1887

oa South African Journal of Geology - The Lesotho 'volcanic' event of February 1983

Volume 101 Number 4
  • ISSN : 1012-0750
  • E-ISSN: 1996-8590

 

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 m3) , 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.


Aanduidings van 'n moontlike vulkaniese erupsie in westelike Lesotho tydens 1 983 word gedokumenteer en evalueer. Daar word tot die slotsom gekom dat 'n klein hoeveelheid hoogs vesikulêre, basaltiese lawa vanuit 'n smal opening geassosieer met 'n breuk in basalt van die Drakensberg Groep ge-ekstrudeer is. Die erupsie het gepaard gegaan met 'n seismiese insident wat 4.3 op die Richter skaal gemeet het. Die nuwe lawa het 'n chemiese samestelling baie dieselfde as die van die herberggesteentes wat 183 Ma oud is, maar is isotropies en glasagtig, met geen kristallyne fases nie. In teenstelling hiermee is die basalt van die Drakensberg Groep feitlik heeltemal kristallyn, alhoewel baie van die primêre minerale chemiese verandering toon, met die mesostasis gedevitrifiseer. Uitgrawing van die opening toon dat dit deur 'n smal pyp of toevoerkanaal gevoed word, die kante waarvan dun oordek is met glasagtige materiaal waarin vloeistrukture terugsakking van 'n baie klein oorblywende gedeelte van die lawa na die erupsie aandui. Die morfologie van die toevoerkanaal, vesikulêre aard van die lawa, die hoeveelheid geproduseer (tussen 0.3 en 1 m3) , en die oënskynlike snelheid waarteen dit ge-ekstrueer is, sluit skynbaar die moontlikheid uit dat die gebeurtenis óf deur 'n weerligstraal óf 'n elektriese ontlading van die nabye kraglyn veroorsaak is.

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1998-12-01
2020-10-01

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