Mount Cayley

Mount Cayley
The Mount Cayley volcanic complex in August 13, 2005. Summits left to right: Pyroclastic Peak, Mount Cayley
Elevation	2,377 m (7,799 ft)
Prominence	674 m (2,211 ft)
Listing	List of volcanoes in Canada List of Cascade volcanoes
Location
Location	Squamish River, British Columbia, Canada
Range	Pacific Ranges
Coordinates	50°07′13″N 123°17′26″W / 50.12028°N 123.29056°W / 50.12028; -123.29056Coordinates: 50°07′13″N 123°17′26″W / 50.12028°N 123.29056°W / 50.12028; -123.29056
Topo map	NTS 92J/03
Geology
Type	Stratovolcano (potentially active)
Age of rock	Pleistocene
Volcanic arc/belt	Canadian Cascade Arc Garibaldi Volcanic Belt
Last eruption	Pleistocene
Climbing
First ascent	1928 E. Brooks, T. Fyles, W. Wheatley
Easiest route	rock/ice climb

Mount Cayley is a potentially active^[1] stratovolcano in Squamish-Lillooet Regional District of southwestern British Columbia, Canada. Located 45 kilometres (28 mi) north of Squamish and 24 kilometres (15 mi) west of Whistler in the Pacific Ranges of the Coast Mountains, it rises 2,264 metres (7,428 ft) above the Squamish River to the west and 1,844 metres (6,050 ft) above the Cheakamus River to the east.^[1]

Mount Cayley consists of ridges, rounded lava domes and sharp eroded rocky pinnacles with the highest reaching 2,377 metres (7,799 ft) in elevation.^[2] It lies at the southern end of a field of glacial ice called the Powder Mountain Icefield.

Human history

To the Squamish, the local indigenous people of this territory, the mountain is called t'aḵ'taḵmu'yin tl'a in7in'a'xe7en.^[3] In their language it means "Landing Place of the Thunderbird".^[3] This name of the mountain refers to the legendary Thunderbird, a creature in North American indigenous peoples' history and culture.^[3] Like Black Tusk further south, the rock was said to have been burnt black by the Thunderbird's lightning. This mountain, like others located in the area, is considered sacred because it plays an important part in their history.

The first recorded ascent of Mount Cayley was made by the mountaineers E.C. Brooks, W.G. Wheatley, B.Clegg, R.E. Knight, and Tom Fyles in 1928.^[4] During this time, the party named the volcano after the late Beverley Cochrane Cayley, who was an ardent mountaineer of the executive committees of the British Columbia Mountaineering Club and the Vancouver section of the Alpine Club for several years.^[4] Beverley Cayley was a friend of those in the climbing party, and died on June 8, 1928, at the age of 29 in Vancouver.^[4] Photographs of Mount Cayley were published in the Canadian Alpine Journal Vol XX in 1931.^[4]

Geology

Like other volcanoes in southwestern British Columbia, Mount Cayley lies within the Coast Plutonic Complex, which is the single largest contiguous granite outcropping in North America.^[5] The intrusive and metamorphic rocks of the Coast Plutonic Complex extend approximately 1,800 kilometres (1,100 mi) along the coast of British Columbia, the Alaska Panhandle and southwestern Yukon. This is a remnant of a once vast volcanic arc called the Coast Range Arc that formed as a result of subduction of the Farallon and Kula Plates during the Jurassic-to-Eocene periods.^[5] In contrast, Mount Cayley, Mount Meager, Mount Garibaldi and Silverthrone Caldera are of recent volcanic origin.^[6]

Mount Cayley consists mostly of an igneous rock with a high iron content called dacite, although another igneous rock, rhyodacite, with intermediate composition of dacite and rhyolite is also present.^[2] Unlike many of the other volcanoes further south, Cayley does not dominate the surrounding landscape, which consists of high, rugged peaks.

Origins

Mount Cayley began erupting about 4 million years ago and has grown steadily since then. Like all of the volcanoes further south, Mount Cayley has its origins in the Cascadia subduction zone—a long convergent plate boundary that stretches from mid-Vancouver Island to Northern California.^[7] The subduction zone separates the Juan de Fuca, Explorer, Gorda and North American Plates. Here, the oceanic crust of the Pacific Ocean sinks beneath North America at a rate of 40 millimetres (1.6 in) per year.^[8] Hot magma upwelling above the descending oceanic plate creates volcanoes, and each individual volcano erupts for a few million years. These volcanoes are commonly referred to as Cascade volcanoes because they were formed by Cascadia subduction.^[9]

The subduction zone has existed for at least 37 million years, and has created a line of volcanoes called the Cascade Volcanic Arc which stretches over 1,000 kilometres (620 mi) along the subduction zone.^[10] Several volcanoes in the arc are potentially active.^[11] Lassen Peak in California, which last erupted in 1917, is the southernmost historically active volcano in the arc, and Mount Meager, just north of Mount Cayley, which erupted about 2,350 years ago, is generally considered the northernmost.^[12] A few isolated volcanic centers northwest of Mount Meager, such as the Silverthrone Caldera, which is a 20 kilometres (12 mi) wide, deeply dissected caldera complex, are considered by some geologists to be the northernmost member of the arc.^[13]

Structure

Mount Cayley is the largest and oldest volcanic complex of the Garibaldi Volcanic Belt.^[2] Like other stratovolcanoes, it is composed of layers of hardened lava, tephra, and volcanic ash. Eruptions are explosive in nature, involving viscous magma, glowing avalanches of hot volcanic ash and pyroclastic flows. The source magma of this rock is classified as acidic, having high to intermediate levels of silica (as in rhyolite, dacite, or andesite).^[14]

Stratovolcanoes are a common feature of subduction zones.^[15] The magma that forms them arises when water, which is trapped both in hydrated minerals and in the porous basalt rock of the upper oceanic crust, is released into mantle rock of the asthenosphere above the sinking oceanic slab. The release of water from hydrated minerals is termed "dewatering", and occurs at specific pressure/temperature conditions for specific minerals as the plate subducts to lower depths. The water freed from the subducting slab lowers the melting point of the overlying mantle rock, which then undergoes partial melting and rises due to its density relative to the surrounding mantle rock, and pools temporarily at the base of the lithosphere. The magma then rises through the crust, incorporating silica rich crustal rock, leading to a final intermediate composition. When the magma nears the surface it pools in a magma chamber under the volcano. The relatively low pressure of the magma allows water and other volatiles (CO₂, S^2-, Cl^-) dissolved in the magma to begin to come out of solution, much like when a bottle of carbonated water is opened.^[16] Once a critical volume of magma and gas accumulates, the obstacle provided by the volcanic cone is overcome, leading to a sudden explosive eruption.^[16]

Eruptive history

Cayley was formed in relatively early Pliocene time compared to most other Cascade volcanoes, which were formed in the past two million years, and mostly in the past one million years or less.^[2] Cayley's first phase of activity began approximately four million years ago with the eruption of lava and ash, such as plagioclase-hypersthene-hornblende-phyric dacite flows, tephra, and pyroclastic breccia, and concluded with the formation of a central lava dome which develop the present summit spires of Mount Cayley.^[2]

Aggluinated vent breccia, enormous lava, and welded breccia with plagioclase-hypersthene-hornblende-biotite-phyric dacite erupted throughout Cayley's second phase of activity, forming the largest of a number of small pinnacles extending from the jagged summit ridge of Pyroclastic Peak called Vulcan's Thumb on the southwestern flank of Mount Cayley about 600,000 years ago.^[2][13]

Lengthy erosion, which removed nearly all of the original outer cone of pyroclastic material, was followed by the eruption of satellitic vents.^[2] This third and final phase of activity began about 300,000 years ago with the eruption of a dacite lava flow into the extant valley of Shovelnose Creek and concluded with the construction of two small satellitic plagioclase-hypersthene-biotite-phyric dacite lava domes about 200,000 years ago.^[2]

Current activity

Mount Cayley is one of the eleven Canadian volcanoes associated with recent seismic activity; the others are Castle Rock,^[17] Mount Edziza,^[17] Mount Garibaldi,^[17] Hoodoo Mountain,^[17] The Volcano,^[17] Crow Lagoon,^[17] Mount Meager,^[17] Wells Gray-Clearwater Volcanic Field^[17] and Nazko Cone.^[18] Seismic data suggests that these volcanoes still contain living magma plumbing systems, indicating possible future eruptive activity.^[19] Although the available data does not allow a clear conclusion, these observations are further indications that some of Canada's volcanoes are potentially active, and that their associated hazards may be significant.^[1] The seismic activity correlates both with some of Canada's most youthful volcanoes, and with long-lived volcanic centers with a history of significant explosive behavior, such as Mount Cayley.^[1]

Several hot springs on Cayley's southwestern flank indicates that magmatic heat is still present, providing further evidence of continuous volcanic activity.^[2] The long history of volcanism in the area, coupled with continued subduction off the British Columbia Coast, indicates that volcanism has not yet concluded in the area. Because of this magmatic heat, Mount Cayley has been a target for geothermal exploration.^[2] Bottom hole temperatures of 50 °C and thermal gradients of less than 100 °C have been measured in shallow boreholes on Cayley's southwest flank.^[2]

Related features

Mount Cayley is surrounded by smaller volcanic features and volcanoes called satellite cones, also known as parasitic cones.^[20] These formed due to Cayley's volcanic vent being heavily blocked by cooled and solidified lava, causing magma to force out of the lines of weakness at the side of the volcano, forming a satellite cone.^[20] They commonly derive material from the same source as the initial volcano, although it may have its own magma chamber system.^[20] Similar volcanic formations are found elsewhere in the Cascade Volcanic Arc, including Mount Shasta in Northern California, which consists of four overlapping volcanic cones and several satellite cones, including Black Butte and Shastina.^[21] The small satellite cones at Mount Cayley become progressively younger from south to north, ranging in age from Pliocene-to-Pleistocene which forms a volcanic field. Because these features are related to the stratovolcano of Mount Cayley, the volcanic field is commonly referred to as the Mount Cayley volcanic field.^[13] The high elevations of the volcanic field, coupled with its cluster of mostly high altitude, non-overlapping vents, have resulted in several eruptions under the Powder Mountain Icefield, creating many ice-contact features.^[13] Due to the volcanic field's remoteness, it has not been studied or mapped in detail.^[13] As a result, the number and age of eruptions remains unknown.^[13]

Ember Ridge, the oldest and southern known parasitic vent, is a subglacial volcano that formed and last erupted during the Pliocene period.^[22] It comprises a chain of steep-sided lava domes with glassy, tortuously jointed lava, such as hornblende-phyric basalt.^[13]

Mount Fee, about 1 kilometre (0.62 mi) north of Ember Ridge, is a parasitic volcanic plug comprising a narrow summit ridge about a kilometre (⅔ mi) long.^[13] It contains several spines reaching heights of 100 to 150 metres (330–490 ft).^[13] Several pyroclastic deposits are found at the volcano, indicating it might have been covered by layers of pyroclastic rocks that have now been mostly worn away by erosion.^[13] The complete denudation of the central spine as well as the absence of till under lava and pyroclastics indicate it is preglacial or Pleistocene age.^[13][23]

North of Mount Cayley lies the parasitic Pali Dome subglacial volcano which is partly covered by glacial ice.^[13] It formed and last erupted during the Pleistocene period, producing coarsely lava flows, such as plagioclase-hypersthene-hornblende-phyric andesite.^[13][24] Proximal sections of lava flows contain vertical, well developed, large-diameter columnar joints, and lie beneath scoriaeous oxidized flow breccia, suggesting a possible subaerial origin.^[13] Distal sections of lava flows are glassy and contain minor diameter columnar joints with horizontal or nearby radiating orientations.^[13] Lava flow terminations appear as subvertical cliffs up to 200 metres (660 ft) in height, which are structures constant with eruptions against glacial ice.^[13]

Northwest of Pail Dome lies a parasitic subglacial volcano called Cauldron Dome which also formed and last erupted during the Pleistocene period.^[25] It consists of coarsely lava flows, such as plagioclase-orthophyroxene-phyric andesite.^[13] Its total geomorphology is comparable to that of a tuya.^[13] However, any precise record of volcanic glass or fine-scale jointing has probably been worn away by erosion.^[13] Two compositionally identical lava flows spread to the southwest from the base of the volcano.^[13] It is likely that Cauldron Dome was formed subglacially and the associated lava flows were erupted within a meltwater conduit.^[13]

Slag Hill, another parasitic subglacial volcano located just north of Mount Cayley, was erupted during the Pleistocene period, producing glassy lava flows, such as augite-phyric basaltic andesite.^[13][26] These lava flows were cooled to form steep-sided, glassy, finely jointed lava domes comparable to those found at Ember Ridge, and one minor, flat-topped bluff.^[13]

Ring Mountain, just north of Slag Hill, is a parasitic tuya composed of plagioclase-hypersthene-phyric andesite.^[13] The highest elevation of the volcano comprises bomb-like fragments of vesicular, oxidized lava, suggesting that the higher elevation lava flows were most likely subaerial.^[13] However, as is the case at volcanoes of comparable morphology elsewhere, lower elevations might have erupted subglacially.^[13]

Little Ring Mountain, also known as Little Ring Peak, is an almost circular, flat-topped, steep-sided volcanic feature about 270 metres (890 ft) in height and 120 metres (390 ft) wide on its top surface.^[13] It is known to be the northernmost parasitic cone and is similar in structure to a flat-topped, steep-sided tuya, although its inner stratigraphy is not yet known because the area has not been studied in detail due to its remoteness.^[13]

Volcanic hazards

Volcanic eruptions in Canada rarely cause fatalities because of their remoteness and low level of activity. The only known fatality due to volcanic activity in Canada occurred at the Tseax Cone in 1775, when a 22.5 kilometres (14.0 mi) long basaltic lava flow traveled down the Tseax and Nass Rivers, destroying a Nisga'a village and killing approximately 2,000 people by volcanic gases.^[27] Many towns and cities near Mount Cayley are home to well over half of British Columbia's human population, and there is a likelihood that future eruptions will cause damage to populated areas, making Mount Cayley and other Garibaldi belt volcanoes a major hazard.^[28] There are significant hazards from Canadian volcanoes that require hazard maps and emergency plains.^[29] Volcanoes which exhibit significant seismic activity, such as Mount Cayley, appear to be most likely to erupt.^[29] A significant eruption of the Garibaldi belt volcanoes would significantly impact Highway 99 and communities like Pemberton, Whistler and Squamish, and possibly Vancouver.^[29]

Eruption scenario

The western flank of the volcanic complex has been the site of several landslides.^[1] Several hot springs exist high up on the western flank of the mountain.^[1] There have been shallow earthquakes close to Mount Cayley since 1985, and seismic studies by seismologists and related geoscientists have discovered a strong mid-crustal reflector below it consistent with an unusually large, solidified, mafic, sill-like intrusion lying approximately 12.5 to 13 kilometres (7.8–8.1 mi) below the mountain.^[1] The eruptive scenario is based on past volcanic activity in the Garibaldi Volcanic Belt, in terms of magnitude and sequence of events, to its neighbor Mount Meager about 2,350 years ago.^[1] This volcanic eruption was similar in size to the 1980 eruption of Mount St. Helens and is Canada's most recent major eruption.^[12]

Significant indicator activity would be expected below the mountain weeks to years before magma penetrates its way through the Earth's crust.^[1] The large amount of seismicity and the sensitivity of the current seismograph in this region would warn the Geological Survey of Canada and would most likely cause an expanded monitoring effort.^[1] As the magma rises to the surface, the mountain would probably swell and the surface would likely rupture, causing greatly increased vigour in the hot springs, and the formation of new hot springs or fumaroles on the mountainside.^[1] In other words, they would get hotter. Minor and possibly large landslides could occur and may perhaps temporarily dam the Squamish River, as happened in the past without earthquake shaking and intrusion-related deformation.^[1] The continued presence of magma near the surface would eventually make contact with surface water, causing phreatic eruptions and debris flows, such as what occurred in the 1980 eruption of Mount St. Helens.^[1] By this time Highway 99 would be closed and Squamish would be at least partially, and possibly entirely, abandoned.^[1]

Eruptive activity itself could continue for years, followed by years of decreasing secondary activity.^[1] Volcanic ash would most likely spread throughout the Pacific Northwest, causing airports to be closed and relevant flights to be diverted or cancelled. The associated ash column would then extend eastward by the prevailing winds and disrupt air traffic throughout Canada from Alberta to Newfoundland and Labrador. The cooling lava would discontinuously spall units to create pyroclastic flows (super-heated mix of gas, ash, and pumice).^[1] The loose volcanic rock and pyroclastic material on Cayley's flanks and in valleys would be periodically reactivated into debris flows.^[1] Considerable structural improvements would have to be made to reclaim use of Highway 99 and Squamish area.^[1]

Monitoring

Currently Mount Cayley is not monitored closely enough by the Geological Survey of Canada to ascertain how active the volcano's magma system is.^[30] The existing network of seismographs has been established to monitor tectonic earthquakes and is too far away to provide a good indication of what is happening beneath the mountain.^[30] It may sense an increase in activity if the volcano becomes very restless, but this may only provide warning of a large eruption.^[30] It might detect activity only once the volcano has started erupting.^[30]

A possible way to detect an eruption is studying Cayley's geological history since every volcano has its own pattern of behavior, in terms of its eruption style, magnitude and frequency, so that its future eruption is expected to be similar to its previous eruptions.^[30]

While there is a likelihood of Canada being critically effected by local or close by volcanic eruptions argues that some kind of improvement program is required.^[1] Cost-benefit thoughts are critical to dealing with natural hazards.^[1] However, a cost-benefit examination needs correct data about the hazard types, magnitudes and occurrences. These do not exist for volcanoes in British Columbia or elsewhere in Canada in the detail required.^[1]

Other volcanic techniques, such as hazard mapping, displays a volcano's eruptive history in detail and speculates an understanding of the hazardous activity that could possibly be expected in the future.^[1] At present no hazard maps have been created for Mount Cayley because the level of knowledge is insufficient due to its remoteness.^[1] A large volcanic hazard program has never existed within the Geological Survey of Canada.^[1] The majority of information has been collected in a lengthy, separate way from the support of several employees, such as volcanologists and other geologic scientists. Current knowledge is best established at Mount Meager just north of Mount Cayley and is likely to rise considerably with a temporary mapping and monitoring project.^[1] Knowledge at Mount Cayley and other volcanoes in the Garibaldi Volcanic Belt is not as established, but certain contributions are being done at least Mount Cayley.^[1] An intensive program classifying infrastructural exposure near young Canadian volcanoes and quick hazard assessment at each individual volcanic edifice associated with recent seismic activity would be in advance and would produce a quick and productive determination of priority areas for further efforts.^[1]

The existing network of seismographs to monitor tectonic earthquakes has existed since 1975, although it remained small in population until 1985.^[1] Apart from a few short-term seismic monitoring experiments by the Geological Survey of Canada, no volcano monitoring has been accomplished at Mount Cayley or at other volcanoes in Canada at a level approaching that in other established countries with historically active volcanoes.^[1] Active or restless volcanoes are usually monitored using at least three seismographs within approximately 15 kilometres (9.3 mi), and frequently within 5 kilometres (3 mi), for better sensitivity of detection and reduced location errors, particularly for earthquake depth.^[1] Such monitoring detects the risk of an eruption, offering a forecasting capability which is important to mitigating volcanic risk.^[1] Currently Mount Cayley does not have a seismograph closer than 41 kilometres (25 mi).^[1] With increasing distance and declining numbers of seismographs used to indicate seismic activity, the prediction capability is reduced because earthquake location accuracy and depth decreases, and the network becomes not as accurate.^[1] The inaccurate earthquake locations in the Garibaldi Volcanic Belt are a few kilometers, and in more isolated northern regions they are up to 10 kilometres (6 mi).^[1] The location magnitude level in the Garibaldi Volcanic Belt is about magnitude 1 to 1.5, and elsewhere it is magnitude 1.5 to 2.^[1] At carefully monitored volcanoes both the located and noticed events are recorded and surveyed immediately to improve the understanding of a future eruption.^[1] Undetected events are not recorded or surveyed in British Columbia immediately, nor in an easy-to-access process.^[1]

In countries like Canada it is possible that small precursor swarms might go undetected, particularly if no events were observed; more significant events in larger swarms would be detected but only a minor subdivision of the swarm events would be complex to clarify them with confidence as volcanic in nature, or even associate them with an individual volcanic edifice.^[1]

See also

• Cascade Volcanoes
• Garibaldi Volcanic Belt
• Cascade Range
• Pacific Ranges
• Volcanism of Canada
• Volcanism of Western Canada
• Geology of the Pacific Northwest

References

External links

Wikimedia Commons has media related to Mount Cayley.

• Volcanoes of Canada Garibaldi Volcanic Belt (Mount Cayley area)
• Catalogue of Canadian volcanoes - Mount Cayley

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