Earth's Mantle convection and seismic activity

Heat rises through the mantle in convection currents like those in a pan of water on a stove. But does semi molten rock circulate through the whole mantle or are there two separate circulations like a double boiler? Density estimates suggest the upper and lower mantles are chemically different and are therefore not mixed by whole mantle circulation. But seismic evidence shows slabs of cold materials descending through the lower mantle: some volcanic rocks carry chemical signatures of the lower mantle. There is a barrier to flow about 650 KM down. To descend further, rocks may denser.

As the seismic waves travel through the Earth’s crustal layer, the velocity of seismic waves is highest in hard, dense rocks. However, seismic waves travel faster through the semi molten mantle than the solid lithosphere above it. This apparent mystery is explained by the Earth’s gravity, which compresses the planet into a ball, both making the interior extremely hot and subjection it to great pressure. The immense pressure generated within the mantle means that the hot rock of which it is composed can remain molten but also be incredible dense at the same time. The density changes encountered with increasing depth can cause traveling seismic waves to be refracted.