Mount Cayley is an eroded but potentially active stratovolcano in the Pacific Ranges of southwestern British Columbia, Canada. Located 45 km (28 mi) north of Squamish and 24 km (15 mi) west of Whistler, the volcano resides on the edge of the Powder Mountain Icefield. It consists of massif that towers over the Cheakamus and Squamish river valleys. All major summits have elevations greater than 2,000 m (6,600 ft), Mount Cayley being the highest at 2,385 m (7,825 ft). The surrounding area has been inhabited by indigenous peoples for more than 7,000 years while geothermal exploration has taken place there for the last four decades. Part of the Garibaldi Volcanic Belt, Mount Cayley was formed by subduction zone volcanism along the western margin of North America. Eruptive activity began about 4,000,000 years ago and has since undergone three stages of growth, the first two of which built most of the volcano. The latest eruptive period occurred sometime in the last 400,000 years with lesser activity continuing into the present day. Future eruptions are likely to threaten neighbouring communities with pyroclastic flows, lahars (volcanically induced mudslides, landslides and debris flows) and floods. To monitor this threat, the volcano and its surroundings are monitored by the Geological Survey of Canada (GSC). Eruption impact would be largely a result of the concentration of vulnerable infrastructure in nearby valleys.
Geography and geology
The volcano resides in the middle of a north–south trending zone of volcanism called the Mount Cayley volcanic field. It consists predominantly of volcanoes that formed subglacially during the Late Pleistocene age, such as Pali Dome, Slag Hill, Ring Mountain and Ember Ridge, but activity continued at Pali Dome and Slag Hill into the Holocene epoch. The Mount Cayley volcanic field is part of the Garibaldi Volcanic Belt, which in turn represents a northern extension of the Cascade Volcanic Arc. Volcanism of the Cascade Arc is largely a result of the Juan de Fuca Plate sliding under the North American Plate at the Cascadia subduction zone. Three main summits comprise the Mount Cayley massif. The highest and northernmost is Mount Cayley with an elevation of 2,385 m (7,825 ft). Its northeastern flank abuts the southern end of the Powder Mountain Icefield. This is a 9 km (5.6 mi) long and 5 km (3.1 mi) wide irregularly-shaped glacier that trends slightly to the northwest. Just southwest of Mount Cayley lies Pyroclastic Peak, 2,341 m (7,680 ft) in elevation. It contains a jagged summit ridge of many slender rock pinnacles, the largest of which is known as the Vulcan's Thumb. Wizard Peak with an elevation of 2,240 m (7,350 ft) is east of Pyroclastic Peak and is the lowest of the three main summits. As a stratovolcano, Mount Cayley is built up of solidified lava and ash from successive volcanic eruptions. It is predominantly dacitic in composition, although rhyodacite is also common. Its original and current volumes remain uncertain. It may have had a volume as large as 13 km3 (3.1 mi3), but erosion has since reduced it to glacially eroded crags. The modern volcano has an estimated volume of 8 km3 (1.9 mi3) and is only a modest fraction of its total output of silicic eruptive products. It has a proximal relief of 550 m (1,800 ft) and a draping relief of 2,070 m (6,790 ft), with a nearly vertical cliff more than 500 m (1,600 ft) high immediately above the Turbid Creek valley.[a] Turbid Creek, Dusty Creek, Avalanche Creek and Shovelnose Creek flow from the slopes of Mount Cayley. Deep seismic profiling 12.5 to 13 km (7.8 to 8.1 mi) below the volcano has identified a large bright spot, a reflector interpreted to be a mid-crustal magma chamber or body of very hot rock. Similar mid-crustal reflectors have been identified under subduction zone volcanoes in Japan.
Volcanic history Mount Cayley has experienced volcanic eruptions sporadically for the last 4,000,000 years, making it one of the most persistent eruptive centres in the Garibaldi Volcanic Belt. Three primary eruptive stages in the history of the volcano have been identified. The Mount Cayley and Vulcan's Thumb stages occurred between 4,000,000 and 600,000 years ago with the construction of the stratovolcano and plug domes. A 300,000-year-long period of quiescence followed, during which prolonged erosion destroyed much of the original volcanic structure. This was followed by the third and final Shovelnose stage about 300,000 to 200,000 years ago with the emplacement of parasitic lava domes and flows. Although one of the Shovelnose domes has been potassium-argon dated at 310,000 years old, this date may be in error from excess argon. The Shovelnose stage rocks could be much younger, perhaps less than 15,000 years old. Eruptions during the three stages produced volcanic rocks of felsic and intermediate compositions, including andesite, dacite and rhyodacite. The lack of evidence for volcano-ice interactions at Mount Cayley implies that all eruptive stages most likely took place prior to glacial periods. This contrasts with many neighbouring volcanoes, which contain abundant volcanic glass and fine-scale columnar jointing from contact with ice during eruptions. Initial volcanic activity of Mount Cayley 4,000,000 years ago corresponded with changes to the regional plate tectonics. This involved the separation of the Explorer and Juan de Fuca plates off the British Columbia Coast, which had some significant ramifications for regional geologic evolution. After this reorganization ceased, volcanism shifted westward from the Pemberton Volcanic Belt to establish the younger and currently active Garibaldi Volcanic Belt. The westward shift in volcanism may have been related to steepening of the Juan de Fuca slab after the formation of the Explorer Plate.
Mount Cayley stage
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