Background Mount Edziza is part of the
Northern Cordilleran Volcanic Province, a broad area of shield volcanoes, lava domes, cinder cones and stratovolcanoes extending from northwestern British Columbia northwards through
Yukon into easternmost
Alaska. The dominant rocks comprising these volcanoes are
alkali basalts and
hawaiites, but
nephelinite,
basanite and
peralkaline phonolite, trachyte and
comendite are locally abundant. These rocks were deposited by volcanic eruptions from 20 million years ago to as recently as a few hundred years ago. Volcanism in the Northern Cordilleran Volcanic Province is thought to be due to
rifting of the
North American Cordillera, driven by changes in relative
plate motion between the
North American and
Pacific plates.
Composition A wide variety of volcanic rocks comprise Mount Edziza, the main
mafic rock being
basalt which comprises lava flows, cinder cones and ash
beds on the flanks of the mountain. Basalt at Mount Edziza is in the form of alkali basalt and hawaiite; the latter is thought to be the product of partial
fractional crystallization in subterranean
magma chambers. Volcanic rocks of
intermediate composition such as
trachybasalt,
tristanite,
mugearite and
benmoreite are restricted to Ice Peak where they form the upper part of this subsidiary peak. Ice Peak is the only location where mugearites and benmoreites are found in the Mount Edziza volcanic complex.
Felsic volcanic rocks such as trachyte and
rhyolite form the central stratovolcano of Mount Edziza, the upper part of Ice Peak and several lava domes and flows, as well as pyroclastic rocks.
Stratigraphy Mount Edziza is subdivided into at least six
geological formations, each the product of a distinct stage of volcanic activity. These periods of volcanic activity occurred during four magmatic cycles of the Mount Edziza volcanic complex; each cycle began with the effusion of alkali basalt and culminated with the eruption of lesser volumes of felsic magma. The oldest geological formation comprising Mount Edziza is the
Pyramid Formation, which formed during a period of volcanic activity 1.1 million years ago at the end of the
second magmatic cycle. Another period of volcanic activity about 1 million years ago during the start of the
third magmatic cycle deposited the
Ice Peak Formation on the southern part of the Pyramid Formation. The third and fourth oldest geological formations comprising Mount Edziza are the Pillow Ridge and
Edziza formations, which were deposited by volcanic eruptions 0.9 million years ago during the later stages of the third magmatic cycle; they both overlie the Ice Peak Formation. A period of volcanic activity at the end of the
fourth magmatic cycle 0.3 million years ago deposited the Kakiddi Formation, the fifth oldest geological formation comprising Mount Edziza which also overlies the Ice Peak Formation. The youngest geological formation is the
Big Raven Formation, which was formed by a period of volcanic activity during the
fifth magmatic cycle in the last 20,000 years.
Pyramid Formation on the northeastern flank of Mount Edziza The Pyramid Formation is exposed along the deeply eroded eastern flank of Mount Edziza where rhyolite and trachyte flows, domes and pyroclastic rocks of this formation comprise ridges and prominent cliffs. A basaltic lava flow sequence up to thick overlies the basal trachytic
surge deposit of the Pyramid Formation; it is included as a part of this formation due to it being coeval with the early stages of Pyramid felsic volcanism.
Potassium–argon dating of the Pyramid Formation has yielded ages of 1.2 ± 0.4 million years and 1.2 ± 0.03 million years from comenditic
glass, as well as 0.94 ± 0.12 million years and 0.94 ± 0.05 million years from trachyte. The Pyramid Formation includes Sphinx Dome, Pharaoh Dome and The Pyramid which were the main sources of the rhyolites and trachytes of this geological formation. The Pyramid is a prominent trachyte dome whose structure has not been greatly modified by erosion, nor has it been buried under younger lavas. In contrast, much of the southern edge of Sphinx Dome has been destroyed by headward erosion of Cook Creek; the western half of this rhyolite dome is also buried under trachyte of the Edziza Formation. From Cartoona Ridge north to Tennaya Creek are isolated remnants of Pharaoh Dome, the main mass of which comprises flow-layered rhyolite and is buried under basalt of the Ice Peak Formation.
Ice Peak Formation The Ice Peak Formation consists of lava and pyroclastic rocks that originated mainly from Ice Peak about south of the summit of Mount Edziza. Two
stratigraphic units comprise this once symmetrical stratovolcano, both of which are
lithologically distinct. The lower stratigraphic unit, which forms much of the volcanic pile, is an assemblage of mostly thin basalt flows. Lavas of intermediate composition such as tristanite, trachybasalt and mugearite are very limited in extent. The upper stratigraphic unit is a highly varied succession of lavas and pyroclastic rocks forming the high, central edifice of Ice Peak. It consists of basalt, trachyte and a variety of intermediate rocks such as tristanite, trachybasalt, benmoreite and mugearite. The Ice Peak Formation also includes the Koosick and Ornostay bluffs, both of which are thick lobes of trachyte that originated under the summit ice cap. Both bluffs are similar in
geomorphology and composition, consisting of several lava flows up to thick. The Neck, which forms a prominent high
buttress on Sorcery Ridge, is also part of the Ice Peak Formation. Potassium–argon dating of
pantelleritic trachyte from the Ice Peak Formation has yielded ages of 1.6 ± 0.2 million years, 1.5 ± 0.4 million years and 1.5 ± 0.1 million years. These dates being older than those of the Pyramid Formation may be due to excess
argon in the Ice Peak Formation; therefore the dates are considered unreliable. Ice Peak Formation basalt flows on the northwestern flank of Mount Edziza are
interbedded with
diamictites recording a regional glaciation that occurred during the
Early Pleistocene. The lowermost basalt flow contains basal
pillows, directly overlies
hyaloclastites and is brecciated and deformed, suggesting that it may have been extruded onto a glacier or an
ice sheet. Its extrusion onto glacial ice is also evident due to the lack of
fluvial and
lacustrine sediments at the base of the basalt flow which suggests that it did not extrude into lakes or streams. The steep sides and unusually large thicknesses of the trachyte flows comprising Koosick and Ornostay bluffs is attributed to them having been extruded through glacial ice.
Pillow Ridge Formation The Pillow Ridge Formation is restricted to Pillow Ridge and Tsekone Ridge on Mount Edziza's northwestern flank, both of which are
glaciovolcanic in origin. Pillow Ridge is a nearly long, northwesterly-trending ridge of basaltic pillow lava, pillow breccia,
tuff breccia and
dikes. Its upper surface is sparsely covered by trachyte of the Edziza Formation whereas the western edge of the ridge overlaps with a large flow of Edziza trachyte. Tsekone Ridge is an isolated volcanic pile adjacent to Pillow Ridge consisting of basaltic pillow lava and tuff breccia that has been cut by vertical north-trending
feeder dikes. This ridge is elliptical in structure, containing a nearly long, north–south trending axis. Nearly surrounding Tsekone Ridge are trachyte flows of the Edziza Formation which is slightly younger than the Pillow Ridge Formation.
Fission track dating of
apatite from partially fused
granitic xenoliths in contaminated Pillow Ridge Formation basalt has yielded ages of 0.9 ± 0.3 million years and 0.8 ± 0.25 million years. In contrast, potassium–argon dating has yielded an anomalously old age of 5.9 ± 0.9 million years which is inconsistent with the ages of the underlying and overlying formations. This date being much older than the fission track dates most likely results from contamination and introduction of excess argon from the partially fused granitic and
gneissic xenoliths in Pillow Ridge Formation basalt.
Edziza Formation The Edziza Formation consists mainly of trachyte that straddles the pantelleritic trachyte and comenditic trachyte boundary. It includes the central stratovolcano of Mount Edziza, as well as several satellitic features on its summit and flanks. Inside the summit crater of the stratovolcano is a succession of at least four lava lakes exposed in the breached eastern crater rim. They are represented by at least four cooling units, the lower two of which are about thick. The two upper cooling units reach thicknesses of about and may have originated from Nanook Dome, the largest of three lava domes consisting of Edziza Formation trachyte. Nanook Dome is about in diameter whose structure appears to be nearly identical to its original form. The other two Edziza Formation trachyte domes, Glacier Dome and Triangle Dome, are elliptical in structure and contain concentric flow layering. Potassium–argon dating of pantelleritic trachyte or comenditic trachyte from the Edziza Formation has yielded an age of 0.9 ± 0.3 million years.
Kakiddi Formation The Kakiddi Formation consists of the remains of thick trachyte flows and associated pyroclastic rocks. They are lithologically and geomorphologically similar to Edziza Formation trachytes, but occur south of the central stratovolcano of Mount Edziza. The remains of a nearly wide, rubble-covered trachyte flow are present on the eastern flank of Ice Peak in Sorcery Valley and in the south fork of Tennaya Valley where it is divided into two tributary branches. In Kakiddi Valley, the lava flow appears to have spread out to form a once continuous, terminal lobe at least wide. Remnants of this terminal lobe are present in the form of isolated
outcrops adjacent to Kakiddi Lake and Nuttlude Lake. The source of this Kakiddi flow remains unknown, but the tributary branch that descended Tennaya Valley probably originated from a vent near the summit of Ice Peak that is now covered by glaciers. Another plausible source is Nanook Dome on the southeastern crater rim of Mount Edziza. A relatively small trachyte flow descended from Punch Cone on the western flank of Ice Peak and spread onto the Big Raven Plateau. Potassium–argon dating of the Kakiddi Formation has yielded ages of 0.31 ± 0.07 million years from mugearite, as well as 0.30 ± 0.02 million years, 0.29 ± 0.02 million years and 0.28 ± 0.02 million years from trachyte.
Big Raven Formation is part of the Snowshoe Lava Field. The Big Raven Formation includes the Desolation Lava Field, the Snowshoe Lava Field, Icefall Cone, Ridge Cone, Cinder Cliff and the
Sheep Track Member. All of these features consist of alkali basalt and hawaiite with the exception of the Sheep Track Member which comprises a small volume of trachyte pumice. Some of the lava flows comprising the Desolation Lava Field issued from vents adjacent to the northern trim line of the summit ice cap where meltwater interacted with the erupting lava to form
tuff rings. These tuff rings, composed of quenched breccia, later transitioned into normal
subaerial cinder cones as the progressing eruptions displaced ice and meltwater. The Snowshoe Lava Field contains
subglacial and subaerial cones, as well as transitional cones which consist of both
subaqueous and subaerial
ejecta. Eruptions on the heavily eroded eastern flank of Mount Edziza created Icefall Cone, Ridge Cone and Cinder Cliff which comprise a separate volcanic zone called the east slope centres. The Sheep Track Member is the product of an
explosive eruption that originated from the southwestern flank of Ice Peak. It was deposited on all lava flows and cinder cones in the Snowshoe Lava Field with the exception of The Saucer which likely postdates the Sheep Track eruption. The source of the Sheep Track pumice is unknown, but it probably originated from a vent hidden under Tencho Glacier.
Holocene in age, the Big Raven Formation has yielded dates of 6520 BCE ± 200 years, 750 BCE ± 100 years, 610 CE ± 150 years and 950 CE ± 6000 years.
Basement Underlying the aforementioned geological formations is the
Tenchen Member of the
Nido Formation, one of many stratigraphic units forming the Big Raven Plateau. Basalt flows and pyroclastic rocks of this
Pliocene geological member are exposed north of
Raspberry Pass on the eastern and western flanks of Mount Edziza. Much of the Tenchen Member as well as the southern edge of the Ice Peak volcanic pile are underlain by the
Armadillo Formation which consists of
Miocene comendite, trachyte and alkali basalt. Most of Mount Edziza is also underlain by Miocene basalt flows of the
Raspberry Formation which form the base of prominent
escarpments east and west of the mountain. These geological formations are underlain by the
Stikinia terrane, a
Paleozoic and
Mesozoic suite of volcanic and
sedimentary rocks that
accreted to the
continental margin of North America during the
Jurassic.
Hazards and monitoring from
Williams Cone on the northeastern flank of Mount Edziza A
Natural Resources Canada report published in 2021 considers Mount Edziza a high threat volcano because it has had the highest eruption rate in Canada throughout the Holocene. However, its extremely remote location makes it less
hazardous than Mounts
Garibaldi,
Price,
Cayley and
Meager in southwestern British Columbia. The hazard rating of Mount Edziza is similar to that of
Mount Churchill in the U.S. state of Alaska, which deposited the
White River Ash across northwestern Canada in the last 2,000 years. Lava flows are a potential hazard at Mount Edziza as they have formerly
dammed local streams. Another potential hazard is the ignition of wildfires by eruptions since the surrounding area contains vegetation. An eruption under the ice cap could produce floods or lahars that may flow into the Stikine or
Iskut rivers, potentially destroying
salmon runs and threatening river bank villages. Mount Edziza trachyte and rhyolite have
silica-rich compositions that are comparable to those associated with the most powerful eruptions around the world; parts of northwestern Canada could be affected by an
ash column if an explosive eruption were to happen from the volcano. Ash columns can drift for thousands of kilometres downwind and often become increasingly spread out over a larger area with increasing distance from an erupting vent. Mount Edziza lies under a major
air route from
Vancouver, British Columbia to
Whitehorse, Yukon, suggesting that the volcano poses a potential threat to air traffic. Volcanic ash reduces visibility and can cause jet engine failure, as well as damage to other aircraft systems. Like other volcanoes in Canada, Mount Edziza is not
monitored closely enough by the Geological Survey of Canada to ascertain its activity level. The
Canadian National Seismograph Network has been established to
monitor earthquakes throughout Canada, but it is too far away to provide an accurate indication of activity under the mountain. It may sense an increase in seismic activity if Mount Edziza becomes highly restless, but this may only provide a warning for a large eruption; the system might detect activity only once the volcano has started erupting. If Mount Edziza were to erupt, mechanisms exist to orchestrate relief efforts. The
Interagency Volcanic Event Notification Plan was created to outline the notification procedure of some of the main agencies that would respond to an erupting volcano in Canada, an eruption close to the
Canada–United States border or any eruption that would affect Canada. ==Human history==