The first suggestion are the so-called ‘thermal models’, and the second are the ‘chemical models’. They may be (likely are) zones of higher temperature (because of the lower shear wave velocity), but they may (likely do) consist of rock with different composition. The explanation of the LLSVPs has been discussed for decades. They are still quite flat structures: the width of perhaps 5000 km or more is several times larger than the height. The African LLSVP is more than twice as high, with its top at 1600 km: it reaches halfway to the crust. The Pacific LLSVP extends 700 km above the core-mantle boundary, with the top about 2300 km below the surface. The African LLSVP also reaches much higher than the Pacific one. The two regions have different shapes: the Pacific LLSVP is roundish and the African LLSVP has a more complex shape, elongated north–south with two extension east-west. They sit on top of the core-mantle boundary, entombed within the lower mantle. One of the regions is located underneath Africa, and the other is almost (but not quite) opposite, underneath the Pacific. Source: Cottaar & Lekic, 2016, Morphology of seismically slow lower-mantle structures, Geophysical Journal International, Volume 207, Pages 1122–1136 Once we know what they are, a more palatable name (not involving piles) will be invented. The plethora of unhelpful names shows that the understanding of them is still lacking. The name ‘thermo-chemical pile’ has been used in scientific papers but this sounds like a severe and socially inhibiting disease. Nowadays it is abbreviated as ‘LLSVP’ which to me still sounds like a quango! Wikipedia calls them ‘superplumes’, whilst the term ‘superswells’ is also in use both names, however, are also used for other (perhaps related) phenomena. (But ‘Large’ and ‘Provinces’ does seem a tautology, saying the same thing twice.) However, it is clearly a name designed by committee. The name kind of makes sense: the waves are shear waves, they are slow, and the regions of slowness are very large and therefore are called ‘provinces’ rather than say ‘spots’. These two areas of tardiness have become known as ‘Large Low Shear Velocity Provinces’. The two regions occupy around 8% of the volume of the mantle. This does not sound dramatic, but it is far more than expected.
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In particular, there are two very large regions in the lower mantle where the shear wave velocity decreases significantly. Other anomalies are not nearly as well understood.
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In these places of subduction, the high shear wave velocity reveals the departed but still cold ocean floor.
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Try kicking a deflated football and compare it to one at full pressure, if you need experimental evidence (always recommended). A hard push gives a fast response, and therefore a cold slab has a higher v s than a warmer, softer slab. Think of the students in the previous post: if they push back hard against the wave coming from behind, they push the person in the line equally hard. The shear wave velocity is particularly sensitive to this: in harder material, it travels faster. The subducting slabs are cold and hard, and therefore resistant to deformation.
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For instance, S-waves in subduction regions show high velocities, indicating low temperatures. There are locations below which the earthquake waves travel unexpectedly fast or slow. But there are deviations: not all places on Earth have the same mantle structure underneath them. Seismography has shown us how the conditions in the mantle change with depth. Now it is time to see what we can do with this knowledge, study the alien life of the Earth – and find the Earth’s heart. In the previous post we went through some of the background to this post: the structure of the Earth, the change in minerals at various depth in the mantle which change the density, and a bit on seismology which can show how the temperature changes with depth.