Unfortunately mantle convection as a direct cause of plate motion just doesnt

Unfortunately mantle convection as a direct cause of

This preview shows page 6 - 8 out of 8 pages.

Unfortunately, mantle convection as a direct cause of plate motion just doesn’t work. There are convection currents within the mantle and asthenosphere, but they don’t appear to directly drive plate motion. These convection currents do act to transfer huge amounts of heat from Earth’s core and heat generated by the decay of radioactive elements within Earth, but the conveyor belt model failed! Ridge Push Ridge push - force developed by higher elevations at the ridge crest of mid- ocean ridges. Gravity causes the elevated lithosphere at the ridge crest to push on the lithosphere on the slopes of the ridge. As the lithosphere moves away from the ridge crest, new hot asthenosphere rises to fill the gap – forming new ocean lithosphere. The upward movement of asthenosphere under the mid-ocean ridge is a consequence of seafloor spreading, not the cause. Slab Pull Slab Pull – force that subducting, downgoing plates apply to oceanic lithosphere at convergent margins. Like a tablecloth sliding off a table. The subducting plate can be pull or sink down into the asthenosphere because it is less denser (cooler). Once the subducting plate starts to sink, it gradually pulls the rest of the plate along behind it. Convecting asthenosphere in some areas may apply some force helping to move the plate down into the subduction zone, but in other places this force may slow subduction. 6
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What Causes Plate Motion? Most likely direct processes are ridge push and slab pull . But, heat transfer by convection in the mantle does greatly affects density (hot = less dense, cooler = more dense). And density affects isostasy, which affects elevation. Hot, less dense ocean lithosphere stands high at mid-ocean ridges. Gravity appears to do the rest! But, do we fully understand plate motion – no! But we’re working on it. To summarize: Theory of Plate Tectonics This theory unifies all aspects of Earth Science and explains all the features of Earth surface and interior. Explains: Positions of continents and oceans Earthquakes Volcanoes Mountains Why organisms are different in different locations (example: fauna in Australia) Wegener had it right all along. Earth changes as plates move. His Pangea was assembled 250 million years ago and then broke apart. A Review Types of Plate Boundaries (Active). Divergent Margins - spreading centers, 2 plates move apart. Example: Mid-Atlantic Ridge. Convergent Margins - 2 plates moving towards each other. One plate sinks beneath other - subduction zone . Example: west coast of South America. Two plates collide - collision zone . Example: Asia - India forming Himalayas. Transform Margin - 2 plates slide past each other. Example: San Andreas Fault System. Plate Tectonics Review. Lithosphere is thin, cool, and hard. 7
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Asthenosphere is weak and hot. Lithosphere broken into large fragments called ‘Plates’. Plates “float” on the asthenosphere. Plates move around and interact with each other. Interactions result in earthquakes, volcanoes, mountains, ocean basins, etc. 8
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