因此，地幔底层既是全地幔对流的起点，又是全地幔对流的终点。
Therefore, the CMB is the beginning and the end of whole mantle convection.
地幔底层由具有高地震波速和高密度的D″区和超低速带（ULVZ）组成。
The lowermost mantle layer is above the core mantle boundary (CMB) and composed of D" region and ultra-low velocity zone (ULVZ)."
地幔底层是地核热能向地幔传播的必经之路，也是地幔中温度和温度梯度最高的地区。
The CMB is not only the medium transferring thermal energy from core to mantle, but also the region in which the temperature and temperature gradient is the highest in the whole mantle.
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