are
hot springs, known as the Belknap Hot Springs, on the northern bank of the McKenzie River. Located in
Lane County, they were discovered by R. S. Belknap in November 1869. They lie to the southwest of the wilderness area, ejecting water at a rate of per minute with a temperature of . The hot springs are indicative of a fault that underlies the Cascade Volcanoes, onto which the east side dropped down and
lava intrusions formed volcanic centers. == Eruptive history ==