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 Large Low-Shear-Velocity Provinces or LLSVPs.

The African LLSVP and the Pacific LLSVP are several thousand km across and are thought to be around one thousand km in depth.

The origins of the two masses are unknown.

The current leading hypothesis is that they are long-term accumulations of oceanic crust slabs forced downwards due toTectonic subductionplugin-autotooltip__plain plugin-autotooltip_bigTectonic subduction

Movements of the 'Tectonic Plates' are believed to have formed almost all of the Earth's current-day crust - relying on a process called 'Subduction'.

In that process, the oceanic lithosphere of a tectonic plate collides with the less dense lithosphere of a second plate. Under pressure, the heavier plate slides under the second plate and sinks down into the mantle. See

Another theory suggests that they could be remnants of Theia, a planet-sized body which is theorised to have collided with Earth at the time of the formation of the Moon.

Further reading : Wikipedia (also showing an animation of a 3-D model of the LLSVPs)

Related : The work of the Mukhopadhyay group at the UC Davis Earth and Planetary Sciences dept.

Also see : Mantle plumesplugin-autotooltip__plain plugin-autotooltip_bigMantle plumes

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.