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, Iceland, 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 main problem with the standard accepted 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

Also see : LLSVPs (superplumes)plugin-autotooltip__plain plugin-autotooltip_bigLLSVPs (superplumes)

In the mid 1980s, seismologists analyzing recordings of reflections of seismic waves in the Earth's crust (caused by earthquakes) found anomalies which suggested the existence of two supersized 'plumes' of semi-molten rocky material in the region between the Earth's crust and its core. Subsequent measurements have confirmed the two continent-sized structures, which are now known as