DESCRIPTION:
Belknap Shield Volcano, Oregon

Belknap84_belknap_shield_volcano_10-01-84.jpg
Belknap Shield Volcano, Oregon. View from McKenzie Pass.
USGS Photograph taken on October 1, 1984, by Lyn Topinka.
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Belknap Shield Volcano

Location: Oregon

Latitude: 44.285 N

Longitude: 121.841 W

Height: 2,095 Meters

Type: Shield Volcano

Remarks: Three principal eruptive episodes between 3,000 and 1,500 years ago ²

Belknap Crater: Focal point of a long-continued and complex episode of Holocene basalt and basaltic andesite volcanism. The broad shield ... is 5 miles in diameter; it is estimated to be 1,700 feet in maximum thickness and 1.3 cubic miles in volume. The volcano probably contains a core of cinders which interfingers with peripheral lavas and whose surface expression is the summit cone. Basaltic andesite issued from vents at the north and south bases of the cone approximately 1,500 years ago. Lava poured 12 miles to the west and ash was ejected from the northernmost of two summit craters. The main bulk of Belknap ash, which has been traced over an area exceeding 100 square miles, was ejected earlier from a larger south crater. Still earlier lavas were basaltic and moved eastward 7 miles from their vents.

Another type of basaltic activity is characterized by the concentration of many tephra and lava-flow eruptions at a central vent and several flank vents. This type of activity has built shield volcanoes typically 5-15 kilometers in diameter and several hundred meters to more than 1000 meters high. Many have summit cinder cones. Belknap in central Oregon is the youngest such shield volcano in the Cascades and has lava flows as young as 1,400 years.

To get to Belknap, from Eugene (Oregon) follow U.S. 126 eastward to the junction with Oregon 242, then on 242 to McKenzie Pass. From Sisters follow 242 westward to McKenzie Pass. Highways are paved but closed by snow during later fall, winter, and spring.

The name Belknap refers to early settlers along the McKenzie River. R.S. Belknap developed Belknap Springs and his son, J.H. Belknap, was involved in the toll road over the McKenzie Pass in the early 1870s.

The Belknap shield volcano and its distal lava tongues cover 98 square kilometers of the crest of the central High Cascades in Oregon. Prior to 2,900 years before present, the first eruptive phase distributed basaltic cinders and ash over a broad area to the northeast and southeast, while basaltic lavas moved 10 kilometers eastward from a growing shield. A second phase, 2,883 years before present (carbon-14), produced an adventive shield of basaltic andesite on the east flank, known as "Little Belknap". The third phase was responsible for the bulk of modern Belknap volcano. It was constructed by effusion of basaltic andesite lavas from the central vent (Belknap Crater), 1,495 years before present (carbon-14), and from a vent 2 kilometers to the south (South Belknap cone), 1,775 years before present (carbon-14). The final eruptions from the northeast base of Belknap Crater sent lavas 15 kilometers westward into the valley of the McKenzie River.

Belknap volcano is a well preserved Holocene example of the type of volcanic process responsible for construction of the Pleistocene High Cascade platform. Eruption of mafic lava and ash from a single vent area produced a broad shield with a core of cinders. Belknap is intermediate in scale between diminutive cinder cones with small lava flows such as Twin Craters or Yapoah Cone (south of Belknap) and larger composite cones on a shield base which may reach elevations of 2,500 meters, such as Mount Washington or Three Fingered Jack (north of Belknap).

Dee Wright Observatory: From the observatory roof the following landmarks are seen proceeding clockwise from true north in azimuthal degrees. ...

282 -- South Belknap Cone:

Cone was formed and breached 1800 years ago, then surrounded by basaltic andesite lava from a nearby vent about 1500 years ago.

286 and 306 -- Unnamed twin steptoes (in foreground lava field):

Glaciated basaltic andesite volcanoes surrounded by lava from Little Belknap.

309 -- Belknap Crater (summit cone on skyline):

Focal point of a long-continued and complex episode of Holocene basalt and basaltic andesite volcanism. The broad shield which fills the northwest view is 5 miles in diameter; it is estimated to be 1,700 feet in maximum thickness and 1.3 cubic miles in volume. The volcano probably contains a core of cinders which interfingers with peripheral lavas and whose surface expression is the summit cone. Basaltic andesite issued from vents at the north and south bases of the cone approximately 1,500 years ago. Lava poured 12 miles to the west and ash was ejected from the northernmost of two summit craters. The main bulk of Belknap ash, which has been traced over an area exceeding 100 square miles, was ejected earlier from a larger south crater. Still earlier lavas were basaltic and moved eastward 7 miles from their vents.

321 -- Little Belknap:

A subsidiary shield volcano, built 2,900 years ago on the east flank of the larger Belknap shield.

Dee Wright Observatory

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