Abstract

Evidence relating to a possible volcanic eruption in western Lesotho during 1983 is documented and evaluated. It is concluded that a small quantity of highly vesicular, basaltic lava was extruded from a narrow orifice associated with a fracture in basalt of the Drakensberg Group. The eruption was accompanied by a seismic event measuring 4.3 on the Richter scale. The new lava has a chemical composition very similar to that of the 183-Ma-old host rock, but is isotropic and glassy, with no crystalline phases. In contrast, the Drakensberg Group basalt is almost completely crystalline, although many of the primary minerals display chemical alteration, with the mesostasis being devitrified. Excavation of the vent shows it to be fed by a narrow pipe or conduit, the sides of which are thinly coated by glassy material in which flow structures record slumping back of a very small residual portion of the lava after the eruption. The morphology of the conduit, vesicular nature of the lava, the quantity produced (between 0.3 and 1 m 3 ), and the apparent rapidity with which it was extruded seem to preclude the event having been occasioned either by a lightning strike or an electrical discharge from the nearby power line.

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