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, it is part of the central Oregon segment of the Cascade Range. 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. The central Belknap shield volcano has a diameter of and a volume of , with a maximum thickness of . The shield has proximal and draping reliefs of , which are the "difference between summit elevation and that of highest exposure of older rocks overlain by the edifice" and the "difference between summit elevation and that of lowest distal lavas of the edifice (not including distal pyroclastic or debris flows)," respectively. According to Hildreth (2007), Belknap Crater is a broad, low-angled shield volcano compared to other mafic (rich in magnesium and iron) volcanic cones. Belknap's summit cone rises about above the base shield volcano, and the volcano itself rises about above its surroundings.
Oregon Route 242, which follows the course of the McKenzie River, passes through the lava fields produced by Belknap Crater, the Yapoah cinder cone volcano, and a number of other volcanic vents. These fields consist of black, basaltic lava and encompass an area of .
Ecology
Belknap forms part of the Mount Washington Wilderness, which encompasses of lava and plains. Lodgepole pine trees are common, and there are 28 lakes within the wilderness area. 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
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
Name Elevation CoordinatesBelknap Crater Inaccessible Cones Little Belknap South Belknap Twin Craters