030311 Volcanoes & Internal Heat Flow-1

030311 Volcanoes & Internal Heat Flow-1 - 1...

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1 03/03/11 Volcanism & Internal Heat Flow Volcanism & Internal Heat Flow Questions to remember and contemplate (!) 1) What drives global-scale tectonics for terrestrial planets? Or, put differently, what are the energy “reservoirs” for providing the power to do the work that is global tectonics? 2) Is global-scale tectonics dominantly horizontal or vertical for each of the Terrestrial Planets? 3) Given that Plate Tectonics is the global-scale explanation for Earth tectonics, does it apply to Venus and Mars? 4) Are the terrestrial planets tectonically active now? Whatever the answer, Why? 5) Also, if the answer is not the same for all terrestrial planets, why? Introduction The dominant geological process for a terrestrial planet is cooling. Tectonicism is really a first-order manifestation (e.g., plate tectonics on the Earth) of this cooling. However, cooling creates other geological structures, such as volcanoes at the planetary surface that provide additional information about the pattern of past and present planetary cooling. We combine this information with our understanding of heatflow for the Earth to provide insight for heatflow past and present for Venus and Mars. Volcanism Three important types of volcanism for Earth (Slides 3 to 5)): Mid-oceanic ridge basalts produced by decompression melting of upwelling asthenosphere are volumetrically most important. They "pave" about 70% of the Earth’s surface (e.g. Slide 4: East Pacific Rise, mid-Atlantic Ridge). Volcanic arcs of composite volcanoes where subduction of oceanic lithosphere releases water, hydrating the upper mantle and producing basalts, andesites, etc. (e.g., Slide 5: Mt. St. Helens, Mt. Pelee, Mt. Vesuvius, Mt. Pinatubo) Hotspot basalts and “large igneous provinces (LIPs)” form mainly above mantle plumes that are mostly rooted to the mantle/core interface. They represent a separate heat-transfer mechanism from "general" mantle convection. (e.g., Slide 5: Hawaii, Deccan Traps, Yellowstone-Snake River Province). Application to Mars and Venus Basalt is the important surface bedrock on Mars and Venus, so volcanic processes related to basalts are important: spreading centers and hotspot magmatism. At present, evidence for andesites or rhyolites is limited or absent, which may have implications for presence or absence of subduction on Mars and Venus. Surface evidence for Volcanism on Mars (slide 6) Mars has three “volcanic provinces” with basalt as the signature rock type: southern highlands, northern lowlands and Tharsis. Tharsis “province” is distinctly different with largest (shield) volcanoes in the solar system. Tharsis is an early feature of Mars, but
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2 younger than the hemispheric difference. Cratering data may indicate eruptions within last 100 m.y.
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