Some Possible Victorian Analogues of the Flat Topped Volcanoes of Venus

Joseph Leach
Department of Infrastructure Engineering, University of Melb

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     Last modified: July 15, 2011

Abstract
The similarity of the small to medium volcanoes on Venus and rhyolite domes on Earth was noted early in the investigation of the Magellan data. These domes are distinctive because of the high viscosity of their lava. There was, however, one class of the volcanoes observed which cannot be explained this way, These are the flat topped, mesa-like domes. The volcanic province of Victoria, in southern Australia, has a range of eruption types covering a wide range of viscosities: from basalts to soda trachites. Nowhere in this sequence, however, do we see the mesa-like features of the flat topped domes – suggesting that viscosity alone was not responsible for their formation. To achieve such a form, something must restrict the lava flow to a specific radius from the eruption point. Some possible terrestrial analogues, such as the flat topped sea mounts seen in GLORIA imagery and the tuyas of Iceland and Alaska, have been suggested. However,, it is hard to see how the processes which formed these types of volcanoes could occur on Venus. There is another class of flat topped volcanic feature which occurs in the South east Australian volcanic province - but they are not primary forms. They occur where late stage lava has erupted into the crater of a pre-existing scoria cone. Here the scoria acts to restrict the lateral flow, resulting in a basalt plug. Once the scoria is eroded, this plug is left as a flat topped hill. Scoria is unlikely to be the restricting material on Venus. However, another material, such as surficial sand deposits, could be a substitute allowing this process to occur. An intriguing possibility is that this material is still there but invisible to the Magellan RADAR.