TSEAX VOLCANO: A DEADLY BASALTIC ERUPTION IN NORTH-WESTERN BRITISH COLUMBIA'S STIKINE VOLCANIC BELT

Tseax Volcano at the south end of the Stikine volcanic belt was the site of a deadly alkali olivine basaltic eruption in 1775. The eruption killed an estimated 2000 Nisga'a first nations people living in a village near the edge of the Nass River, approximately 20 km downstream from the vent area. Significant oral history documents the eruption and describes the death of some of the villagers from “poison smoke.” Investigations are now underway to test various hypotheses as to how the villagers were killed. Field work during the summer of 2006 revealed several previously unknown details. Samples were collected from the proximal to the distal reaches of the flow (over 30km). Analyses of volatiles in the samples will help constrain whether CO2 was present in high enough concentrations to be a potential cause of death. Chemistry will also be used to estimate the viscosity of the flow. An orthophoto with a resolution of 1 m, and a DEM with a resolution of 5.6 m, were created of the flow surface. These will be used to evaluate flow dynamics along with detailed observations of surface morphology. The lower reaches of the flow interacted with water of some depth, possibly as much as 10 m. This left a pillowed, glassy surface close to where the flow debouches from the side valley and there is a change in slope. Proximal to the area displaying the pillows, and downstream, the lava surface is deeply crevassed and extremely irregular. Sections through the flow indicate large tubes, up to 10 m in diameter, with gas explosion vents marginal to the tubes. The lower sections of tubes rest on a pillowed sequence of unknown thickness. In rare cases, the flows are seen to rest on silt and fluvial gravels. One hypothesis is that as the flow debouched from the side valley it rapidly advanced into a lake-like widening of the river known to exist at this location at that time. People in the doomed village sought refuge on the opposite side of the broad river valley. They paddled across the river, possibly at the same time the flows were entering the water. We speculate that they may have been swept away in sudden waves and turbulence caused by the lava entering the river, but more work needs to be done to evaluate this hypothesis.