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USGS CMG InfoBank: Upper and Lower Mantle

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Comment: 18:18 - 19:53 (01:35)
Source: Annenberg/CPB Resources - Earth Revealed - 6. Plate Dynamics
Keywords: "Don L. Anderson", mantle, plate, subduction, "Pacific Ocean", earthquake, depth, seismic, convection, viscosity, "mid-oceanic ridge", basalt, lithosphere, "plate tectonics"

Our transcription: I think the mantle's split into at least two layers that we call the "upper" mantle and the "lower" mantle.
The evidence for this is the plates that subduct around the ring of fire, around the Pacific Ocean, and generate mostly earthquakes.
We can trace these earthquakes down to about 700 kilometers depth; then, the earthquakes stop.
We also use seismic images to find out where these slabs are, and they appear, the slabs themselves appear to stop at 700 kilometers.
Below that we have the lower mantle which is also convecting and to some extent influencing convection in the shallow mantle.
But it probably has a higher viscosity; it probably has a higher density, and it's likely that the slabs cannot sink into the lower mantle but have to stop at the 700 kilometer boundary, which is a very sharp seismic discontinuity.
So, in my view, we have two layers.
The upper mantle provides mid-ocean ridge basalts.
Then, as the lithosphere cools and gets denser, it sinks back down to the bottom of the lower mantle; then eventually heats back up again and comes up to the surface.
The lower mantle is convecting very, very slowly, but because it's convecting very slowly, the parts of the lower mantle that are particularly hot, stay hot for a long time and tend to heat up the upper mantle.
So I think the lower mantle is definitely influencing convection in the upper mantle on the locations of where there's hot upwelling material.
But I don't think any material's coming directly from the lower mantle into the upper mantle, nor is upper mantle material sinking into the lower mantle.

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