page_title: Mount Cayley volcanic field
section_title: Eruptions
hierarchical_section_title: Mount Cayley volcanic field / Volcanic hazards / Eruptions
caption_attribution_description: English: Map of the Mount Cayley volcanic field in southwestern British Columbia, Canada.
context_page_description: The Mount Cayley volcanic field is a remote volcanic zone on the South Coast of British Columbia, Canada, stretching 31 km from the Pemberton Icefield to the Squamish River. It forms a segment of the Garibaldi Volcanic Belt, the Canadian portion of the Cascade Volcanic Arc, which extends from Northern California to southwestern British Columbia. Most of the Cayley volcanoes were formed during periods of volcanism under sheets of glacial ice throughout the last glacial period. These subglacial eruptions formed steep, flat-topped volcanoes and subglacial lava domes, most of which have been entirely exposed by deglaciation. However, at least two volcanoes predate the last glacial period and both are highly eroded. The field gets its name from Mount Cayley, the largest and most persistent volcano, located at the southern end of the Powder Mountain Icefield. This icefield covers much of the central portion of the volcanic field and is one of the several glacial fields in the Pacific Ranges of the Coast Mountains.
  Eruptions along the length of the field began between 1.6 and 5.3 million years ago. At least 23 eruptions have occurred throughout its eruptive history.
context_section_description: Eruptive activity in the Mount Cayley volcanic field is typical of past volcanism elsewhere in the Garibaldi Belt. Large earthquakes would occur under the volcanic field weeks to years in advance as molten rock intrudes through the Earth's rocky lithosphere. The extent of earthquakes and the local seismographs in this region would warn the Geological Survey of Canada and possibly cause an upgrade in monitoring. While molten rock breaks through the crust, the size of the volcano vulnerable to an eruption would probably swell and the area would rupture, creating much more hydrothermal activity at the regional hot springs and the formation of new springs or fumaroles. Small and probably significant rock avalanches may result and could dam the nearby Squamish River for a limited amount of time, such as those that occurred in the past without seismic activity and deformation related to magmatic activity. At some point the subsurface magma will produce phreatic eruptions and lahars. At this time Highway 99 would be out of service and the residents of Squamish would have to travel away from the eruptive zone.
  While molten rock comes closer to the surface it would most likely cause more fragmentation, triggering an explosive eruption that could produce an eruption column with an elevation of 20 km (12 mi) and may be sustained for 12 hours. A well-documented explosive eruption in the Garibaldi Belt with such force is the eruption of the Mount Meager massif 2,350 years ago, which deposed ash as far east as Alberta. This would endanger air traffic and would have to take another route away from the eruptive zone. Every airport buried under pyroclastic fall would be out of service, including those in Vancouver, Victoria, Kamloops, Prince George and Seattle. The tephra would destroy power transmission lines, satellite dishes, computers and other equipment that operates on electricity. Therefore, telephones, radios and cell phones would be disconnected. Structures not built for holding heavy material would likely demolish under the weight of the tephra. Ash from the eruption plume would subside above the vent area to create pyroclastic flows and would travel east and west down the nearby Cheakamus and Squamish river valleys. These would likely have significant impacts on salmon in the associated rivers and would cause considerable melting of glacial ice to produce debris flows that may extend into Daisy Lake and Squamish to cause significant damage. The eruption column would then travel eastward and extract air travel throughout Canada from Alberta to Newfoundland and Labrador.
  Explosive eruptions may decrease and be followed by the eruption of viscous lava to form a lava dome in the newly formed crater. Precipitation would frequently trigger lahars and these would continuously create problems in the Squamish and Cheakamus river valleys. If the lava dome continues to grow, it would eventually rise above the crater rim. The lava would be cooling and expanding then may produce landslides to create a massive zone of blocky talus in the Squamish river valley. While the dome of lava grows, it would frequently subside to create large pyroclastic flows that would again travel down the adjacent Squamish and Cheakamus river valleys. Tephra swept away from the pyroclastic flows would create ash columns with elevations of at least 10 km (6.2 mi), repeatedly depositing tephra on the communities of Whistler and Pemberton and again disrupting regional air traffic. Lava of the unstable dome may occasionally create minor pyroclastic flows, explosions and eruption columns. The community of Squamish would be abandoned, Highway 99 would be out of service and destroyed, and traffic adjacent to Vancouver, Pemberton and Whistler would remain forced to travel along a route to the east that is more lengthy than Highway 99.
  Eruptions would likely continue for a period of time, followed by years of decreasing secondary activity. The solidifying lava would occasionally collapse portions of the volcano to create pyroclastic flows. Rubble on the flanks of the volc
caption: The extent of the Mount Cayley volcanic field. Also shown is the lateral ice-covered mountain ridge the volcanic field lies on.
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