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Our transcription: The depth at which mantle convection occurs is another controversial issue within the geologic community. A single convection cell extending throughout part or all of the mantle was first proposed as part of the sea floor spreading hypothesis. Most current models, however, use a two tiered approach known as the "boundary layer" theory of convection. According to this theory, one set of cells in the upper mantle is driven by another set in the lower mantle. Although the issue of mantle convection is still unresolved, evidence from seismic studies of the Earth's interior points toward a two tiered process.
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