BackgroundThe Mount Edziza volcanic complex contains a large basal shield or plateau (65 km long and 20 km wide) consisting of predominantly basaltic lava flows, with four large stratovolcanoes built on top of the plateau. The complex was constructed during 5 major magmatic cycles, each beginning with eruption of alkali basalts and ending with felsic and basaltic eruptions:
Phase 1: Armadillo Peak eruptive period
Armadillo Peak stratovolcano was built 7.5 million years ago at the south end of the complex. Armadillo Peak has a largely glacially eroded caldera and is overlapped by the Ice Peak central volcano. Armadillo Peak is one of the 4 central volcanoes of the complex.
Phase 2: Spectrum Range eruptive period
The second phase of activity began 3 million years ago, emplacing rhyolitic magma 150 metres thick and 13 metres long during a single event of activity.
A broad circular lava domecalled the Spectrum Range was built on the southwestern flank of Armadillo Peak and north of the Arctic Lake Plateau. This is the southernmost of the 4 central volcanoes. It is over 10 km wide and up to 650 m thick and overlies an older basal shield volcano and contains deeply carved circular valleys. The deeply carved circular valleys also display the bounding faults of a buried caldera approximately 4.5 km across.
More than 100 km3 of rhyolite and trachyte was erupted during the Spectrum Range dome eruptive period, with its activity ending 2,500,000 years ago.
Spectrum Range is known for its colorful alterated rocks.
Phase 3: Ice Peak eruptive period
Ice Peak, 2,500 metres (8,200 ft) high, overlaps the northern flank of Armadillo Peak. It began to form 1,600,000 years ago when the regional Cordilleran Ice Sheet began retreating. It is a stratovolcano that was constructed when large areas of the Edziza lava plateau were free from glacial ice and now enclosed by glacial deposits. The volcanic activity produced lava flows and pyroclastic flows, which mixed with meltwater to produce debris flows.
As Ice Peak began to form, basic lava flowed into meltwater lakes, creating pillow lava and solidified rubble. At the end of its activity 1.5 million years ago, 3 viscous lava domes formed on its western side during and developed nearly all of the steep, higher flanks of the volcano.
Two cinder cones also exist on Ice Peak's southern flank called Camp Hill and Cache Hill.
During the long period of Ice Peak activity, high-altitude glaciers developed and melted cutting valleys into the volcano. The current summit of Ice Peak is a remnant of the western rim of a small summit caldera. Near the end of Ice Peak activity 1,500,000 years ago, this high-altitude glacial ice combined with the regional ice forming part of the Cordilleran Ice Sheet. It is likely that only the tallest mountains might have been visible over the Cordilleran Ice Sheet which was at least 2,285 metres (7,497 ft) thick. A small volume of intermediate lava was erupted from Ice Peak compared to the other central volcanoes.
Phase 4: Mount Edziza eruptive period
1 million years ago when the Cordilleran Ice Sheet retreated from the upper flanks of the adjacent lava plateau, Mount Edziza proper started to form, the northernmost of the 4 central volcanoes.
It is a steep-sided stratovolcano and the largest and highest of the peaks. It is made up of fine-grained trachyte from several lava domes. Its smooth northern and western flanks, only slightly channeled by erosion, curve up to a circular 2,700-metre (8,900 ft) summit ridge which surrounds a central, ice-filled caldera 2 km in diameter.
Many glaciers cover Mount Edziza, including the Tencho Glacier on its southern flank. Depressions on the east flank expose the remnants of numerous lava lakes which ponded in the caldera 900,000 years ago.
Piles of pillow lava and hyaloclastite, formed by subglacial eruptions, are found on the flanks of Mount Edziza and nearby Ice Peak, as well as on the surface of the surrounding shield volcano. Pillow Ridge on Edziza's northwest flank was formed when basaltic lava erupted beneath the regional Cordilleran Ice Sheet when it was close to its greatest thickness.
Phase 5: Central volcano flank eruptive period
The last phase of eruptive activity started 10,000 years ago and occurred from flank vents on the 4 central volcanoes. This activity created numerous explosion craters and 30 small cinder cones, primarily of basaltic composition, including Mess Lake Cone, Kana Cone, Cinder Cliff, Icefall Cone, Ridge Cone, Williams Cone, Walkout Creek Cone, Moraine Cone, Sidas Cone, Sleet Cone, Storm Cone, Triplex Cone, Twin Cone, Cache Hill, Camp Hill, Cocoa Crater, Coffee Crater, Nahta Cone, Tennena Cone, The Saucer and the well-preserved Eve Cone.
Some of these cinder cones were formed no more than 1300 years ago based on the age of burnt plant stems. The Snowshoe lava field, on the southern end of the Big Raven Plateau, and the Desolation lava field on the northern end of the Big Raven Plateau are two young lava fields. Desolation lava field is the largest of young lava flows, covering an area of 150 km2, and has lava flows reaching 12 km length. This volcanic activity was followed by at least 2 younger, but still undated eruptions, one of which was a major explosive event that deposited an up to 1.5 m thick layer of pumice, which is still free of vegetation and covers an area of 77 km2.
- Souther, J.G., Armstrong, R.L., and Harakal, J. (1984) "Chronology of the peralkaline, late Cenozoic Mount Edziza Volcanic Complex, northern British Columbia, Canada", GSA Bulletin; March 1984; v. 95; no. 3; p. 337-349
- Souther, J.G. (1992) "The Late Cenozoic Mount Edziza Volcanic Complex, British Columbia", Geological Survey of Canada Memoir 420
- Mount Edziza on Wikipedia
- Catalogue of Canadian volcanoes (Natural Resources Canada website, Edziza)
See also: Sentinel hub | Landsat 8