Figure A16. Tseax Cone lava flows Lava flows from the Tseax cone ponded near the Nass River, forming a vast, flat plain underlain by the lava flows. The fluid lava, travelling over 22 km from the cone, filled in any preexisting topographic depressions, then spread out and solidified. The flows most likely dammed the Nass River for a short period of time. (Photograph by C.J. Hickson (Geological Survey of Canada)) larger image [JPEG, 32.7 kb, 400 X 263]

The Nisga'a Memorial Lava Beds Provincial Park is approximately 60 km north of Terrace, British Columbia, and contains cinder cones and accompanying lava flows of the Aiyansh-Tseax River volcano. The cinder cones are situated in a valley above and east of the Tseax River, about 20 km south of the river's junction with the larger Nass River. The most recent cone, 290 m in diameter at the base, rests on the remains of an earlier and somewhat larger, dissected, 460 m diameter cone. The cones formed in the narrow confines of a tributary of the Tseax River and comprise basaltic bombs and cinder. The most recent lava flowed 22.5 km from the vent to the Nass River where, according to legend of the Nisga'a people, it blocked the flow of the Nass River (Figure A16). Along the south edge of the Nass River, the exposed lava contains structures indicative of lava that has flowed over wet ground. The provincial park is a must for volcano enthusiasts because it is easily accessible and has a wide range of lava-flow features, including pahoehoe and aa lava flows.

The eruption of Tseax cone is the only eruption in Canada for which legends of First Nations people have been verified. The Nisga'a (Tsimshian people) tell of a prolonged period of disruption by the volcano, including the destruction of their village on the Nass River and the death of some people (reportedly 2000) from "poisonous smoke" (Barbeau, 1935, p. 223). The vent was active at least twice (220 and 650 years ago) and remnants of other, older lava flows exist in the area. Nisga'a Memorial Lava Beds Park became a reality in 1993 when the province of British Columbia set aside the area to honour the dead and to preserve the unique region.

If Tseax cone were to erupt again, modern monitoring techniques and an understanding of volcanic hazards would help prevent a repeat of the 'poisonous smoke' disaster. The poison smoke was most likely odourless, heavier than air, CO2 gas. The CO2 replaces air in depressions and poorly ventilated structures. People sleeping in these areas would have had no knowledge or warning of the danger. Because of our knowledge of this previous disaster, modern monitoring would include studies of the gases emitted by the volcano and a warning to people living downslope from the volcano. Future eruptions could start forest fires and potentially dam local rivers (as happened to the Tseax and Nass rivers) if the volume of lava were significant enough. If lava were to again reach the Nass River, it could have disastrous short-term consequences for the important salmon fisheries on the Nass River system.

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