The Mount Cayley volcanic field is a remote volcanic zone on the South Coast of British Columbia, Canada, stretching 31 km (19 mi) 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. This volcanic activity ranged from effusive to explosive, with magma compositions ranging from basaltic to rhyolitic. Because the Mount Cayley volcanic field 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 800 m (2,600 ft) 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 field 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 are situated in 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 past 10,000 years.
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