A Mantle Plume is a proposed convection mechanism of abnormally hot rock within the Earth's mantle. In which huge 'mushroom' shaped plumes of molten rock rise, by thermal convection, towards the surface. From depths of around 3,000Km, and creating volcanic 'hotspots' - e.g Hawaii, the Azores, Icleand, Yellowstone etc.
The concept of deep-mantle plumes is typically embedded in Earth science courses at all levels. It is commonly presented as a problem-free fact in most textbooks and popular science books though its basic theory is usually neglected.
Source : MantlePlumes.org
The hypothesis is, however, not universally accepted by geologists. The main problem with the theory is that, since its proposal, many studies have shown that real-world examples often don't meet the key characteristics of the model. Especially in chains of volcanic islands that have been found to originate from different time periods.
Two other prominent theories to explain volcanic hotspots are the 'Plate Hypothesis' and the 'Impact Hypothesis' - see Wikipedia
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