Abstract

The Aiyansh alkali basalt lava flow is one of the youngest volcanic features of British Columbia, about 220 years old. It issued from a vent area (55° 7′ N, 128° 54′ W) in a narrow tributary valley of the Tseax River, flowed 14 miles (22.5 km) to the Nass Valley, and there spread out in a lava plain 6 miles (9.7 km) long, forcing the river to the northern margin of the valley. The Aiyansh flow has an area of about 15 miles2 (38.8 km2) and a volume of about 0.1 mile3 (0.455 km3).The lava flow is a single cooling unit. Its surface consists of pahoehoe, slab, and block lava in a pattern related to slope, with low slopes favoring preservation of level pahoehoe. Piping and collapse were important about the margins of the lava plain. The eruption must have terminated with a series of lava fountains and explosions in the vent area which built one main and several small cones of bombs and cinders.The Aiyansh flow is entirely a fresh black alkali basalt varying only slightly in crystallinity and texture. Holocrystalline specimens from the interior of the lava plain consist of about 50% plagioclase (An55±5), 10% olivine (Fa35), 30% pyroxene, and 10% opaques. A new chemical analysis confirms that it is a high-iron, low-magnesium, alkali basalt.

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