UNDERSTANDING OUR PLANET Q/A #1- Why some parts of the lower mantle hotter than other parts? – First of all, the earth temperature is higher proportionally direct according to the pressure. This explains why the core is hotter than the lower mantle and the lower mantle is hotter than the upper mantle. The major factor causing heat beneath the earth is fusion reactions in the core. This reaction is involving some radioactive element such as uranium (U), thorium (Th), potassium (K).
In subduction zone crustal material goes down deep into the mantle. It is assumed that the subducted crustal materials will be uniformly distributed in the whole mantle. Yet, based on many studies, there are geophyscial as well as geochemical evidence that some of these crustal material can go as deep as core-mantle boundary. Later on, the experts found particular zones where some of these crustal materials get concentrated. As mentioned before, these crustal materials are strongly enriched in heat producing elements like those three radioactive elements. As the time goes by, catalyzed by dynamics movements of plate, the area of subducted crustal material in the lower mantle will be really hotter than the surrounding and cause melting of the mantle in that area.