Graduate Thesis Or Dissertation

 

The flow mechanics and resulting erosional and depositional features of explosive volcanic density currents on earth and Mars Public Deposited

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  • Straight, radial, erosional channels and fan-like deposits of tephra develop around steep-sided cones and composite volcanoes, but are not characteristic features of shield volcanoes. The channels and deposits normally are formed by explosive volcanic density currents which originate as nuées adrentes or base surges. The physical processes of explosive volcanic density current transport, erosion, and deposition are modeled using the equations developed to describe turbidity currents and grain flows, similar species of sediment gravity flows. The model predicts reasonable values for the flow densities and thicknesses of two photometrically timed nuée ardent eruptions, and illustrates that a nuée ardente probably develops from a single layer, highly mobile, autosuspended flow into a two layer system consisting of a high concentration, pseudo-laminar underflow and an overriding turbulent cloud. The model also makes possible quantitative comparisons between the driving forces of nuées ardentes, deep-sea turbidity currents, rivers and catastrophic floods such as that from Pleistocene Lake Missoula. These calculations indicate that the ero­sive potentials of nuées ardentes are far greater than river erosion but very similar to turbidity currents and catastrophic flooding. A similar analysis for such flows on Mars shows that the velocities and driving forces compare with terrestrial nuées ardentes and base surges, but martian pyroclastic flows probably travel and disperse debris over greater distances. The presence of a permafrost terrain suggests the most plausible generating mechanism for explosive volcanic density currents in a martian environment is by hydromagmatic explosions. This mechanism may explain the steeper slopes, broad calderas, radial channels and blanketing flank deposits on a number of martian volcanoes (specifically Ceraunius Tholus, Uranius Tholus, Uranius Patera and Hecates Tholus) that were heretofore puzzling. Crater age data indicate these volcanoes probably represent an early to intermediate period of martian volcanism, on the order of 0.5 to 2 billion years older than the quietly erupted flank surfaces on the giant Tharsis shields and basal plains. These data are consistent with the view that martian atmospheric conditions in the past may have been more favorable for periodic burial of water and other volatiles in the vicinity of volcanic conduits, hence producing an extensive permafrost layer conducive to phreatic explosions and geothermal seepage. It also allows for explosive volcanism on a planet without moving plates.
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