Broken Top
Broken Top | |
---|---|
Highest point | |
Elevation | 9,177 ft (2,797 m) NAVD 88[1] |
Prominence | 2,175 ft (663 m) |
Coordinates | 44°04′58″N 121°41′59″W / 44.0828931°N 121.6997595°W[2] |
Geography | |
Location | Deschutes County, Oregon, U.S. |
Parent range | Cascade Range |
Topo map | USGS Broken Top |
Geology | |
Age of rock | Pleistocene |
Mountain type | Stratovolcano |
Volcanic arc | Cascade Volcanic Arc |
Last eruption | About 100,000 years ago[3] |
Broken Top is a glacially eroded
Diverse species of flora and fauna inhabit the area, which is subject to frequent snowfall, occasional rain, and extreme temperature variation between seasons. Broken Top and its surrounding area constitute popular destinations for hiking, climbing, and scrambling.
Geography
Broken Top lies in the
Wilderness
The Three Sisters Wilderness covers an area of 281,190 acres (1,137.9 km2), making it the second-largest
Physical geography
Weather varies greatly in the area due to the rain shadow caused by the Cascade Range. Air from the Pacific Ocean rises over the western slopes, which causes it to cool and dump its moisture as rain (or snow in the winter). Precipitation increases with elevation. Once the moisture is wrung from the air, it descends on the eastern side of the crest, which causes the air to be warmer and drier. On the western slopes, precipitation ranges from 80 to 125 inches (200 to 320 cm) annually, while precipitation over the eastern slopes varies from 40 to 80 inches (100 to 200 cm) in the east. Temperature extremes reach 80 to 90 °F (27 to 32 °C) in summers and −20 to −30 °F (−29 to −34 °C) during the winters.[10]
Before settlement of the area at the end of the 19th century,
Geology
Broken Top is a complex stratovolcano. It lies northwest of Ball Butte and southeast of South Sister (part of the Three Sisters complex volcano), which is located at longitude 121.7° W and latitude 44.08° N. It has an elevation of 9,186 feet (2,800 m), with a volume of 2.4 cubic miles (10 km3).[1] Like other Cascade volcanoes, Broken Top was fed by magma chambers produced by the subduction of the Juan de Fuca tectonic plate under the western edge of the North American tectonic plate.[17] The mountain was also shaped by the changing climate of the Pleistocene Epoch, during which multiple glacial periods occurred and glacial advance eroded local mountains.[18]
Broken Top joins several other volcanoes in the eastern segment of the Cascade Range known as the High Cascades, which trends from north–south.[19] This includes the Three Sisters complex, Belknap Crater, Mount Washington, Black Butte, and Three Fingered Jack, and Mount Bachelor.[20] Constructed towards the end of the Pleistocene epoch, these mountains are underlain by more ancient volcanoes that sank within parallel north–south trending faults in the surrounding region.[21] The Three Sisters form the centerpiece of a region of closely grouped volcanic peaks, an exception to the typical 40-to-60-mile (64 to 97 km) spacing between volcanoes elsewhere in the Cascades. This vicinity is among the most active volcanic areas in the Cascades and one of the most densely populated volcanic centers in the world,[22] as well as the second largest volcanic field of silicic rock within the Quaternary Cascades.[23] The 193 square miles (500 km2) area from the Three Sisters to Broken Top and Mount Bachelor features at least 50 eruptive vents for rhyolitic and rhyodacitic lava.[23]
East of Broken Top, the c, a 115.8 square miles (300 km2) area of andesitic and mafic scoria cones, features similarly rhyolitic and rhyodacitic lava deposits. The
Eruptive history
Broken Top's initial eruptions, beginning about 300,000 years ago during the middle of the
Broken Top's
Glaciation and flash floods
During the late Pleistocene, Broken Top and its surroundings were almost entirely covered by glaciers. The only features in the area not buried by glaciers were the summit of Tumalo Mountain and the summit of Broken Top, including the northwest ridge crest. No geologic evidence suggests that any eruptive activity took place subglacially.[31]
Currently, the Bend and Crook glaciers, located at elevations of 7,762 feet (2,366 m)[32] and 8,209 feet (2,502 m),[33] respectively, continue to erode the mountain's interior contents. Both small ice streams, they have decreased significantly in size over the past few decades,[29] and retreat continues at a rapid rate.[34]
When Little Ice Age glaciers retreated during the 20th century, water filled in the spaces left behind, forming moraine-dammed lakes, which are more common in the Three Sisters Wilderness than anywhere else in the contiguous United States.[35] The local area has a history of flash floods, including at Broken Top itself, where a flash flood took place on October 7, 1966. Analysis of the event by forest ranger David Rasmussen determined that it had originated from a lake on Broken Top, located at an elevation of 8,000 feet (2,400 m) with an area of 11 acres (0.045 km2). This lake sits at the foot of Crook Glacier and is fed by Bend Glacier. The event was likely initiated by glacial ice that fell into the lake, spawning a large wave that breached the moraine and drained 50,000,000 US gallons (190,000,000 L) of water into a channel underneath the lake.[36] The resulting flood reached depths of 10–15 feet (3.0–4.6 m),[37] cascading down the eastern and southern flanks of the mountain and reaching the Cascade Lakes Scenic Byway.[36] The flood moved logs, rocks, and mud to the Sparks Lake meadow basin; it covered between 35 and 45 percent of the lake's basin with silt and deposited at least 10,000–20,000 short tons (9,100–18,100 t) of sediment in the lake and surrounding meadow.[36]
Additional cones
Todd Lake volcano, which erupted
As Pleistocene glaciers retreated, basaltic cinder cone volcanoes formed south of
Ball Butte refers to an additional cone, 8,091 feet (2,466 m) in elevation, which is considered one of Broken Top's subfeatures.
During the late Pleistocene, six cinder cone and auxiliary vents between Broken Top and Tumalo Mountain erupted, yielding glomeroporphyritic basaltic andesite. They all likely derived from the same magma source.[27]
Recent history and potential hazards
When the first geological reconnaissance of the surrounding region was published in 1925, its author, Edwin T. Hodge, suggested that the Broken Top, the Three Sisters, and several other mountains in the area constituted the remains of an enormous collapsed volcano that had been active during the Miocene or early Pliocene epochs. Naming this ancient volcano Mount Multnomah, Hodge theorized that it had collapsed to form a caldera just as Mount Mazama collapsed to form Crater Lake.[42] In the 1940s, Howel Williams completed an analysis of the vicinity and concluded that Multnomah had never existed, instead demonstrating that each volcano in the area possessed its own individual eruptive history.[42][43] Williams' 1944 paper, titled "Volcanoes of the Three Sisters Region, Oregon Cascades",[43] defined the basic outline of the region, though he lacked access to chemical techniques and radiometric dating.[44] Oregon State University geologist and volcanologist Edward Taylor's analysis in 1978[45] determined that the current Broken Top cone was constructed on basaltic andesite lavas that built a platform.[29] As dikes and sills invaded the edifice over time, it formed a volcanic cone made of different lavas including basaltic andesite, dacite, and rhyodacite, as well as tephra and pyroclastic materials from more explosive eruptions.[29]
Broken Top poses significant hazards to the surrounding area, especially within the proximal hazard zone with a diameter of 12 miles (19 km) that surrounds Broken Top and the Three Sisters. A landslide at Broken Top, or an eruption from the nearby South Sister, could initiate a lahar (rapid flows of water, rock, and mud) that could cascade down river valleys in the surrounding area.[46] While smaller landslides of less than 350,000,000 cubic feet (10,000,000 m3) are more probable than huge avalanches, they can still cause significant damage and cause lahars. Larger landslide events would most likely be the result of magma intrusion, and thus they would be predicted by seismometers and volcanic surveying devices.[47]
An eruption from South Sister would pose a threat to nearby life, as the proximal danger zone extends 1.2 to 6.2 miles (2 to 10 km) from the volcano's summits.[47] During an eruption, tephra could accumulate to 1 to 2 inches (25 to 51 mm) in the city of Bend, and mudflows and pyroclastic flows could run down the sides of the mountain, threatening any life in their paths.[48]
Over the course of the past century, five or more lahars have occurred at the Three Sisters and Broken Top, either as a result of glacier-outburst flooding or the failure of
Recreation
Broken Top and the nearby Three Sisters represent a popular climbing destination for hikers and mountaineers. The United States Forest Service requires permits (of which a limited number are available) for use during the busier parts of the year, and dogs must be kept on leashes on the Green Lakes, Moraine Lakes, South Sister, Soda Creek, Todd Lake, and Crater Ditch trails from July 15 through September 15. Horses are prohibited, and ice axes are sensible during the winter climbing season for safety reasons.[51] Motor vehicles and other mechanical means of transport such as bicycles, wagons, motorboats, and helicopters are generally prohibited within the Three Sisters Wilderness under the rules of the National Wilderness Preservation System.
The hiking trail on Broken Top begins at the Green Lakes Basin at the trailhead east of Bend, running 12 miles (19 km) and gaining 3,450 feet (1,050 m) in elevation. It can be
See also
Footnotes
- [a] ^ Sources disagree on the number of cirques that carved into Broken Top; Wood and Kienle (1992) say three cirques,[6] but Williams (1944) claims that five cirques altered the volcano's core.[28]
Notes
- ^ a b c Hildreth 2007, p. 7.
- ^ "Broken Top". Geographic Names Information System. United States Geological Survey, United States Department of the Interior. Retrieved 2008-03-31.
- ^ "Broken Top Volcano". Deschutes & Ochoco National Forests. USDA Forest Service. 2003-11-26. Archived from the original on 2010-11-09. Retrieved 2008-03-18.
- ^ "Three Sisters Wilderness". Willamette National Forest. United States Forest Service. 2004-07-29. Archived from the original on 2010-08-06. Retrieved 2008-05-18.
- ^ Taylor 1978, p. 3.
- ^ a b c d e f g h Wood & Kienle 1992, p. 183.
- ^ a b "Three Sisters Wilderness: General". Wilderness Connect. U.S. Forest Service and the University of Montana. Archived from the original on 2014-04-01. Retrieved 2014-08-15.
- ^ "Eagle Cap Wilderness: General". Wilderness Connect. U.S. Forest Service and the University of Montana. Retrieved 2018-05-29.
- ^ Joslin 2005, p. 34.
- ^ Joslin 2005, pp. 32–33.
- ^ a b "Central Oregon Fire Environment". U.S. Forest Service. Retrieved 2017-11-28.
- OregonLive.com. Advance Publications. Retrieved November 24, 2017.
- ^ Gannett Company. Retrieved 2017-11-23.
- ^ "Lightning-sparked wildfires prompt large closure of Three Sisters Wilderness east of Eugene". The Register-Guard. Guard Publishing Co. 2017-08-16. Retrieved 2017-11-23.
- ^ "Fire forces trail closures in Three Sisters Wilderness". The Bulletin. Bend, Oregon. 2017-08-15. Retrieved 2017-11-28.
- ^ "Prescribed Fire in Central Oregon". U.S. Forest Service. Retrieved 2017-11-28.
- ^ Joslin 2005, p. 29–30.
- ^ Joslin 2005, pp. 30–31.
- ^ Joslin 2005, p. 30.
- ^ Harris 2005, p. 179.
- ^ Joslin 2005, p. 31.
- ^ Riddick & Schmidt 2011, p. 1.
- ^ a b Hildreth 2007, p. 26.
- ^ Hildreth 2007, p. 27.
- ^ a b c d "Eruption History for Broken Top". Cascades Volcano Observatory. United States Geological Survey. November 15, 2013. Retrieved November 15, 2017.
- ^ a b c Hildreth 2007, p. 28.
- ^ a b c Taylor 1978, p. 5.
- ^ a b c d e Williams 1944, p. 45.
- ^ a b c d e Harris 2005, p. 185.
- ^ Wood and Kienle 1992, p. 183.
- ^ a b Taylor 1978, p. 8.
- ^ "Bend Glacier". Geographic Names Information System. United States Geological Survey, United States Department of the Interior. Retrieved 2017-12-11.
- ^ "Crook Glacier". Geographic Names Information System. United States Geological Survey, United States Department of the Interior. Retrieved 2017-12-11.
- ^ Marcott et al. 2009, p. 183.
- ^ Joslin 2005, p. 44.
- ^ a b c Nolf 1966, p. 182.
- ^ Nolf 1966, p. 188.
- ^ Williams 1944, p. 52.
- ^ a b Williams 1944, p. 54.
- ^ Fierstein, Hildreth & Calvert 2011, p. 171.
- ^ Venzke, E., ed. (2013). "Broken Top: Synonyms and Subfeatures". Global Volcanism Program. Smithsonian Institution. Retrieved 2017-12-11.
- ^ a b Harris 2005, p. 182.
- ^ a b Williams 1944, p. 37.
- ^ Hildreth, Fierstein & Calvert 2012, p. 1.
- ^ Taylor 1978, p. 1.
- ^ Scott et al. 2000, p. 1.
- ^ a b Scott et al. 2000, p. 8.
- ^ Sherrod et al. 2004, p. 11.
- ^ Scott et al. 2000, p. 6.
- ^ Scott et al. 2000, p. 8–9.
- ^ "South Sister Climber Trail". U.S. Forest Service. Retrieved 2014-08-15.
- Gannett Company. Retrieved 2017-12-04.
- ^ Bishop & Allen 2004, p. 144.
References
- Bishop, E. M.; Allen, J. E. (2004). Hiking Oregon's Geology. The Mountaineers Books. ISBN 978-0898868470.
- .
- Harris, S. L. (2005). "Chapter 13: The Three Sisters". Fire Mountains of the West: The Cascade and Mono Lake Volcanoes (Third ed.). Missoula, Montana: Mountain Press Publishing Company. ISBN 978-0-87842-511-2.
- Hildreth, W. (2007). Quaternary Magmatism in the Cascades, Geologic Perspectives. United States Geological Survey. Professional Paper 1744. Retrieved 2017-11-29.
- Hildreth, W.; Fierstein, J.; Calvert, A. T. (2012). Geologic map of Three Sisters volcanic cluster, Cascade Range, Oregon: U.S. Geological Survey Scientific Investigations Map 3186, pamphlet. United States Geological Survey. Retrieved 2017-11-29.
- Joslin, L. (2005). The Wilderness Concept and the Three Sisters Wilderness: Deschutes and Willamette National Forests, Oregon. Bend, Oregon: Wilderness Associates. ISBN 978-0-9647167-4-2.
- Marcott, S. A.; Fountain, A. G.; O'Connor, J. E.; Sniffen, P. J.; Dethierd, D. P. (2009). "A latest Pleistocene and Holocene glacial history and paleoclimate reconstruction at Three Sisters and Broken Top Volcanoes, Oregon, U.S.A.". Quaternary Research. 71 (2): 181–189. S2CID 129150777.
- Nolf, B. (1966). "Broken Top breaks; flood released by erosion of glacial moraine". Ore Bin. 28 (10). Oregon Department of Geology and Mineral Industries: 182–188.
- Riddick, S. N.; Schmidt, D. A. (2011). "Time-dependent changes in volcanic inflation rate near Three Sisters, Oregon, revealed by InSAR". Geochemistry, Geophysics, Geosystems. 12 (12): n/a. S2CID 134308941.
- Scott, W. E.; Iverson, R. M.; Schilling, S. P.; Fisher, B. J. (2000). "Volcano Hazards in the Three Sisters Region, Oregon". Open-File Report 99-437. U.S. Geological Survey. Retrieved 2008-05-01.
- Sherrod, D. R.; Taylor, E. M.; Ferns, M. L.; Scott, W. E.; Conrey, R. M.; Smith, G. A. (2004). "Geologic Map of the Bend 30-×60-Minute Quadrangle, Central Oregon" (PDF). Geologic Investigations Series I–2683. U.S. Geological Survey. Retrieved 2017-11-29.
- Taylor, E. M. (1978). "Field geology of S. W. Broken Top quadrangle, Oregon" (PDF). Department of Geology and Mineral Industries. State of Oregon.
- Wood, C. A.; Kienle, J. (1992). Volcanoes of North America. ISBN 978-0-521-43811-7.
- Williams, H. (1944). Louderback, G. D.; Anderson, C. A.; Camp, C. L.; Chaney, R. W. (eds.). "Volcanoes of the Three Sisters Region, Oregon Cascades" (PDF). Bulletin of the Department of Geological Sciences. 27 (3). University of California Press: 37–84. Retrieved 2017-11-26.
External links
- "Broken Top". SummitPost.org. Retrieved 2011-05-07.
- "Broken Top". SkiingtheBackcountry.org. Archived from the original on 2009-12-12. Retrieved 2009-12-07.