Snowshoe Lava Field

Template:Short description Template:Use Canadian English The Snowshoe Lava Field (SLF) is a largely buried volcanic field at Mount Edziza in British Columbia, Canada. It reaches an elevation of and engulfs more than of the Big Raven Plateau and adjacent valleys with blocky lava flows. The Snowshoe Lava Field is the southernmost of two lava fields on the Big Raven Plateau, the other being the smaller Desolation Lava Field at the northern end of the plateau. It is in Mount Edziza Provincial Park and is part of the Mount Edziza volcanic complex, the latter of which consists of several other volcanic landforms such as shield volcanoes, stratovolcanoes, lava domes and cinder cones. Sezill Creek, Shaman Creek, Taweh Creek and several other unnamed streams in the Stikine River watershed drain the Snowshoe Lava Field. Access to the lava field is only by aircraft or by a network of horse trails from surrounding roads.

The Snowshoe Lava Field issued from several eruptive centres during the Holocene, most of which are clustered on the southern and western flanks of Ice Peak at elevations more than . Volcanism in the lava field began with the eruption of three subglacial volcanoes when outlet glaciers of Mount Edziza's ice cap extended to lower elevations during the climax of a glacial advance. This was followed by the creation of four volcanoes whose eruptions were at first subaqueous and then transitioned subaerially. The youngest volcanoes in the Snowshoe Lava Field are completely subaerial in origin and issued most of the blocky lava flows comprising much of the current surface of the lava field. Alkali basalt and hawaiite of the Big Raven Formation are the main volcanic rocks comprising the Snowshoe Lava Field, but most of them are obscured by trachyte pumice of the Sheep Track Member.

Geography

The Snowshoe Lava Field is at the southern end of the Big Raven Plateau which is bounded by Mess Creek valley in the west, Kakiddi Creek valley in the east and the Klastline River valley in the north. This intermontane plateau is one of the principal physiographic features of the Mount Edziza volcanic complex, a group of overlapping shield volcanoes, stratovolcanoes, lava domes and cinder cones that have formed over the last 7.5 million years. The Snowshoe Lava Field reaches an elevation of on the plateau, but remnants of the lava field decrease in elevation to near Mess Creek. It is one of two lava fields on the Big Raven Plateau, the other being the larger Desolation Lava Field at the northern end of the plateau. Mount Edziza Provincial Park surrounds the Snowshoe Lava Field; it was founded in 1972 to showcase the volcanic landscape.

Landforms

The Snowshoe Lava Field consists of more than of blocky lava flows that issued from 12 eruptive centres. Most of these eruptive centres are more than in elevation and are located on the southern and western flanks of Ice Peak, the prominent south peak of Mount Edziza. Cocoa Crater, Coffee Crater, Keda Cone, Tennena Cone and The Saucer are the only named eruptive centres in the Snowshoe Lava Field. Tennena Cone on the upper western side of Ice Peak is the highest with an elevation of , although the Global Volcanism Program gives a lower elevation of for the cone. Cocoa Crater and Coffee Crater on the western and southern flanks of Ice Peak attain elevations of , respectively, and are so-named for their deep colours. Keda Cone, in elevation, is just south of Coffee Crater on the southern side of upper Taweh Creek. The Saucer is a low, circular mound of lava in elevation on the southern flank of Ice Peak bounded by concentric ridges of broken lava slabs.

Drainage

As a part of the Mount Edziza volcanic complex, the Snowshoe Lava Field is drained entirely by streams within the Stikine River watershed. Taweh Creek flows northwest along the southern edge of the lava field and is a tributary of Mess Creek. At the northwestern end of the Snowshoe Lava Field is Sezill Creek, a northwesterly flowing tributary of Taweh Creek. Shaman Creek at the southeastern end of the lava field flows east and north into Kakiddi Lake which is an expansion of Kakiddi Creek. The source of Sezill Creek, Shaman Creek and several unnamed tributaries of Sezill Creek and Taweh Creek is Tencho Glacier which lies at the northeastern end of the Snowshoe Lava Field.

Geology

Diagram showing the locations of volcanoes and lava fields of a geological formation.

Geological map of the Big Raven Formation showing the extent of the Snowshoe Lava Field

The Snowshoe Lava Field is the second largest of the Holocene volcanic features on the Big Raven Plateau, succeeded only by the Desolation Lava Field. It is also the second largest of three Holocene lava fields in the Mount Edziza volcanic complex; the smaller Mess Lake Lava Field covers about . The lava flows and volcanic cones comprising the Snowshoe Lava Field consist mainly of alkali basalt and hawaiite of the Big Raven Formation, the youngest geological formation of the Mount Edziza volcanic complex. Nearly all of the surficial details of the Snowshoe Lava Field are obscured by pyroclastic fall of the Sheep Track Member; this is the only named geological member of the Big Raven Formation. The pyroclastic fall consists of granular trachyte pumice that was deposited by a small, but violent VEI-3 eruption from the southwestern flank of Ice Peak around 950 CE.

Individual eruptive centres of the Snowshoe Lava Field have been given numeronyms ranging from SLF-1 to SLF-12; greater numbers indicate a younger age. SLF-1, SLF-2 and SLF-3 are alkali basaltic and formed subglacially when outlet glaciers of Mount Edziza's ice cap extended to lower elevations during the climax of a glacial advance. SLF-4, SLF-5, SLF-6, SLF-7 and SLF-8 are volcanic cones formed when eruptions were at first subaqueous and then transitioned subaerially. They consist of alkali basalt and minor hawaiite which are in the form of subaqueous and subaerial ejecta. SLF-9, SLF-10, SLF-11 and SLF-12 are completely subaerial in origin and consist mainly of hawaiite. They are in the form of pyroclastic cones with the exception of the youngest eruptive centre, SLF-12.

The order of eruptions that formed the Snowshoe Lava Field are based on the degree of erosion and vegetation cover of the lava flows and volcanic cones, as well as the order in which the lava flows overlap. Most of the lava ponded onto the surrounding Big Raven Plateau in the form of broad, thick sheets, but some of it also travelled into neighbouring valleys; the largest of these valley-filling lava flows is located at the head of Taweh Creek. The transition from subglacial to subaerial volcanism in the Snowshoe Lava Field is attributed to retreat of glaciers from lower elevations. These glaciers are now away from their trim lines, but during their maximum advance, they had an ice surface that rose above their current levels.

As a part of the Mount Edziza volcanic complex, the Snowshoe Lava Field lies within a broad area of volcanoes and lava flows called the Northern Cordilleran Volcanic Province, which extends 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.

Subglacial centres

A black cone-shaped mountain rising over glacial ice in the foreground

Tennena Cone is the oldest eruptive centre in the Snowshoe Lava Field

SLF-1 is Tennena Cone which is one of the few glaciovolcanic features at Mount Edziza that have been investigated in detail. It has been mapped into four subdivisions, all of which are exposed on the eastern, southern and western flanks of the cone. The first subdivision is massive and crudely bedded tuff breccia exposed in near-vertical cliffs on the flanks of Tennena Cone. Exposed in scarps on the eastern and southern flanks of Tennena Cone is lapilli tuff of the second subdivision which forms beds. Two wide dikes comprise the third subdivision, both of which consist of fragmented plagioclase-phyric rock. The first dike forms a high remnant and is exposed on the eastern flank of Tennena Cone whereas the second dike is exposed to the south. In addition to occurring on the eastern flank, the second dike is also exposed on the western flank and along the summit ridge of Tennena Cone. The fourth subdivision consists of pillow and fluidal lavas that overlie tuff breccia in the northern section of the cone.

About south of Tennena Cone is SLF-2, a smaller crescent-shaped nunatak protruding through the western portion of Tencho Glacier. It may be the remains of a parasitic cone closely related to Tennena Cone or it may be an extension of Tennena Cone since it consists of similar geology. SLF-3 at the southern terminus of Tencho Glacier is a crescent-shaped ridge that has been overridden by glacial ice. Crudely bedded tuff breccia and quenched flow fragments comprise the entire ridge, which may be the remains of a tuff ring that formed in a meltwater lake. In contrast to Tennena Cone which was constructed on a steep slope, SLF-3 is underlain by flat-lying terrain of the Big Raven Plateau. This likely resulted in their differing geomorphology; the Tennena Cone eruption sent both lava and meltwater down the steep western flank of Ice Peak whereas ejecta from the SLF-3 eruption ponded inside a meltwater lake when Tencho Glacier extended onto the flat plateau surface.

Transitional centres

File:Coffee Crater (Snowshoe Lava Field, North America).jpg

Coffee Crater from the south

SLF-4 and SLF-5 are located on the western and southern margins of Tencho Glacier, respectively. They are within the trim lines of this glacier and have been reduced to low, drumlin-like ridges from glacial ice overriding them. A short distance beyond the trim lines of Tencho Glacier are SLF-6, SLF-7 and SLF-8; SLF-7 is Coffee Crater. These eruptive centres contrast from SLF-4 and SLF-5 in that they have not been overridden by glacial ice, which has allowed them to retain their central craters and their original conical forms. However, their inner structures have been partially exposed due to sufficient erosion. All five eruptive centres consist of a lower succession of brown subaqueous tuff breccia and an upper sequence of subaerial cinders, spatter and bombs which have been stained red due to extreme oxidation.

Lava flows from the transitional centres have been mostly buried under younger basalt of the subaerial centres, as well as pumice of the Sheep Track Member. However, they are exposed where streams have eroded the overlying volcanic deposits. The lava flows are also locally exposed along streams that have cut through older lava flows below.

Subaerial centres

File:Edziza042909-- 055-8.jpg

Keda Cone in the foreground with flat-topped Mount Edziza in the background; Coffee Crater, SLF-11 and SLF-8 are visible north and northeast of Keda Cone.

Most of the blocky lava flows comprising much of the current surface of the Snowshoe Lava Field originated from eruptive centres SLF-9, SLF-10 and SLF-11, all of which are pyroclastic cones. These cones consist of agglutinated lapilli, bombs and spatter which have been stained red due to extreme oxidation. SLF-9 is Keda Cone at the southern end of the lava field whereas SLF-10 is Cocoa Crater near the centre of the lava field. Cocoa Crater issued a wide lava flow that cascaded into upper Sezill Creek canyon, but the most voluminous lava flows originated from Keda Cone and SLF-11. SLF-11 is the only subaerial eruptive centre in the Snowshoe Lava Field with no name.

Lava flows from Keda Cone and SLF-11 engulfed a more than wide and long area on the Big Raven Plateau prior to entering the upper valley of Taweh Creek. At the head of Taweh Valley, the Keda Cone and SLF-11 flows converged to form a relatively narrow tongue of lava that travelled for at least another to near Mess Creek. Erosion has removed nearly all of the final of this lava tongue; small remnants are present along the banks of lower Taweh Creek near the junction with Mess Creek. None of the lava flows from these three subaerial eruptive centres display any evidence of having been quenched by water, nor do any of the pyroclastic cones.

The youngest subaerial eruptive centre, SLF-12, is The Saucer which appears to be the remains of a fissure instead of a pyroclastic cone. This vent south of Tencho Glacier issued lava that travelled eastward and westward; the long eastward lava flow entered the head of Shaman Creek whereas the relatively thick westward lava flow spread onto the Big Raven Plateau in the form of broad lobes. In contrast to the neighbouring Coffee Crater lava flows which are overlain by thick drifts and pockets of Sheep Track pumice, The Saucer and its lava flows are only sparsely covered with very fine pumice of the Sheep Track Member. The sparsity of this very fine pumice on The Saucer and its lava flows suggests they were extruded sometime after the explosive Sheep Track eruption which may have occurred around 950 CE. Because very fine pumice is susceptible of being blown by wind, it could easily be wind-blown material carried in from neighbouring pumice beds long after the Sheep Track eruption had occurred.

Accessibility

File:SLF-11.jpg

SLF-11 from the south

As part of the Mount Edziza volcanic complex, the Snowshoe Lava Field is in a remote location with no established road access. The closest roads are the Stewart–Cassiar Highway to the east and the Telegraph Creek Road to the northwest, both of which extend within of the lava field. Extending from these roads are horse trails that provide access to the Mount Edziza volcanic complex. From Telegraph Creek, the Buckley Lake Trail extends about southeast along Mess Creek and Three Mile Lake. It then traverses about northeast along Dagaichess Creek and Stinking Lake to the northeastern end of Buckley Lake where it meets with the Klastline River Trail and the Buckley Lake to Mowdade Lake Route.

To the east, the roughly long Klastline River Trail begins at the community of Iskut on the Stewart–Cassiar Highway. It extends northwest and west along the Klastline River for much its length. The trail enters Mount Edziza Provincial Park at about where Kakiddi Creek drains into the Klastline River. After entering Mount Edziza Provincial Park, the Klastline River Trail traverses northwest along the Klastline River for about and then crosses the river north of the Big Raven Plateau. From there, the Klastline River Trail traverses west for about to the northeastern end of Buckley Lake where it meets with the Buckley Lake Trail and Buckley Lake to Mowdade Lake Route.

File:SLF-8.jpg

SLF-8 from the south

The Buckley Lake to Mowdade Lake Route traverses south from Buckley Lake along Buckley Creek and gradually climbs onto the northern end of the Big Raven Plateau where Tsekone Ridge and landforms of the Desolation Lava Field such as Eve Cone and Sidas Cone are visible along the route. Most of the Buckley Lake to Mowdade Lake Route is marked by a series of rock cairns from Tsekone Ridge onwards. At the southern end of the Buckley Lake to Mowdade Lake Route is the Snowshoe Lava Field where Tennena Cone, Coffee Crater and Keda Cone occur. The route continues east through Shaman Creek valley and then turns southeast to the northern end of Mowdade Lake.

The distance between Buckley Lake and Mowdade Lake is about , but the hiking length between these two lakes varies depending on the route taken; it can take a minimum of 7 days to hike the Buckley Lake to Mowdade Lake Route. The weather can change extremely fast along this hiking trail. Buckley Lake is large enough to be used by float-equipped aircraft, but landing on this lake with a private aircraft requires a letter of authorization from the BC Parks Stikine Senior Park Ranger. Mowdade Lake about southeast of the Snowshow Lava Field is also of significant size and does not require a letter of authorization from the BC Parks Stikine Senior Park Ranger to land on it with a private aircraft. Alpine Lakes Air and BC Yukon Air are the only air charter companies permitted to provide access to this area via aircraft.

See also

• Volcanism of the Mount Edziza volcanic complex
• List of volcanic fields

References

Sources

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External links

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