Belknap Crater is a shield volcano in the Cascade Range in the U.S. state of Oregon. Located in Linn County, it is associated with lava fields and numerous subfeatures including the Little Belknap and South Belknap volcanic cones. It lies north of McKenzie Pass and forms part of the Mount Washington Wilderness. Belknap is not forested and most of its lava flows are not vegetated, though there is some wildlife in the area around the volcano, as well as a number of tree molds formed by its eruptive activity.
Belknap was named for J. H. Belknap, whose father R. S. Belknap developed Belknap Springs. Early routes through the area extended near Belknap and its lava fields, and in the early 20th century, herds of sheep were moved to the two steptoes that lie among the Little Belknap lava flows to graze. The Oregon Skyline Trail, which runs to the west of Belknap's lava flows, follows paths used by Native American populations, who harvested huckleberries in the area. Today, there are a number of trails that run near Belknap, including ones that extend to Little Belknap and Belknap Crater. Belknap can be seen by tourists at the Dee Wright Observatory, which was built in-part with lava blocks from Belknap Crater.
A precise determination for Belknap's age has not been made, as its early history remains obscure. Belknap has likely been built up by eruptive activity over a long period of time. Belknap Crater has had four Holocene eruptive periods confirmed by geological evidence. In total, the Belknap shield and its multiple vents were formed in less than 1,500 years, its last eruptive episode finishing about 1,500 years ago. Belknap formed on the lower slopes of Mount Washington, a highly eroded volcano, and is one of the larger mafic (rich in magnesium and iron) volcanoes in the Sisters Reach.
Belknap consists of a shield volcano and pyroclastic cone and consists of basaltic and basaltic andesite lava with sub-alkaline composition, and it is characteristic of High Cascade volcanism. Well-preserved, its core is made of cinder materials; its eruptive deposits have well-preserved pressure ridges (tumuli) and levees. There are a number of subfeatures including the Inaccessible Cones, Little Belknap, South Belknap, and Twin Craters, as well as the Belknap hot springs. Postglacial, mafic eruptions are more common in the Sisters Reach — which includes Belknap — than anywhere in the Cascade volcanic arc. However, the Volcano Hazards Program of the United States Geological Survey considers it unlikely that Belknap will erupt again soon.
Geography
Belknap Crater lies to the north of the Three Sisters, in the U.S. state of Oregon. Located within Linn County, L. A. McArthur and L. L. McArthur (1984) described Belknap as "one of the important features of the Cascade Range." It lies to the north of McKenzie Pass and can be seen from McKenzie Highway.
Hildreth (2007) lists Belknap Crater's summit elevation at . McArthur and McArthur (1984) list its elevation at , while according to the U.S. National Geodetic Survey, Belknap reaches an elevation of , and has a "bald" appearance. Belknap itself is not forested and its lava flows are generally not vegetated, though there were trees growing on its lava flow deposits in the 1960s according to Taylor (1965). He observed only sparse growth of trees on lava flows from Little Belknap in a report published in the 1980s, noting that soil from Belknap was about thick and made up of lapilli and volcanic ash, which came mostly from Belknap Crater. Along the trail to Little Belknap Crater, there is a tree island with lodgepole pine, mountain hemlock, and true fir. There are also ground squirrels running around on lava flows from Belknap. Along the Oregon Skyline Trail, which passes west of Belknap, there are black-tailed and mule deer as well as lakes stocked with Eastern brook trout
Along the western flows produced by Belknap, there are several dozen tree molds, which range from in diameter and in depth. Some fell into the lava, forming hemi-cylinder-shaped trenches up to in length. Taylor (1965) identified a system of radial roots that were charred from the lava under soil at one of the molds. The material underwent radiocarbon dating to determine an age of 360 ± 160 years A.D.
Geology
thumb|upright=1.6|Belknap formed on the lower slopes of [[Mount Washington (Oregon)|Mount Washington (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]]
thumb|upright=1.2|Belknap viewed from space in 1973
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,
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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
Eruptions at Belknap Crater built up the main shield volcano over repeated activity from vents surrounding a composite summit cone. The lava had a fluid character, leading to inundation of an area that encompassed more than . Rather than forming extensive streams, the lava settled into shorter channels that intersected, leaving complicated drainage patterns. The volcano sits on top of the thick and extensive lava deposits left behind. Belknap Crater has been the focal point for Holocene volcanism producing basaltic and basaltic andesitic lava in its vicinity, which was complex and sustained over a long period of time.
According to the Global Volcanism Program, Belknap Crater had four Holocene eruptive periods confirmed by geological evidence including corrected radiocarbon dating and magnetism; their durations are not known. The first took place in 5050 BCE, producing the Tamolitch lava flow. The next eruptive event, which took place in 1030 BCE ± 300 years, had a VEI of 0 and took place at Little Belknap. The subsequent eruption at South Belknap and Twin Craters occurred in 800 BCE ± 300 years and had a VEI of 2 (Strombolian/Vulcanian); it was followed by the most recent identified event at Belknap Crater in 480, which also had a VEI of 2. According to the Global Volcanism Program, these four eruptions took place during the Holocene: