The Regional Context of the McBride Basalt Province and the Formation of the Undara Lava Flows, Tubes, Rises and Depressions

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Conference: 14th International Symposium on VulcanospeleologyAt: Undara Volcanic National Park, QLD, Australia.Volume: Proceedings of the 14th International Symposium on Vulcanospeleology


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The McBride Basalt Province covers approximately 5500 km 2 and is one of 11 discontinuous regions of Cenozoic volcanic rocks in north Queensland. These provinces are the northernmost part of a chain of volcanics that extend down the entire eastern coast of Australia, from the Torres Strait to southern Tasmania. These volcanics can be subdivided into central volcano provinces, lava field provinces and lesser leucitite provinces. The central volcano and leucitite provinces contain some felsic volcanics and form hotspot trails, whereas the lava field provinces are entirely mafic and do not have a well defined age-latitude relationship. All of the north Queensland provinces are of the lava field type and are centred around the Great Divide, suggesting a genetic relationship between the divide and the volcanic provinces. 164 eruption centres have been identified within the McBride Basalt Province. The majority of flows are less than 3 Ma, with the youngest volcano (Kinrara) being less than 50,000 y. Remnants of older flows of around 8 Ma occur in the southwest. A hiatus in eruptions is evident from 8 Ma to 3 Ma. Lavas from the 190,000 year old Undara volcano, cover about 1550 km 2. Undara crater is nearly circular with a diameter of around 330 m and is about 60 m deep. It forms the highest point of the McBride Basalt Province, at 1020 m a.s.l., although its low rim rises only 20 m above the surrounding lava field. Unusually for the McBride Province, the Undara vent does not have associated pyroclastic material. The longest of the Undara flows is some 160 km, with an average gradient of just 0.3°. Thermal insulation provided by the solidification of the crust above the actively flowing lava is required to produce flows of such length. In the distal regions, the confinement of the initial flows within pre-existing drainage channels also assists the development of such long flows. Lava tube caves occur up to 30 km from Undara crater, where the gradient is > 0.6°. Up to five successive flow units, demarcated by pahoehoe surfaces, can be seen in the walls of some caves. The caves developed by erosion through the lower units. Entrance to the caves is by steep collapse structures. Lava inflation features, such as lava-rise ridges and lava-rise pits, are common in the Undara flows. A particularly long lava-rise ridge, known as " The Wall " , extends for some 40 km. It is up to 20 high, and around 200 m wide. Formation was due to the inflation of a flow in a confined channel. Where a section of a flow solidifies, it is not underlain by actively flowing lava, so can not inflate. Where the surrounding region may undergo inflation, a depression is left at the level of the pre-inflation surface. Such lava inflation pits have more gently sloping sides than lava tube collapses and may contain pahoehoe floors that continue up their flanks. A series of lava-rise pits occur in close alignment to the main lava tube lines at Undara. In places, lava tubes head towards a depression, then are deflected around the solidified obstruction.




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