The Lesotho 'volcanic' event of February 1983

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 associa ted with a fracture in basalt of the Drakensberg Group. The eruption was ac companied by a seismic event measuring 4.3 on the Richter scale. The new la va has a chemical composition very similar to that of the 183-Ma-old host r ock, bur is isotropic and glassy, with no crystalline phases. In contrast, the Drakensberg Group basalt is almost completely crystalline, although man y of the primary minerals display chemical alteration, with the mesostasis being devitrified. Excavation of the vent shows it to be fed by a narrow pi pe or conduit, the sides of which are thinly coated by glassy material in w hich flow structures record slumping back of a very small residual portion of the lava after the eruption. The morphology of the conduit, vesicular na ture of the lava, the quantity produced (between 0.3 and 1 m(3)), and the a pparent rapidity with which it was extruded seem to preclude the event havi ng been occasioned either by a lightning strike or an electrical discharge from the nearby power line.