Mount Cayley
Mount Cayley is an eroded but potentially active stratovolcano in the Pacific Ranges of southwestern British Columbia, Canada. Located north of Squamish and 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, Mount Cayley being the highest at. 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 and floods. To monitor this threat, the volcano and its surroundings are monitored by the Geological Survey of Canada. 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. Its northeastern flank abuts the southern end of the Powder Mountain Icefield. This is a long and wide irregularly-shaped glacier that trends slightly to the northwest. Just southwest of Mount Cayley lies Pyroclastic Peak, 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 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, but erosion has since reduced it to glacially eroded crags. The modern volcano has an estimated volume of and is only a modest fraction of its total output of silicic eruptive products. It has a proximal relief of and a draping relief of, with a nearly vertical cliff more than high immediately above the Turbid Creek valley. Turbid Creek, Dusty Creek, Avalanche Creek and Shovelnose Creek flow from the slopes of Mount Cayley.
Deep seismic profiling 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
The early Mount Cayley stage was characterized by the eruption of felsic lava flows and pyroclastic rocks onto a crystalline basement. Initial volcanism formed a southwesterly-dipping prism of dacite flows and tephra cut by several dikes and sills. These rocks have been hydrothermally altered to varying degrees and are light yellow or red in colour. They are well exposed in the prominent southwestern cliffs of the volcano.Subsequent activity deposited a series of massive dacite flows up to thick, which form the summit and northern slope of Wizard Peak. The Mount Cayley stage culminated with the emplacement of a central plug dome that forms the narrow jagged summit ridge of Mount Cayley. This edifice consists of similar intrusive dacite.
Vulcan's Thumb stage
The next eruptive period, the Vulcan's Thumb stage, built an edifice that grew upon the southwestern slope of the ancestral Mount Cayley stratovolcano. This began with the eruption of massive dacite flows and blocky agglutinated breccias onto basement and older volcanic rocks of the Mount Cayley stage. These rocks partially form a ridge south of Wizard Peak and comprise the prominent summit ridge pinnacles of Pyroclastic Peak, including the Vulcan's Thumb.Later activity produced an overlying wide and long southwest-trending lobe of unconsolidated or poorly consolidated tephra. The tephra consists of ash and lapilli-sized fragments that have been heavily eroded to form vertical cliffs and ridges. Volcanism also deposited a thick sequence of blocky dacitic tuff breccia between Wizard Peak and Mount Cayley.
Shovelnose stage
Volcanic activity of the final Shovelnose stage involved the eruption of two lava domes at the east and southeast margins of Mount Cayley in the upper Shovelnose Creek valley. The southeast dacite dome forms high cliffs of small diameter columnar joints. It was the source of a long dacite flow that extends down the Shovelnose and Turbid creek valleys to near the Squamish River. The east lava dome was built upon blocky bedded tephra overlying basement rocks and consists of a steep-sided columnar jointed mass of dacite.Recent activity
Although Mount Cayley is not known to have had historical volcanic eruptions, low-level activity has continued into recorded history. Shallow earthquakes have occurred in the vicinity since 1985 and the Shovelnose and Turbid creek valleys contain two and three hot springs, respectively. The GSC therefore considers the volcano to be potentially active. Temperatures ranging from have been measured from the hot springs.The existence of hot springs indicate that magmatic heat is still present. Extensive tufa and sinter deposits inhabit the main hot springs while bright red ferruginous ochre precipitates from several cold seeps in the vicinity. The springs are confined around dacite cupolas and dikes that were emplaced during the Vulcan's Thumb stage.
Landslide history
Because Mount Cayley is rich in coarse proximal pyroclastic deposits, some of them hydrothermally altered, it is especially prone to slope failure and debris avalanches. At least three major debris avalanches have occurred from the western slope in the last 10,000 years, all of which blocked the Squamish River and formed temporary lakes upstream. The first and largest event about 4,800 years ago produced a debris fan exposed along the Squamish River. A thick sequence of silts, sands and pebbles interbedded in the debris fan suggests that it may be the product of two major, closely spaced, debris avalanches rather than a single event. Another large debris avalanche about 1,100 years ago deposited material immediately upstream of the mouth of Turbid Creek. The third event followed about 500 years ago with the deposition of two diamicton units along Turbid Creek and was the smallest of the three major prehistoric debris avalanches. A lack of organic and paleosol horizons between the two units implies that they most likely represent separate surges within the same debris avalanche event.At least three smaller scale debris avalanches have occurred in historic time. A landslide occurred in 1963 with the failure of a large volcanic block consisting of poorly consolidated tuff breccia and columnar-jointed dacite. The mass slid into Dusty Creek where it quickly fragmented into an aggregate then travelled roughly downstream where it entered the broader flatter valley of Turbid Creek for an additional. Both creeks were blocked by the event, resulting in the creation of lakes that eventually overtopped and breached the landslide dam to produce floods and possibly debris flows which in turn swept down Turbid Creek far beyond the landslide terminus. In June 1984, a major rockslide and debris flow resulted from a collapse at the head of Avalanche Creek. The debris flow reached the mouth of Turbid Creek where it destroyed a logging road bridge and blocked the Squamish River, introducing massive quantities of sediment. The third event took place along Turbid Creek in June 2014 and involved a debris flow that removed part of the Squamish River Forest Service Road.