Mount Edziza

Mount Edziza ( ; ) is a volcanic mountain in Cassiar Land District of northwestern British Columbia, Canada. It is located on the Big Raven Plateau of the Tahltan Highland which extends along the western side of the Stikine Plateau. Mount Edziza has an elevation of , making it the highest point of the Mount Edziza volcanic complex and one of the highest volcanoes in Canada. However, it had an elevation of at least before its formerly cone-shaped summit was likely destroyed by a violent eruption in the geologic past; its current flat summit contains an ice-filled, crater. The mountain contains several lava domes, cinder cones and lava fields on its flanks, as well as an ice cap containing several outlet glaciers which extend to lower elevations. All sides of Mount Edziza are drained by tributaries of Mess Creek and Kakiddi Creek which are situated within the Stikine River watershed.

Mount Edziza consists of several types of volcanic rocks and at least six geological formations that formed during six distinct stages of volcanic activity. The first stage 1.1 million years ago produced basalt flows and a series of rhyolite and trachyte domes. Basalt flows and smaller amounts of trachyte, tristanite, trachybasalt, benmoreite and mugearite produced during the second stage about 1 million years ago comprise Ice Peak, a glacially eroded stratovolcano forming the south peak of Mount Edziza. The third and fourth stages 0.9 million years ago created basalt ridges and the central trachyte stratovolcano of Mount Edziza, respectively. Thick trachyte flows were issued during the fifth stage 0.3 million years ago, most of which have since eroded away. The sixth stage began in the last 20,000 years with the eruption of cinder cones, basalt flows and minor trachyte ejecta. Renewed volcanism could block local streams with lava flows, disrupt air traffic with volcanic ash and produce floods or lahars from melting glacial ice.

Indigenous peoples have lived adjacent to Mount Edziza for thousands of years and it is a sacred mountain to the Tahltan people who historically used volcanic glass from it to make tools and weaponry. Mineral exploration just southeast of Mount Edziza had commenced by the 1950s where gold, silver and other metals were discovered. This mineral exploration was conducted by several mining companies into the early 1990s. Mount Edziza and the surrounding area was made into a large provincial park in the early 1970s to showcase the volcanic landscape. The mountain and provincial park can only be accessed by aircraft or by a network of horse trails from surrounding roads.

Name and etymology

The mountain was labelled ''Edziza Peak'' on Geological Survey of Canada maps as early as 1926. This name for the mountain was adopted by the Geographical Names Board of Canada on September 24, 1945, as identified on the 1926 Geological Survey of Canada map sheet 309A. ''Edziza Mountain'' appeared in the 1930 BC Gazetteer, in which the name was erroneously spelled ''Edzia''. On December 3, 1974, the form of name was changed from ''Edziza Peak'' to ''Mount Edziza'' in accordance with a 1927 British Columbia Land Surveyors report, two world aeronautical charts published in 1950, and three British Columbia maps published in 1931, 1933 and 1943. The form of the name was also changed to reflect entrenched local usage and in conformation with Mount Edziza Provincial Park, which was established in 1972. To the local Tahltan people, Mount Edziza is called , which translates to .

A number of explanations have been made regarding the origin of the name ''Edziza''. A 1927 report by J. Davidson of the British Columbia Land Surveyors claims that ''Edziza'' means in the Tahltan language, referring to the deep volcanic ash deposits or pumice-like sand covering large portions of the Big Raven Plateau around Mount Edziza. According to David Stevenson of University of Victoria's Anthropology Department, or is instead translated as in Tahltan. An explanation listed in the BC Parks brochure is that ''Edziza'' means in the Tahltan language. Another explanation proposed by Canadian volcanologist Jack Souther is that ''Edziza'' is a corruption of ''Edzerza'', the name of a local Tahltan family. Obsolete spellings of ''Edziza'' include ''Eddziza'', ''Eddiza'', ''Edidza'', ''Edzia'' and ''Etseza''.

Geography and geomorphology

Location and climate

Mount Edziza rises from within the middle of the Big Raven Plateau, a barren plateau in Cassiar Land District bounded on the west by Mess Valley, on the north by Klastline Valley, on the east by Kakiddi Valley and on the south by Chakima and Walkout valleys. It lies at the northern end of the Mount Edziza volcanic complex which also includes the smaller Arctic Lake and Kitsu plateaus to the south. This complex of shield volcanoes, stratovolcanoes, lava domes, calderas and cinder cones forms a broad, intermontane plateau at the eastern edge of the Tahltan Highland, a southeast-trending upland area extending along the western side of the Stikine Plateau.

Mount Edziza is in the Southern Boreal Plateau Ecosection which consists of several upland summits, wide river valleys and deeply incised plateaus. It is one of seven ecosections comprising the Boreal Mountains and Plateaus Ecoregion, a large ecological region of northwestern British Columbia encompassing high plateaus and rugged mountains with intervening lowlands. Boreal forests of black and white spruce occur in the lowlands and valley bottoms of this ecoregion whereas birch, spruce and willow form forests on the mid-slopes. Extensive alpine altai fescue covers the upper slopes, but barren rock is abundant at higher elevations.

The region is characterized by warm summers and cold, snowy winters; temperatures are warmest in mid-summer during the day when they may hit the range. However, temperatures can drop below freezing during summer nights, making snow or freezing rain a possibility at any time of the year. The closest weather stations to Mount Edziza are located at Telegraph Creek and Dease Lake, which lie about to the northwest and to the northeast, respectively.

Glaciation

Mount Edziza is covered with snow year-round, containing a long and wide ice cap which covers an area of . Several small outlet glaciers extending down to elevations of drain the ice cap. Outlet glaciers on the western side of the ice cap spread in broad lobes onto the Big Raven Plateau whereas distributary glaciers on the eastern side drape down steep slopes to form discontinuous icefalls. The Mount Edziza glacier complex is the only one worthy of note on the Stikine Plateau.

Four outlet glaciers of the ice cap are named, and all have names of Tahltan origin. Idiji Glacier descends from the eastern side of the ice cap near the head of Tennaya Creek. At the head of Tenchen Creek is Tenchen Glacier, a debris-covered glacier on the eastern side of the ice cap. Tencho Glacier at the southern end of the ice cap is the largest outlet glacier. At the head of Tennaya Creek on the eastern side of the ice cap is Tennaya Glacier.

As a part of the Mount Edziza volcanic complex, Mount Edziza was covered by a regional ice sheet during the Pleistocene which receded and advanced periodically until about 11,000 years ago when deglaciation was essentially complete in a steadily warming climate. This warming trend ceased about 2,600 years ago, causing glaciers to advance from Mount Edziza and elsewhere along the volcanic complex as a part of the neoglaciation. The present trend towards a more moderate climate put an end to the neoglacial period in the 19th century. This has resulted in rapid glacial recession throughout the Mount Edziza volcanic complex. This rapid glacial recession is apparent from the lack of vegetation on the barren, rocky ground between the glaciers and their trim lines which are up to apart.

Structure

Mount Edziza has an elevation of , making it the highest point of the Mount Edziza volcanic complex. It has been considered by some to be the highest or tallest volcano in Canada, but others have given higher elevations of for the Silverthrone volcanic complex in southwestern British Columbia. The nearly flat summit of Mount Edziza contains a circular ridge that surrounds an ice-filled, in diameter crater. This ridge is partially exposed above the ice cap as a discontinuous series of spires and serrated nunataks. Spires forming the southern end of the ridge are the highest and consist of greenish grey, sparsely porphyritic trachyte. They comprise well-formed, small diameter rock columns that rise nearly vertically more than above the ice cap. Nunataks elsewhere on the summit ridge are more subdued, consisting of pyroclastic debris that has been glacially reworked. The eastern side of the ridge has been breached by active cirques where remnants of several lava lakes are exposed inside the crater. Formation of the summit crater was likely caused by a violent eruption at the zenith of the mountain's growth, although it may have also formed during the eruption of the Kakiddi Formation lava flows or the effusion of voluminous lava during dome construction. Prior to its formation, the summit of the mountain was at least higher than it is today, having possibly risen as a narrower summit cone truncated with a smaller crater.

The central, high edifice of Mount Edziza is a nearly symmetrical stratovolcano, its symmetry having been broken by several steep-sided lava domes. Its eastern flank has been eroded by a narrow cirque which is bounded by near-vertical headwalls that breach the eastern summit crater rim. A system of radial meltwater channels has moderately eroded the upper flanks and summit crater rim elsewhere. Lesser modification by erosion has taken place on the southern and northwestern flanks of the stratovolcano. Along the north side of Tenchen Valley on the eastern flank of the stratovolcano are high cliffs exposing explosion breccias, trachyte lavas and landslide or lahar deposits. Although Mount Edziza is surrounded by relatively flat terrain of the Big Raven Plateau to the north, west and south, the terrain east of the mountain is characterized by a series of ridges with intervening valleys. Among these ridges are Idiji Ridge and Sorcery Ridge which are the namesakes of Idiji Glacier and Sorcery Creek.

About south of the summit is Ice Peak, the south peak of Mount Edziza. This prominent pyramid-shaped horn has an elevation of and is the glacially eroded remains of an older stratovolcano whose northern flank is buried under the younger edifice of Mount Edziza. The southern and western flanks are approximal to those of the original stratovolcano whereas the eastern flank has been almost completely destroyed by headward erosion of glacial valleys. At its climax, the stratovolcano had a symmetrical profile and contained a small crater at its summit; the current peak is an erosional remnant etched from the eastern crater rim.

Subfeatures

High on the eastern rim of Ice Peak are two glaciated volcanic cones called Icefall Cone and Ridge Cone, both with elevations of about . Punch Cone on the western flank of Ice Peak protrudes through Mount Edziza's ice cap whereas Koosick Bluff and Ornostay Bluff, also on the western flank of Ice Peak, extend westward onto the surrounding Big Raven Plateau. The northeastern side of Mount Edziza contains The Pyramid, a pyramid-shaped lava dome in elevation. Pillow Ridge on the northern side of Mount Edziza has an elevation of whereas Tsekone Ridge northwest of Mount Edziza has an elevation of . High on the western side of Mount Edziza is Triangle Dome, an elliptical lava dome in elevation. Glacier Dome reaches an elevation of on Mount Edziza's lower northeastern flank.

A circular lava dome on the southeastern crater rim of Mount Edziza called Nanook Dome has an elevation of . Sphinx Dome, in elevation, is a partially buried lava dome on the northeastern flank of Mount Edziza. Remnants of a volcanic pile called Pharaoh Dome occur along the eastern flank of Mount Edziza. They lie at an elevation of between Tennaya Creek and Cartoona Ridge. Cinder Cliff is a high barrier of volcanic rocks on the eastern side of Mount Edziza at an elevation of in the north fork of Tenchen Creek. The Neck, in elevation, is a circular volcanic plug on the southeastern flank of Ice Peak.

The Snowshoe Lava Field on the west flank of lce Peak contains at least 12 volcanic cones, a handful of which are named. Tennena Cone is a symmetrical volcanic cone high on the west side of Ice Peak. It has an elevation of and is almost completely surrounded by ice. Cocoa Crater is the largest cone in the Snowshoe Lava Field and is in elevation. To the southeast is Coffee Crater which has an elevation of . Keda Cone, in elevation, lies just south of Coffee Crater on the south side of upper Taweh Creek. A saucer-shaped mound of lava called The Saucer is in elevation and has a diameter of about .

The Desolation Lava Field on the northern flank of Mount Edziza contains at least 10 cinder cones, most of which are clustered near the northern trim line of Mount Edziza's ice cap. Sleet Cone and Storm Cone are rounded, mostly soil-covered, conical mounds that reach elevations of and , respectively. North of Storm Cone are the Triplex Cones, a group of three eroded circular mounds reaching an elevation of . Twin Cone, in elevation, is a pyroclastic cone whose southeastern side has been breached. Moraine Cone has an elevation of nearly and has been nearly destroyed by alpine glaciation. The northeastern side of Mount Edziza contains Williams Cone, a prominent cinder cone in elevation. Eve Cone, in elevation, is a symmetrical cone between Buckley Lake and Mount Edziza. The northernmost cinder cone in the Desolation Lava Field is Sidas Cone which consists of two symmetrical halves and reaches an elevation of .

Drainage

Mount Edziza is drained on all sides by streams within the Stikine River watershed. Elwyn Creek is a westward-flowing stream originating from the northwestern flank of Mount Edziza. It contains one named tributary, Kadeya Creek, which flows northwest from Mount Edziza. Sezill Creek is a westward-flowing stream originating from the western flank of Mount Edziza. It is a tributary of Taweh Creek which flows northwest from just south of Mount Edziza. Elwyn Creek and Taweh Creek are tributaries of Mess Creek which flows northwestward into the Stikine River.

Tsecha Creek is a northeast-flowing stream originating from the northern flank of Mount Edziza. Nido Creek flows northeastward from the eastern side of Mount Edziza into Nuttlude Lake. Flowing from the eastern flank of Mount Edziza just southeast of The Pyramid is Tenchen Creek. Shaman Creek flows east and north into Kakiddi Lake from the southern flank of Mount Edziza. Tennaya Creek flows northeastward from the eastern side of Mount Edziza into Nuttlude Lake. All five streams are tributaries of Kakiddi Creek, a north-flowing tributary of the Klastline River which flows north into the Stikine River.

Geology

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. The cause of volcanic activity 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

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 and are therefore 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 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 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

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

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