Tseax Cone ( ) is a small volcano in the Nass Ranges of the Hazelton Mountains in northwestern British Columbia, Canada. It has an elevation of and lies within an east–west valley through which a tributary of the Tseax River flows. The volcano consists of two nested structures and was the source of four lava flows that descended into neighbouring valleys. A secondary eruptive centre lies just north of Tseax Cone on the opposite side of Melita Lake. It probably formed simultaneously with Tseax Cone, but the timing of volcanism at the two eruptive centres is not precisely known; both were formed by volcanic activity sometime in the last 800 years.
The exact timing of volcanism at Tseax Cone has been a subject of controversy because there are no direct written accounts; radiocarbon dating of plants killed by lava or ejecta from the volcano has yielded ages as old as 625 ± 70 years to as young as 190 ± 15 years. There is also controversy over whether the volcano was formed during one or more distinct episodes of eruptive activity. The single eruptive episode hypothesis has been proposed by researchers as early as 1923 whereas a multi-eruption hypothesis was proposed in 1978. Most research suggests that Tseax Cone was formed during one episode of eruptive activity; new data supporting this hypothesis was reported in 2020.
Tseax Cone is the subject of legends told by the local indigenous people. They describe the destruction of villages along the Nass River by the volcano and the death of several people from inhaling volcanic fumes, although other causes of death may have been involved. As many as 2,000 people are claimed to have been killed by an eruption from Tseax Cone; this would make it the deadliest geological disaster in Canada and the second-worst natural disaster in Canadian history by death toll. Tseax Cone has therefore been described as the deadliest volcano in Canada. Renewed eruptions from the volcano could start wildfires and block local streams with lava flows.
Tseax Cone lies within an ecoregion characterized by mountainous terrain and several streams. Rainforests occur at the volcano, as well as numerous species of mammals. Lichens and mosses cover most of the lava flows that have issued from Tseax Cone, although rainforests and waterbodies also obscure them. After at least 20 years of pleas for protection, the volcano and lava flows were established as Nisga'a Memorial Lava Bed Provincial Park in 1992. Tseax Cone and its lava flows can be accessed via provincial highways and backcountry roads.
Names and etymology Tseax Cone has been variously called Aiyansh Volcano, Aiyansh River Volcano, Tseax River Cone and Tseax Volcano. Aiyansh comes from a Nisga'a word meaning or , whereas Tseax comes from a Nisga'a word meaning . Tseax is possibly a reference to the disturbed drainage patterns of the Tseax River caused by a volcanic eruption from the cone. The well-established local name for the volcano, Tseax Cone, became official on December 13, 1991, and was adopted on the National Topographic System map 103P/2. To the local Nisga'a people, Tseax Cone is known as ; in their language, it means , which is a reference to the volcanic eruption that sent lava spewing out of the volcano.
Geography Location and climate Tseax Cone is about north of Terrace near the Nisga'a villages of Gitwinksihlkw and Gitlaxt'aamiks in Cassiar Land District of northwestern British Columbia, Canada. It lies within a steep-sided, long, east–west valley penetrating the Nass Ranges of the Hazelton Mountains. Tseax Cone is situated at the outlet of Melita Lake, an expansion of Crater Creek which flows west into the Tseax River. Crater Creek gets its name from being in association with Tseax Cone which is located on the eastern side of the creek. The Nass Mountains Ecosection is the main ecosection at the cone.
The area has a climate that is somewhat transitional between those of coastal and continental regimes. It is wetter than other areas in the Nass Ranges Ecoregion due to air entering from the Pacific Coast. Much of this Pacific air enters via the Skeena River valley or flows over the Kitimat Ranges, resulting in cloud cover and heavy rain. Short periods of extreme cold temperatures and deep snow occasionally occur as a result of cold Arctic air invading from the north.
Plants and animals
Lichens and mosses cover large portions of lava flows originating from Tseax Cone. They range in colour from green to yellow, reach thicknesses of a few centimetres and give the illusion that the lava flows are covered in fur. Also present on the lava flows in very small quantities are grasses and smaller shrubs. In the Tseax River valley, the lava flows have been almost completely covered by dense rainforest; coastal western hemlock and subalpine mountain hemlock form rainforests in the area. Wildlife in the area includes marmots, goats, bears and moose.
Despite being covered by lichens, mosses and rainforests, the lava flows are easily recognizable from aerial and satellite imagery, as well as field observations. However, this may change by the end of the 21st century as lodgepole pine and cottonwood forests continue to develop on the lava flows in an increasingly wetter and milder climate. The growth of these forests is bolstered by the deposition of silt on the lava flows by local streams, providing soil for vegetation.
Geology and geomorphology Background Tseax Cone is one of the southernmost volcanoes in the Northern Cordilleran Volcanic Province. This is 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 compossing these volcanoes are alkali basalts and hawaiites, but nephelinites, basanites and peralkaline phonolites, trachytes and comendites 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.
Petrology
Tseax Cone and its eruptive products are basanitic, trachybasaltic or alkali basaltic in composition. They are rich in iron-titanium oxides and were produced by a low degree of partial melting below the surface in the upper crust. Olivine, plagioclase and the iron-titanium oxides are in the form of phenocrysts whereas clinopyroxene occurs as a matrix mineral. The lack of clinopyroxene phenocrysts in Tseax Cone lavas is a phenomenon commonly observed in other mafic lavas throughout the Northern Cordilleran Volcanic Province. Tseax Cone and its eruptive products rest unconformably on sedimentary rocks of the Bowser Lake Group, a geological group of Jurassic and Cretaceous age consisting of grey sandstones and dark grey and black conglomerates, siltstones and mudstones.
Lava and tephra from Tseax Cone cover about and have a total volume of around ; the volume of this volcanic material is similar to that produced during Mauna Loa's 1984 eruption and Kīlauea's fissure 8 eruption in 2018. The Tseax Cone lavas are thought to have been emplaced at high speed during a short period of time, which may have been partially due to eruption rate and topography. Their liquidus viscosities are comparable to basalts of Mount Etna and the Hawaiian Islands, as well as the foidite lavas of Mount Nyiragongo and the tephrite lavas of Nyamuragira. It is possible that the lava erupted from Tseax Cone rose along the same faults as those at the Lakelse Hot Springs south of Terrace, which are the hottest geothermal springs in Canada.
Structure Tseax Cone has an elevation of and consists of two nested structures: a smaller inner cone and a larger external spatter rampart. The younger inner cone is high and in diameter, consisting mainly of black ejecta such as scoria, ballistics and lapilli. It contains an approximately deep volcanic crater with a diameter of . The older external spatter rampart, which has also been described as a cone, is about high and in diameter. It consists of spatter and scoria that ranges in colour from reddish to brownish and black to grey. Each structure was formed by a different style of volcanic activity; the spatter rampart was created by Hawaiian-style lava fountaining whereas the inner cone was created by low-intensity Strombolian explosions. The entire structure of Tseax Cone has been variously described as a cinder cone, a pyroclastic cone or a tephra cone.
About north of Tseax Cone and north of Melita Lake is a much smaller, unnamed asymmetrical satellite cone. It is about high, in diameter and heavily oxidized, containing a deep and in diameter summit crater. Extending southwest of the satellite cone is an eruptive fissure consisting of three or four tephra mounds. These mounds are a few metres high and are completely covered by black tephra. Red oxidized tephra beneath the black tephra was deposited by lava fountaining at the larger satellite cone.
Air-fall tephra layer The inner cone was the source of an elongated tephra layer that extends to the northeast, suggesting a southwesterly wind at the time of eruption. It contains millimetre-sized sandstones and siltstones derived from the underlying Bowser Lake Group, as well as fine-grained metamorphic and plutonic rocks; these clasts make up about 0.5% of the total volume of the tephra layer. The tephra layer has an estimated volume of , suggesting that it was deposited by a VEI-2 eruption. The volume of this tephra deposit is the first to be calculated for a mafic eruption in Canada.
Lava flows Tseax Cone was the source of four distinct lava flows, all of which were probably erupted over a timespan of weeks to a few months. The first flow is the longest and most voluminous, accounting for about 84% of the total volume of lava erupted from Tseax Cone. It travelled through Crater Creek and Tseax River valleys to the Nass River where it forms a wide and long lava plain with an elevation of about . This lava flow was most likely voluminous enough to block the Nass River for a short period of time. The second flow, representing about 13% of the total volume of lava, travelled through Crater Creek valley to near the mouth of the Tseax River valley. Both of these lava flows are in the form of pāhoehoe and are poor in phenocrysts, having issued from the spatter rampart. They are among the longest lava flows in the Canadian Cordillera; lava flows more than long in this cordillera are usually basaltic in composition. The third flow accounts for less than 2% of the total volume of lava erupted from Tseax Cone, having travelled through Crater Creek valley to near Ross Lake in the Tseax River valley. The fourth flow is the shortest and least voluminous of the four lava flows; it represents about 1% of the total lava volume and travelled Crater Creek for . These latter two lava flows are in the form of ʻaʻā and are rich in phenocrysts, having issued from the inner cone.