031011 Terrestrial Planetary Tectonics

031011 Terrestrial Planetary Tectonics - 1 03/10/11 Mantle...

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1 03/10/11 Mantle Convection, Terrestrial Planet Tectonics Mantle Convection, Terrestrial Planetary Tectonics Considering Convection on Earth We will begin class today by improving your understanding of convection within a terrestrial planet as convection is a key planetary-scale heat transfer process and ultimately should drive tectonics. Typically, you are taught to consider convection in the Earth to be analogous to convection in a beaker of water heated from the base (slide 3). Mantle convection DIFFERS significantly from this analogy, and the differences are important to understanding mantle convection and hence, tectonics (Slides 4, 5). The Differences : Characteristic Convecting Water Convecting Mantle “Container geometry” Upright cylinder with top and base of same size Concentric shell with a top/outside that is much larger than base Nature of “top” Liquid like the convecting liquid Rigid, segmented mobile lid or unsegmented stagnant lid – Colder, compositionally less dense rock Heating Heat applied at base of container Heat applied at base of “container, but 80% of heat from internal radioactive decay in the mantle Applying the differences to understanding to tectonics of Earth (Slides 4, 5, 6) Plate tectonics is driven by whole-mantle or near whole-mantle convection where the majority of the cooling is from internal heating of the convecting “liquid”. Consequently, the upper thermal boundary is the driving thermal boundary of the system because more heat is transferred out of the outside of the Earth as compared to heat added across the core-mantle boundary. Thus, the downwelling portion of the system is stronger than the upwelling because of the greater heat loss through the upper thermal boundary. Downgoing colder, subducting plates both strengthen and focus the downwelling. In contrast, the upwelling mantle is passive and diffuse, so spreading centers do NOT correlate to narrow powerful sites of upwelling (Slides 4 and 6). Class Activity: Question: How do you start downwelling with a rigid lid for a terrestrial planet?
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2 Given a more useful representation of convection for the Earth, let’s consider the implications for Venus and Mars. Successfully triggering downwelling (active-lid planetary tectonics) One may consider the Earth with plate tectonics to be an example of a planet with “active lid” behavior where downwelling is occurring. For active-lid tectonics with Earth-like subduction or even “drip tectonics (Slide 7)”, the
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This note was uploaded on 11/10/2011 for the course GEOL 380 taught by Professor Mcsween during the Spring '11 term at University of Tennessee.

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031011 Terrestrial Planetary Tectonics - 1 03/10/11 Mantle...

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