A mantle plume is a localized buoyant upwelling in a planetary mantle. Hot material can rise because thermal expansion lowers its density relative to its surroundings; compositional differences may strengthen or oppose that buoyancy. As it rises and decompresses, it can cross a rock's solidus and produce melt without requiring heat to be added during ascent.
A volcanic chain, a broad topographic swell or a seismic anomaly can motivate a plume interpretation, but they measure different consequences. Plate motion, background mantle flow, melt transport and crustal structure influence the surface expression. A plume hypothesis is tested by combining those observations rather than reading a unique source depth from a hotspot's position.
The Yellowstone hotspot is the region of anomalous volcanism and subsurface thermal structure associated with Yellowstone and the younger end of the eastern Snake River Plain. The observed volcanic progression constrains a relative history of the surface plate and the source of heat; it does not by itself prove an immobile vertical conduit.
Laboratory experiments have tested how subduction-driven mantle flow can distort a buoyant mantle plume and produce contrasting surface thermal progressions. This provides a physical mechanism to compare with the Snake River Plain and High Lava Plains patterns, while remaining conditional on the model geometry and flow properties.[ref]
Bifurcation of the Yellowstone plume driven by subduction-induced mantle flow by Christopher Kincaid and others. Abstract: laboratory plume–subduction interaction and opposite volcanic progressions
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A mantle plume is an upwelling of hot mantle material from deep within the Earth's mantle, typically originating from a boundary layer between the mantle and core or from the lower mantle. These plumes are thought to be caused by thermal convection within the Earth, where hot, less dense material rises while cooler, denser material sinks.