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Aiyansh lava flow, British Columbia

1969; NRC Research Press; Volume: 6; Issue: 6 Linguagem: Inglês

10.1139/e69-149

1480-3313

A. Sutherland Brown,

Geological formations and processes

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 miles 2 (38.8 km 2 ) and a volume of about 0.1 mile 3 (0.455 km 3 ).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 (An 55±5 ), 10% olivine (Fa 35 ), 30% pyroxene, and 10% opaques. A new chemical analysis confirms that it is a high-iron, low-magnesium, alkali basalt.