Tennena Cone

Tennena Cone, alternatively Icebridge Cone, is a small volcanic cone in Cassiar Land District of northwestern British Columbia, Canada. It has an elevation of 2390 m and lies on the western flank of Ice Peak, the prominent south peak of Mount Edziza. The cone is almost completely surrounded by glacial ice of Mount Edziza's ice cap which covers an area of around 70 km2. Tennena Cone is 200 m high, 1200 m long and up to 600 m wide, its symmetrical structure resembling a black pyramid. The cone and the surrounding area are in Mount Edziza Provincial Park which also includes the Spectrum Range to the south.

Tennena Cone is part of the Mount Edziza volcanic complex, a group of overlapping volcanoes that have formed over the last 7.5 million years. It overlies four geological formations of this volcanic complex that formed during the Miocene, Pliocene and Pleistocene epochs, all of which consist of several types of volcanic rocks. Tennena Cone consists of alkali basalt pillow lavas, tuff breccias and lapilli tuffs of the younger Big Raven Formation which were deposited by a small eruption under glacial ice. The exact timing of this eruption is unknown but radiometric dating of volcanic rocks from Tennena Cone has yielded ages as old as 0.011 ± 0.033 million years.

Name and etymology
The name of the volcanic cone was officially adopted on January 2, 1980, after being submitted to the BC Geographical Names office by the Geological Survey of Canada. It was required for geology reporting purposes since Jack Souther, a volcanologist of the Geological Survey of Canada, was studying the area in detail between 1970 and 1992. Tennena is a combination of the Tahltan words ten and nena, which mean ice and bridge, respectively. It was chosen because Tennena Cone is almost completely surrounded by glacial ice in an alpine environment. Tennena Cone and the associated volcanic rocks have collectively been called the Tennena volcanic centre.

Geography
Tennena Cone is located in Cassiar Land District of northwestern British Columbia, Canada, and resembles a symmetrical, 200 m, 1200 m and up to 600 m black pyramid. Its northern, eastern and southern flanks are mantled by the roughly 70 km2 Mount Edziza ice cap and rises about 150 m above the ice surface. Tennena Cone lies at the northern end of Tencho Glacier and reaches an elevation of 2390 m on the upper western flank of Ice Peak, the prominent south peak of Mount Edziza.

At lower elevations, Tennena Cone is surrounded by Ornostay Bluff in the northwest and by Koosick Bluff in the southwest. Between these two bluffs is the head of Sezill Creek which flows northwest from the surrounding Big Raven Plateau and then drains into Taweh Creek, a tributary of Mess Creek. The Big Raven Plateau is the northernmost subdivision of the Mount Edziza volcanic complex which consists of a group of overlapping shield volcanoes, stratovolcanoes, lava domes and cinder cones that have formed over the last 7.5 million years. At the southwestern end of the Big Raven Plateau is the Snowshoe Lava Field, of which Tennena Cone is a part.

Tennena Cone lies in Mount Edziza Provincial Park southeast of the community of Telegraph Creek. With an area of 266180 ha, Mount Edziza Provincial Park is one of the largest provincial parks in British Columbia and was established in 1972 to preserve the volcanic landscape. It includes not only the Mount Edziza area but also the Spectrum Range to the south from which it is separated by Raspberry Pass.

Geology
Tennena Cone is part of the Northern Cordilleran Volcanic Province, a broad area of volcanoes and lava flows 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.

Lithology
Tennena Cone consists mainly of Big Raven Formation alkali basalt that can be 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 10 to 30 cm beds. Two 1 m dikes comprise the third subdivision, both of which consist of fragmented plagioclase-phyric rock. The first dike forms a 5 m remnant and is exposed on the eastern flank of Tennena Cone while the second dike is exposed 50 m 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.

At the southwestern base of Tennena Cone are elongated mounds of pillow lava that cover about 0.45 km2 of hummocky topography. They have a maximum basal diameter of 75 m and range from 3 to 20 m high, decreasing in height to the southwest. The orientation of these mounds suggest that they were formed by a fissure eruption. Just west of these pillow lava mounds are massive non-pillowed lava flows which are exposed over an area of around 0.4 km2 across gently sloping terrain. Extending west of Tennena Cone north of the pillow lava mounds and massive non-pillowed lava flows is a 4.4 km pillowed lava flow that terminates at the head of Sezill Creek valley. It contains pillows that range from less than 1 m to more than 1 m in diameter, as well as vertically oriented pillow-like lava bodies.

Pumice of the Sheep Track Member is present on Tennena Cone and other volcanoes in the Snowshoe Lava Field. This is air-fall tephra deposited by a small but violent VEI-3 eruption from the southwestern flank of Ice Peak 950 CE ± 6,000 years ago. Sheep Track pumice is lithologically distinct from the rest of the Big Raven Formation, consisting mainly of comenditic trachyte rather than alkali basalt or hawaiite. The source of the pumice is unknown but it probably originated from a vent under Tencho Glacier, the largest glacier of the Mount Edziza volcanic complex.

Formation
Tennena Cone was one of the first volcanoes to erupt during the fifth magmatic cycle of the Mount Edziza volcanic complex. Its formation began when basaltic magma issued from a vent under 500 – of glacial ice where it was quenched to create the pillow lavas, tuff breccias and lapilli tuffs comprising Tennena Cone. This volcanic material accumulated inside a depression melted in the ice and did not breach the ice surface, resulting in the formation of a subglacial mound. Lava flows from Tennena Cone travelled west through tunnels created by eruption-generated meltwater escaping at the base of the enclosing ice.

The longest lava flow at the head of Sezill Creek valley 4.3 km west of Tennena Cone travelled to the western edge of the enclosing ice, causing a violent steam explosion. This explosive interaction between meltwater and lava spilled over the terminal moraine and spread onto the Big Raven Plateau beyond the ice. Although the lava flow was quenched by meltwater throughout its entire length, it has a thickness of 2 – and travelled into small depressions of the current topography. This suggests that the lava flow was relatively fluid at the time of eruption, resulting in higher mobility.

The precise age of Tennena Cone is unknown but it may have formed during the Last Glacial Maximum between 23,000 and 18,000 years ago when the Mount Edziza volcanic complex was covered by the Cordilleran Ice Sheet. Another possibility is that Tennena Cone formed under an expansion of the Mount Edziza ice cap during the Younger Dryas between 12,900 and 11,600 years ago or during a more recent glacial advance. Argon–argon dating of glassy pillows from Tennena Cone has yielded ages of 0.011 ± 0.033 million years and 0.005 ± 0.033 million years, but further work is required to improve the precision of these ages.

Since its eruption under glacial ice, Tennena Cone has been modified by glacial erosion. This includes the steepening of its flanks and the formation of its 500 m summit ridge, the latter of which is covered with morainal detritus. The degree of glacial erosion and the deposition of morainal detritus on Tennena Cone suggest that the volcano was overlain by significantly thick ice. This ice may have also overlain the summit of Mount Edziza which is 397 m higher than that of Tennena Cone.

Basement
Tennena Cone overlies the Armadillo, Ice Peak, Nido and Raspberry formations, all of which are older stratigraphic units of the Mount Edziza volcanic complex. The Ice Peak Formation is the youngest of the four geological formations, consisting of an upper assemblage of Pleistocene tristanite, trachyte and comenditic trachyte lava flows and a lower assemblage of Pleistocene alkali basalt and hawaiite lava flows with minor tristanite, trachybasalt and mugearite lava flows and pyroclastic breccia. Underlying the Ice Peak Formation are alkali basalt lava flows and flow breccia of the Tenchen Member of the Nido Formation which were erupted from multiple volcanoes during the Pliocene.

Miocene alkali basalt and minor sparsely porphyritic hawaiite of the Armadillo Formation underlies the Nido Formation and are in the form of lava flows, flow breccia and agglutinate. The oldest geological formation underlying Tennena Cone is the Raspberry Formation which consists of Miocene alkali basalt and minor hawaiite and mugearite. These volcanic rocks are in the form of lava flows, flow breccia and agglutinate, although pillow lava and tuff breccia occur locally. Underlying the Raspberry Formation are sedimentary, volcanic and metamorphic rocks of the Stikinia terrane which are Paleozoic and Mesozoic in age.

Significance
Tennena Cone and its eruptive products are of geological significance because they contain an unusually wide range of features characteristic of a small-volume eruption under thick glacial ice. These features include ordinary pillow lavas and vertically-oriented, distended pillow lavas, as well as massive non-pillowed lavas and interbedded gravelly sands and poorly consolidated sandstone. The subglacially emplaced lavas erupted from Tennena Cone are also of geological significance because they can be traced more than 3 km away from the vent area. Their well-preserved textures and geomorphological structures can be used to help identify other subglacially-emplaced lava flows on Earth and on other terrestrial bodies such as Mars. Tennena Cone is one of two glaciovolcanic features at Mount Edziza that have been investigated in detail, the other being Pillow Ridge on the northwestern side of the mountain.