The Mount Cayley volcanic field (MCVF) is a remote volcanic zone on the South Coast of British Columbia, Canada, stretching 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 MCVF 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, a volcanic peak 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 MCVF began between 1.6 and 5.3 million years ago. At least 23 eruptions have occurred throughout its eruptive history. This volcanic activity ranged from effusive to explosive, with magma compositions ranging from basaltic to rhyolitic. Because the MCVF has a high elevation and consists of a cluster of mostly high altitude, non-overlapping volcanoes, subglacial activity is likely to have occurred under less than of glacial ice. The style of this glaciation promoted meltwater escape during eruptions. The steep profile of the volcanic field and its subglacial landforms support this hypothesis. As a result, volcanic features in the MCVF that interacted with glacial ice lack rocks that display evidence of abundant water during eruption, such as hyaloclastite and pillow lava.
Of the entire volcanic field, the southern portion has the most known volcanoes. Here, at least 11 of them are situated on top of a long narrow mountain ridge and in adjacent river valleys. The central portion contains at least five volcanoes situated at the Powder Mountain Icefield. To the north, two volcanoes form a sparse area of volcanism. Many of these volcanoes were formed between 0.01 and 1.6 million years ago, some of which show evidence of volcanic activity in the last 10,000 years.
Geology
Formation
The MCVF formed as a result of the ongoing subduction of the Juan de Fuca Plate under the North American Plate at the Cascadia subduction zone along the British Columbia Coast. This is a long fault zone running off the Pacific Northwest from Northern California to southwestern British Columbia. The plates move at a relative rate of over per year at an oblique angle to the subduction zone. Because of the very large fault area, the Cascadia subduction zone can produce large earthquakes of magnitude 7.0 or greater. The interface between the Juan de Fuca and North American plates remains locked for periods of roughly 500 years. During these periods, stress builds up on the interface between the plates and causes uplift of the North American margin. When the plate finally slips, the 500 years of stored energy are released in a massive earthquake.
Unlike most subduction zones worldwide, there is no deep oceanic trench along the continental margin of Cascadia. The reason is that the mouth of the Columbia River empties directly into the subduction zone and deposits silt at the bottom of the Pacific Ocean, burying this large depression. Massive floods from prehistoric Glacial Lake Missoula during the Late Pleistocene also deposited large amounts of sediment into the trench. However, in common with other subduction zones, the outer margin is slowly being compressed, similar to a giant spring. When the stored energy is suddenly released by slippage across the fault at irregular intervals, the Cascadia subduction zone can create very large earthquakes, such as the magnitude 9.0 Cascadia earthquake on January 26, 1700. However, earthquakes along the Cascadia subduction zone are less common than expected and there is evidence of a decline in volcanic activity over the last few million years. The probable explanation lies in the rate of convergence between the Juan de Fuca and North American plates. These two tectonic plates currently converge to per year. This is only about half the rate of convergence from seven million years ago.
Scientists have estimated that there have been at least 13 significant earthquakes along the Cascadia subduction zone in the last 6,000 years. The most recent, the 1700 Cascadia earthquake, was recorded in the oral traditions of the First Nations people on Vancouver Island. It caused considerable tremors and a massive tsunami that traveled across the Pacific Ocean. The significant shaking associated with this earthquake demolished houses of the Cowichan Tribes on Vancouver Island and caused several landslides. Shaking due to this earthquake made it too difficult for the Cowichan people to stand, and the tremors were so lengthy that they were sickened. The tsunami created by the earthquake ultimately devastated a winter village at Pachena Bay, killing all the people that lived there. The 1700 Cascadia earthquake caused near-shore subsidence, submerging marshes and forests on the coast that were later buried under more recent debris.
Subglacial volcanoes Lying in the middle of the MCVF is a subglacial volcano named Slag Hill. At least two geological units compose the edifice. Slag Hill itself consists of andesite lava flows and small amounts of pyroclastic rock. Lying on the western portion of Slag Hill is a lava flow that likely erupted less than 10,000 years ago due to the lack of features indicating volcano-ice interactions. The Slag Hill flow-dominated tuya northeast of Slag Hill consists of a flat-topped, steep-sided pile of andesite. It protrudes through remnants of volcanic material erupted from Slag Hill, but it represents a separate volcanic vent due to its geographical appearance. This small subglacial volcano possibly formed between 25,000 and 10,000 years ago throughout the waning stages of the Fraser Glaciation.
Cauldron Dome, a subglacial volcano north of Mount Cayley, lies west of the Powder Mountain Icefield. Like Slag Hill, it is composed of two geological units. Upper Cauldron Dome is a flat-topped, oval-shaped pile of at least five andesite lava flows that resembles a tuya. The five andesite flows are columnar jointed and were likely extruded through glacial ice. The latest volcanic activity might have occurred between 10,000 and 25,000 years ago when this area was still influenced by glacial ice of the Fraser Glaciation. Lower Cauldron Dome, the youngest unit comprising the entire Cauldron Dome subglacial volcano, consists of a flat-topped, steep-sided pile of andesite lava flows long and a maximum thickness of . These volcanic rocks were extruded about 10,000 years ago during the waning stages of the Fraser Glaciation from a vent adjacent to upper Cauldron Dome that is currently buried under glacial ice.
Ring Mountain, a flow-dominated tuya lying at the northern portion of the MCVF, consists of a pile of at least five andesite lava flows lying on a mountain ridge. Its steep-sided flanks reach heights of and are composed of volcanic rubble. This makes it impossible to measure its exact base elevation or how many lava flows constitute the edifice. With a summit elevation of , Ring Mountain had its last volcanic activity between 25,000 and 10,000 years ago when the Fraser Glaciation was close to its maximum. Northwest of Ring Mountain lies a minor andesite lava flow. Its chemistry is somewhat unlike other andesite flows comprising Ring Mountain, but it probably erupted from a volcanic vent adjacent to or at Ring Mountain. The part of it that lies higher in elevation contains some features that indicate lava-ice interactions, whereas the lower-elevation portion of it does not. Therefore, this minor lava flow was likely extruded after Ring Mountain formed but when glacial ice covered a broader area than it does to this day, and that the lava flowed beyond the region in which glacial ice existed at that time.
To the north lies Little Ring Mountain, another flow-dominated tuya lying at the northern portion of the MCVF. It consists of a pile of at least three andesite lava flows lying on a mountain ridge. Its steep-sided flanks reach heights of and are composed of volcanic rubble. This makes it impossible to measure its exact base elevation or how many lava flows comprise the edifice. With a summit elevation of , Little Ring Mountain had its last volcanic activity between 25,000 and 10,000 years ago when the Fraser Glaciation was close to its maximum.
Ember Ridge, a mountain ridge between Tricouni Peak and Mount Fee, consists of at least eight lava domes composed of andesite. They were likely formed between 25,000 and 10,000 years ago when lava erupted beneath glacial ice of the Fraser Glaciation. Their current structures are comparable to their original forms due to the minimal degree of erosion. As a result, the domes display the shapes and columnar joints typical of subglacial volcanoes. The random shapes of the Ember Ridge domes are the result of erupted lava taking advantage of former ice pockets, eruptions taking place on uneven surfaces, subsidence of the domes during volcanic activity to create rubble and separation of older columnar units during more recent eruptions. The northern dome, known as Ember Ridge North, covers the summit and eastern flank of the mountain ridge. It comprises at least one lava flow that reaches a thickness of , as well as the thinnest columnar units in the MCVF. The small size of the columnar joints indicates that the erupted lava was cooled immediately and are mainly located on the dome's summit. Ember Ridge Northeast, the smallest subglacial dome of Ember Ridge, comprises one lava flow that has a thickness no more than . Ember Ridge Northwest, the most roughly circular subglacial dome, comprises at least one lava flow. Ember Ridge Southeast is the most complex of the Ember Ridge domes, consisting of a series of lava flows with a thickness of . It is also the only Ember Ridge dome that contains large amounts of rubble. Ember Ridge Southwest comprises at least one lava flow that reaches a thickness of . It is the only subglacial dome of Ember Ridge that contains hyaloclastite. Ember Ridge West comprises only one lava flow that reaches a thickness of .
Mount Brew, southwest of the resort town of Whistler, is a high lava dome composed of andesite or dacite that probably formed subglacially between 25,000 and 10,000 years ago. It contains two possible ice-marginal lava flows that have not been studied in detail. They may have formed during the same time period as the Ember Ridge subglacial domes due to their similar structures, columnar joints and compositions.
Eroded volcanoes